X-Git-Url: https://git.dogcows.com/gitweb?a=blobdiff_plain;f=doc%2Ftar.texi;h=ddfbac5d9644d4be91af12a55fa6db884be211e7;hb=d01f65092c3ecf3dc8b5f443a542975249afa86c;hp=12b9ad2649f047d2be3f0814e67c3949db186eb5;hpb=8835bb37e9f1286f3e808279ddae58b8e090f29a;p=chaz%2Ftar diff --git a/doc/tar.texi b/doc/tar.texi index 12b9ad2..ddfbac5 100644 --- a/doc/tar.texi +++ b/doc/tar.texi @@ -1,3675 +1,8914 @@ -\input texinfo @c -*-texinfo-*- -@c %**start of header +\input texinfo @c -*-texinfo-*- +@comment %**start of header @setfilename tar.info -@settitle The Tar Manual: DRAFT +@include version.texi +@settitle GNU tar @value{VERSION} @setchapternewpage odd + +@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 +@include rendition.texi +@include value.texi -@c Search for comments marked with !! or <<< (or >>>) +@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 -@smallbook +@defindex op +@syncodeindex op cp -@iftex -@c finalout -@end iftex +@copying -@ifinfo -This file documents @code{tar}, a utility used to store, backup, and -transport files. +This manual is for @acronym{GNU} @command{tar} (version +@value{VERSION}, @value{UPDATED}), which creates and extracts files +from archives. -Copyright (C) 1992 Free Software Foundation, Inc. DRAFT! -@c Need to put distribution information here when ready. -@end ifinfo +Copyright @copyright{} 1992, 1994, 1995, 1996, 1997, 1999, 2000, 2001, +2003, 2004 Free Software Foundation, Inc. -@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 +@quotation +Permission is granted to copy, distribute and/or modify this document +under the terms of the GNU Free Documentation License, Version 1.1 or +any later version published by the Free Software Foundation; with the +Invariant Sections being "GNU General Public License", with the +Front-Cover Texts being ``A GNU Manual,'' and with the Back-Cover Texts +as in (a) below. A copy of the license is included in the section +entitled "GNU Free Documentation License". + +(a) The FSF's Back-Cover Text is: ``You are free to copy and modify +this GNU Manual. Buying copies from GNU Press supports the FSF in +developing GNU and promoting software freedom.'' +@end quotation +@end copying + +@dircategory Archiving +@direntry +* Tar: (tar). Making tape (or disk) archives. +@end direntry -@author Michael I. Bushnell and Amy Gorin +@dircategory Individual utilities +@direntry +* tar: (tar)tar invocation. Invoking @GNUTAR{}. +@end direntry + +@shorttitlepage @acronym{GNU} @command{tar} + +@titlepage +@title @acronym{GNU} tar: an archiver tool +@subtitle @value{RENDITION} @value{VERSION}, @value{UPDATED} +@author John Gilmore, Jay Fenlason et al. @page @vskip 0pt plus 1filll -Copyright @copyright{} 1992 Free Software Foundation, Inc. - -@sp 2 -This draft is not yet ready for distribution. +@insertcopying @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. +@ifnottex +@node Top +@top @acronym{GNU} tar: an archiver tool -@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 +@insertcopying -@c <<< The menus need to be gone over, and node names fixed. -@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 +@cindex file archival +@cindex archiving files -@chapter Tutorial Introduction to @code{tar} +The first part of this master menu lists the major nodes in this Info +document. The rest of the menu lists all the lower level nodes. +@end ifnottex -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. +@c The master menu, created with texinfo-master-menu, goes here. +@c (However, getdate.texi's menu is interpolated by hand.) @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 +* Introduction:: +* Tutorial:: +* tar invocation:: +* operations:: +* Backups:: +* Choosing:: +* Date input formats:: +* Formats:: +* Media:: + +Appendices + +* Genfile:: +* Snapshot Files:: +* Free Software Needs Free Documentation:: +* Copying This Manual:: +* Index:: -@section What @code{tar} Does +@detailmenu + --- The Detailed Node Listing --- -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. +Introduction -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. +* Book Contents:: What this Book Contains +* Definitions:: Some Definitions +* What tar Does:: What @command{tar} Does +* Naming tar Archives:: How @command{tar} Archives are Named +* Current status:: Current development status of @GNUTAR{} +* Authors:: @GNUTAR{} Authors +* Reports:: Reporting bugs or suggestions -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. +Tutorial Introduction to @command{tar} -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. - -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. - -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}. +* assumptions:: +* stylistic conventions:: +* basic tar options:: Basic @command{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:: -@section How to Create Archives +Two Frequently Used Options -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: +* file tutorial:: +* verbose tutorial:: +* help tutorial:: -@example -tar --create --file=afiles.tar apple angst asparagus -@end example +How to Create Archives -The order of the arguments is not important. You could also say: +* prepare for examples:: +* Creating the archive:: +* create verbose:: +* short create:: +* create dir:: -@example -tar apple --create angst --file=afiles.tar asparagus -@end example +How to List Archives -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. +* list dir:: -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: +How to Extract Members from an Archive -@example -tar --create --file=empty-archive.tar -@end example +* extracting archives:: +* extracting files:: +* extract dir:: +* failing commands:: -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. +Invoking @GNUTAR{} -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: +* Synopsis:: +* using tar options:: +* Styles:: +* All Options:: +* help:: +* verbose:: +* interactive:: -@example -tar --create --file=bfiles.tar ./balloons baboon ./bodacious -@end example +The Three Option Styles -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}. +* Mnemonic Options:: Mnemonic Option Style +* Short Options:: Short Option Style +* Old Options:: Old Option Style +* Mixing:: Mixing Option Styles -If you want to see the progress of tar as it writes files into the -archive, you can use the @samp{--verbose} option. +All @command{tar} Options -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. +* Operation Summary:: +* Option Summary:: +* Short Option Summary:: -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: +@GNUTAR{} Operations -@example -tar: can't open /dev/rmt8 : No such device or address -tar: can't open /dev/rsmt0 : I/O error -@end example +* Basic tar:: +* Advanced tar:: +* create options:: +* extract options:: +* backup:: +* Applications:: +* looking ahead:: -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. +Advanced @GNUTAR{} Operations -@section How to List Archives +* Operations:: +* append:: +* update:: +* concatenate:: +* delete:: +* compare:: -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: +How to Add Files to Existing Archives: @option{--append} -@example -apple -angst -asparagus -@end example +* appending files:: Appending Files to an Archive +* multiple:: -The archive @file{bfiles.tar} would list as follows: +Updating an Archive -@example -./baloons -baboon -./bodacious -@end example - -(Of course, @samp{tar --list --file=empty-archive.tar} would produce no -output.) - -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. - -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. +* how to update:: -@section How to Extract Members from an Archive +Options Used by @option{--create} -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. +* Ignore Failed Read:: -@section How to Add Files to Existing Archives +Options Used by @option{--extract} -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}. +* Reading:: Options to Help Read Archives +* Writing:: Changing How @command{tar} Writes Files +* Scarce:: Coping with Scarce Resources -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. +Options to Help Read Archives -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): +* read full records:: +* Ignore Zeros:: -@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 +Changing How @command{tar} Writes Files -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 +* Dealing with Old Files:: +* Overwrite Old Files:: +* Keep Old Files:: +* Keep Newer Files:: +* Unlink First:: +* Recursive Unlink:: +* Modification Times:: +* Setting Access Permissions:: +* Writing to Standard Output:: +* remove files:: -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. +Coping with Scarce Resources -@chapter Invoking @code{tar} +* Starting File:: +* Same Order:: -The usual way to invoke tar is +Performing Backups and Restoring Files -@example -@code{tar} @var{options}... [@var{file-or-member-names}...] -@end example +* Full Dumps:: Using @command{tar} to Perform Full Dumps +* Incremental Dumps:: Using @command{tar} to Perform Incremental Dumps +* 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 -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. +Setting Parameters for Backups and Restoration -@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 +* General-Purpose Variables:: +* Magnetic Tape Control:: +* User Hooks:: +* backup-specs example:: An Example Text of @file{Backup-specs} -@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 +Choosing Files and Names for @command{tar} -@item --create -@item -c -Create a new archive +* 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 -@item --catenate -@item --concatenate -@item -A -Add the contents of one or more archives to another archive +Reading Names from a File -@item --append -@item -a -Add files to an existing archive +* nul:: -@item --list -@item -t -List the members in an archive +Excluding Some Files -@item --delete -Delete members from an archive +* controlling pattern-patching with exclude:: +* problems with exclude:: -@item --extract -@item --get -@item -x -Extract members from an archive +Crossing Filesystem Boundaries -@item --compare -@item --diff -@item -d -Compare members in an archive with files in the file system +* directory:: Changing Directory +* absolute:: Absolute File Names -@item --update -@item -u -Update an archive by appending newer versions of already stored files -@end itemize +Date input formats -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. +* General date syntax:: Common rules. +* Calendar date items:: 19 Dec 1994. +* Time of day items:: 9:20pm. +* Time zone items:: @sc{est}, @sc{pdt}, @sc{gmt}, ... +* Day of week items:: Monday and others. +* Relative items in date strings:: next tuesday, 2 years ago. +* Pure numbers in date strings:: 19931219, 1440. +* Seconds since the Epoch:: @@1078100502. +* Authors of get_date:: Bellovin, Eggert, Salz, Berets, et al. -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}. +Controlling the Archive Format -@section An Old, but Still Supported, Syntax for @code{tar} Commands +* Portability:: Making @command{tar} Archives More Portable +* Compression:: Using Less Space through Compression +* Attributes:: Handling File Attributes +* Standard:: The Standard Format +* Extensions:: @acronym{GNU} Extensions to the Archive Format +* cpio:: Comparison of @command{tar} and @command{cpio} -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: +Making @command{tar} Archives More Portable -@example -@code{tar} @var{option-letters}... [@var{option-arguments}...] [@var{file-names}...]@refill -@end example +* Portable Names:: Portable Names +* dereference:: Symbolic Links +* old:: Old V7 Archives +* posix:: @acronym{POSIX} archives +* Checksumming:: Checksumming Problems +* Large or Negative Values:: Large files, negative time stamps, etc. -@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 Less Space through Compression -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}. +* gzip:: Creating and Reading Compressed Archives +* sparse:: Archiving Sparse Files -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. +Tapes and Other Archive Media -@chapter Basic @code{tar} Operations +* 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:: -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. +Blocking -@section Creating a New Archive +* Format Variations:: Format Variations +* Blocking Factor:: The Blocking Factor of an 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. +Many Archives on One Tape -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}. +* Tape Positioning:: Tape Positions and Tape Marks +* mt:: The @command{mt} Utility -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. +Using Multiple Tapes -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}.) +* Multi-Volume Archives:: Archives Longer than One Tape or Disk +* Tape Files:: Tape Files -There are various ways of causing @code{tar} to skip over some files, -and not archive them. @xref{Specifying Names to @code{tar}}. +GNU tar internals and development -@section Adding to an Existing Archive +* Genfile:: +* Snapshot Files:: -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. +Copying This Manual -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}. +* GNU Free Documentation License:: License for copying this manual -This operation cannot be performed on some tape drives, unfortunately, -due to deficiencies in the formats thoes tape drives use. +@end detailmenu +@end menu -@section Combining Archives +@node Introduction +@chapter Introduction -The @samp{--catenate} (or @code{--concatenate}, or @code{-A}) causes -@code{tar} to add the contents of several archives to an existing -archive. +@GNUTAR{} creates +and manipulates @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. +The name ``tar'' originally came from the phrase ``Tape ARchive'', but +archives need not (and these days, typically do not) reside on tapes. -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}. +@menu +* Book Contents:: What this Book Contains +* Definitions:: Some Definitions +* What tar Does:: What @command{tar} Does +* Naming tar Archives:: How @command{tar} Archives are Named +* Current status:: Current development status of @GNUTAR{} +* Authors:: @GNUTAR{} Authors +* Reports:: Reporting bugs or suggestions +@end menu -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 +@node Book Contents +@section What this Book Contains + +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 @GNUTAR{} +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 @command{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 @command{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 @command{tar} options and option syntax. + +@FIXME{this sounds more like a @acronym{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 @acronym{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 @command{tar} sources. + +In general, we give both 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 +@section Some Definitions + +@cindex archive +@cindex tar archive +The @command{tar} program is used to create and manipulate @command{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 @command{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 @command{ls}, @command{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. -This operation cannot be performed on some tape drives, unfortunately, -due to deficiencies in the formats thoes tape drives use. - -@section Removing Archive Members +@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 performed using @command{tar}. + +@node What tar Does +@section What @command{tar} Does + +@cindex tar +The @command{tar} program provides the ability to create @command{tar} +archives, as well as various other kinds of manipulation. For example, +you can use @command{tar} on previously created archives to extract files, +to store additional files, or to update or list files which were already +stored. + +Initially, @command{tar} archives were used to store files conveniently on +magnetic tape. The name @command{tar} comes from this use; it stands for +@code{t}ape @code{ar}chiver. Despite the utility's name, @command{tar} can +direct its output to available devices, files, or other programs (using +pipes). @command{tar} may even access remote devices or files (as archives). + +@FIXME{the following table entries need a bit of work..} + +You can use @command{tar} archives in many ways. We want to stress a few +of them: storage, backup, and transportation. + +@table @asis +@item Storage +Often, @command{tar} archives are used to store related files for +convenient file transfer over a network. For example, the +@acronym{GNU} Project distributes its software bundled into +@command{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 @command{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 @command{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 @command{tar} can be used to move archives in +all dimensions, even time!) + +@item Backup +Because the archive created by @command{tar} is capable of preserving +file information and directory structure, @command{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. +@GNUTAR{} 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 -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 Naming tar Archives +@section How @command{tar} Archives are Named + +Conventionally, @command{tar} archives are given names ending with +@samp{.tar}. This is not necessary for @command{tar} to operate properly, +but this manual follows that convention in order to accustom readers to +it and to make examples more clear. + +@cindex tar file +@cindex entry +@cindex tar entry +Often, people refer to @command{tar} archives as ``@command{tar} files,'' and +archive members as ``files'' or ``entries''. For people familiar with +the operation of @command{tar}, this causes no difficulty. However, in +this manual, we consistently refer to ``archives'' and ``archive +members'' to make learning to use @command{tar} easier for novice users. + +@node Current status +@section Current development status of @GNUTAR{} + +@GNUTAR{} is currently in the process of active development, whose +primary aims are: + +@itemize @bullet +@item Improve compatibility between @GNUTAR{} and other @command{tar} +implementations. +@item Switch to using @acronym{POSIX} archives. +@item Revise sparse file handling and multiple volume processing. +@item Merge with the @acronym{GNU} @code{paxutils} project. +@end itemize -@section Listing Archive Members +Some of these aims are already attained, while others are still +being worked upon. From the point of view of an end user, the +following issues need special mentioning: -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. +@table @asis +@item Use of short option @option{-o}. -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}. +Earlier versions of @GNUTAR{} understood @option{-o} command line +option as a synonym for @option{--old-archive}. -@section Extracting Archive Members +@GNUTAR{} starting from version 1.13.90 understands this option as +a synonym for @option{--no-same-owner}. This is compatible with +UNIX98 @command{tar} implementations. -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. +However, to facilitate transition, @option{-o} option retains its +old semantics when it is used with one of archive-creation commands. +Users are encouraged to use @value{op-format-oldgnu} instead. -Various options cause @code{tar} to extract more than just file -contents, such as the owner, the permissions, the modification date, and -so forth. +It is especially important, since versions of @acronym{GNU} Automake +up to and including 1.8.4 invoke tar with this option to produce +distribution tarballs. @xref{Formats,v7}, for the detailed discussion +of this issue and its implications. -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. +Future versions of @GNUTAR{} will understand @option{-o} only as a +synonym for @option{--no-same-owner}. -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 +@item Use of short option @option{-l} -@section Updating an Archive +Earlier versions of @GNUTAR{} understood @option{-l} option as a +synonym for @option{--one-file-system}. Such usage is deprecated. +For compatibility with other implementations future versions of +@GNUTAR{} will understand this option as a synonym for +@option{--check-links}. -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. +@item Use of options @option{--portability} and @option{--old-archive} -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. +These options are deprecated. Please use @option{--format=v7} instead. -@section Comparing Archives Members with Files +@item Use of option @option{--posix} -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. +This option is deprecated. Please use @option{--format=posix} instead. +@end table -@chapter Specifying Names to @code{tar} +@node Authors +@section @GNUTAR{} Authors + +@GNUTAR{} was originally written by John Gilmore, +and modified by many people. The @acronym{GNU} enhancements were +written by Jay Fenlason, then Joy Kendall, and the whole package has +been further maintained by Thomas Bushnell, n/BSG, Fran@,{c}ois +Pinard, Paul Eggert, and finally Sergey Poznyakoff with the help of +numerous and kind users. + +We wish to stress that @command{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 @GNUTAR{} 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 @GNUTAR{} +manual, borrowing notes from the original man page from John Gilmore. +This was withdrawn in version 1.11. Thomas Bushnell, n/BSG and Amy +Gorin worked on a tutorial and manual for @GNUTAR{}. +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}. + +In July, 2003 @GNUTAR{} was put on CVS at savannah.gnu.org +(see @url{http://savannah.gnu.org/projects/tar}), and +active development and maintenance work has started +again. Currently @GNUTAR{} is being maintained by Paul Eggert, Sergey +Poznyakoff and Jeff Bailey. + +Support for @acronym{POSIX} archives was added by Sergey Poznyakoff. + +@node Reports +@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-tar@@gnu.org}. + +When reporting a bug, please be sure to include as much detail as +possible, in order to reproduce it. @FIXME{Be more specific, I'd +like to make this node as detailed as 'Bug reporting' node in Emacs +manual}. + +@node Tutorial +@chapter Tutorial Introduction to @command{tar} + +This chapter guides you through some basic examples of three @command{tar} +operations: @option{--create}, @option{--list}, and @option{--extract}. If +you already know how to use some other version of @command{tar}, then you +may not need to read this chapter. This chapter omits most complicated +details about how @command{tar} works. -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. +@menu +* assumptions:: +* stylistic conventions:: +* basic tar options:: Basic @command{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 -@section Reading Names from a File +@node assumptions +@section Assumptions this Tutorial Makes + +This chapter is paced to allow beginners to learn about @command{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, and the +differences between relative and absolute path names. @FIXME{and 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 @command{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 +@command{tar} archives with tape drives. + +@FIXME{this is a cop out. need to add some simple tape drive info.} +@end itemize -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}). +@node stylistic conventions +@section Stylistic Conventions + +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}. + +@c When we have lines which are too long to be +@c displayed in any other way, we will show them like this: + +@node basic tar options +@section Basic @command{tar} Operations and Options + +@command{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 @command{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 @command{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 @command{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 @command{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 @GNUTAR{} for compatibility with Unix +@command{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 @command{tar} commands easier. +For example, instead of typing + +@smallexample +@kbd{tar --create --verbose --file=afiles.tar apple angst aspic} +@end smallexample -@section Excluding Some Files +@noindent +you can type +@smallexample +@kbd{tar -c -v -f afiles.tar apple angst aspic} +@end smallexample -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 +or even +@smallexample +@kbd{tar -cvf afiles.tar apple angst aspic} +@end smallexample -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. +@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 @command{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 @command{tar} ``commands''. +A @command{tar} @dfn{command} is the entire command line of user input +which tells @command{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 @command{tar} command''. When +the word ``command'' is used specifically like this, a person is usually +referring to the @command{tar} @emph{operation}, not the whole line. +Again, use context to figure out which of the meanings the speaker +intends. + +@node frequent operations +@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 @option +@item --create +@itemx -c +Create a new @command{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 -@xref{Regular Expressions} for more information on the syntax and -meaning of regular expressions. +@node Two Frequent Options +@section Two Frequently Used Options -@section Operating Only on New Files +To understand how to run @command{tar} in the three operating modes listed +previously, you also need to understand how to use two of the options to +@command{tar}: @option{--file} (which takes an archive file as an argument) +and @option{--verbose}. (You are usually not @emph{required} to specify +either of these options when you run @command{tar}, but they can be very +useful in making things more clear and helping you avoid errors.) -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. +@menu +* file tutorial:: +* verbose tutorial:: +* help tutorial:: +@end menu -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. +@node file tutorial +@unnumberedsubsec The @option{--file} Option -You should never use this option for making incremental dumps. To learn -how to use @code{tar} to make backups, @ref{Making Backups}. +@table @option +@item --file=@var{archive-name} +@itemx -f @var{archive-name} +Specify the name of an archive file. +@end table -@section Crossing Filesystem Boundaries +You can specify an argument for the @value{op-file} option whenever you +use @command{tar}; this option determines the name of the archive file +that @command{tar} will work on. -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. +If you don't specify this argument, then @command{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 @command{tar} will print an error message. The error message might +look roughly like one of the following: -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. +@smallexample +tar: can't open /dev/rmt8 : No such device or address +tar: can't open /dev/rsmt0 : I/O error +@end smallexample -@chapter Changing the Names of Members when Archiving +@noindent +To avoid confusion, we recommend that you always specify an archive file +name by using @value{op-file} when writing your @command{tar} commands. +For more information on using the @value{op-file} option, see +@ref{file}. -@section Changing Directory +@node verbose tutorial +@unnumberedsubsec The @option{--verbose} Option -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 -@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. - -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. - -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.) - -@section Absolute Path Names - -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}. +@table @option +@item --verbose +@itemx -v +Show the files being worked on as @command{tar} is running. +@end table -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}. +@value{op-verbose} shows details about the results of running +@command{tar}. This can be especially useful when the results might not be +obvious. For example, if you want to see the progress of @command{tar} as +it writes files into the archive, you can use the @option{--verbose} +option. In the beginning, you may find it useful to use +@option{--verbose} at all times; when you are more accustomed to +@command{tar}, you will likely want to use it at certain times but not at +others. We will use @option{--verbose} at times to help make something +clear, and we will give many examples both using and not using +@option{--verbose} to show the differences. + +Sometimes, a single instance of @option{--verbose} on the command line +will show a full, @samp{ls} style listing of an archive or files, +giving sizes, owners, and similar information. @FIXME{Describe the +exact output format, e.g., how hard links are displayed.} +Other times, @option{--verbose} will only show files or members that the particular +operation is operating on at the time. In the latter case, you can +use @option{--verbose} twice in a command to get a listing such as that +in the former case. For example, instead of saying + +@smallexample +@kbd{tar -cvf afiles.tar apple angst aspic} +@end smallexample -If you use the @samp{--absolute-paths} option, @code{tar} will do -neither of these transformations. +@noindent +above, you might say -@section Symbolic Links +@smallexample +@kbd{tar -cvvf afiles.tar apple angst aspic} +@end smallexample -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. +@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: -@chapter Making @code{tar} More Verbose +@smallexample +$ @kbd{tar --create --verbose --verbose @dots{}} +@end smallexample -Various options cause @code{tar} to print information as it progresses -in its job. +@noindent +Note that you must double the hyphens properly each time. -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. +Later in the tutorial, we will give examples using @w{@option{--verbose +--verbose}}. -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}.) +@node help tutorial +@unnumberedsubsec Getting Help: Using the @option{--help} Option -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. +@table @option +@item --help -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. +The @option{--help} option to @command{tar} prints out a very brief list of +all operations and option available for the current version of +@command{tar} available on your system. +@end table -@chapter Input and Output +@node create +@section How to Create Archives +@UNREVISED + +One of the basic operations of @command{tar} is @value{op-create}, which +you use to create a @command{tar} archive. We will explain +@option{--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 @option{--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 +@command{tar} works. -@section Changing the Archive Name +@menu +* prepare for examples:: +* Creating the archive:: +* create verbose:: +* short create:: +* create dir:: +@end menu -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. +@node prepare for examples +@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 @command{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 @command{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}, @command{tar} will erase the current +contents of the file named by @value{op-file} if it exists. @command{tar} +will not tell you if you are about to overwrite an archive 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 +@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: + +@smallexample +$ @kbd{tar --create --file=collection.tar blues folk jazz} +@end smallexample + +The order of the arguments is not very important, @emph{when using long +option forms}. You could also say: + +@smallexample +$ @kbd{tar blues --create folk --file=collection.tar jazz} +@end smallexample -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. +@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 +@command{tar}, to avoid errors). + +Note that the part of the command which says, +@w{@option{--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: @option{--create} +is the operation which creates the new archive +(@file{collection.tar}), and @option{--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 @option{--create} operation). +@FIXME{xref here to the discussion of file name args?}Now that they are +in the archive, they are called @emph{archive members}, not files. +(@pxref{Definitions,members}). + +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, @GNUTAR{} will complain. + +If you now list the contents of the working directory (@command{ls}), you will +find the archive file listed as well as the files you saw previously: + +@smallexample +blues folk jazz collection.tar +@end smallexample -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. - -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). - -@section Extracting Members to Standard Output - -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. - -@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 (@pxref{Making @code{tar} More Verbose}) -causes @code{tar} to print the name of each file or archive member as it -is processed. - -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 - -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. +@noindent +Creating the archive @samp{collection.tar} did not destroy the copies of +the files in the directory. + +Keep in mind that if you don't indicate an operation, @command{tar} will not +run and will prompt you for one. If you don't name any files, @command{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. + +@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}. + +@node create verbose +@subsection Running @option{--create} with @option{--verbose} + +If you include the @value{op-verbose} option on the command line, +@command{tar} will list the files it is acting on as it is working. In +verbose mode, the @code{create} example above would appear as: + +@smallexample +$ @kbd{tar --create --verbose --file=collection.tar blues folk jazz} +blues +folk +jazz +@end smallexample + +This example is just like the example we showed which did not use +@option{--verbose}, except that @command{tar} generated the remaining lines +@iftex +(note the different font styles). +@end iftex +@ifinfo +. +@end ifinfo -@section Blocking +In the rest of the examples in this chapter, we will frequently use +@code{verbose} mode so we can show actions or @command{tar} responses that +you would otherwise not see, and which are important for you to +understand. + +@node short create +@subsection Short Forms with @samp{create} + +As we said before, the @value{op-create} operation is one of the most +basic uses of @command{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: + +@smallexample +$ @kbd{tar -cvf collection.tar blues folk jazz} +blues +folk +jazz +@end smallexample -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. +@noindent +As you can see, the system responds the same no matter whether you use +long or short option forms. -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. +@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: +@smallexample +$ @kbd{tar -cfv collection.tar blues folk jazz} +@end smallexample +@noindent +In this case, @command{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 @option{-f} option, and +is thus taken to be the chosen archive file name. @command{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, @command{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 @command{tar} will add this file to the archive. +Because the @option{-v} option did not get registered, @command{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, + +@smallexample +$ @kbd{tar blues --create folk --file=collection.tar jazz} +@end smallexample +@noindent +is confusing as it is. When shown using short forms, however, it +becomes much more so: +@smallexample +$ @kbd{tar blues -c folk -f collection.tar jazz} +@end smallexample -XXXX MIB XXXX +@noindent +It would be very easy to put the wrong string of characters +immediately following the @option{-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 @command{tar} to +report an error if you have set the shell environment variable +@env{POSIXLY_CORRECT}.) + +@node create dir +@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 @command{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: + +@smallexample +$ @kbd{cd ..} +$ +@end smallexample +@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: +@smallexample +$ @kbd{tar --create --verbose --file=music.tar practice} +@end smallexample -@node Wizardry, Archive Structure, Tutorial, Top -@chapter Wizardry +@noindent +@command{tar} should output: + +@smallexample +practice/ +practice/blues +practice/folk +practice/jazz +practice/collection.tar +@end smallexample + +Note that the archive thus created is not in the subdirectory +@file{practice}, but rather in the current working directory---the +directory from which @command{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 @command{tar}. For example, you will probably +not be able to store your home directory in an archive by invoking +@command{tar} from the root directory; @value{xref-absolute-names}. (Note +also that @file{collection.tar}, the original archive file, has itself +been archived. @command{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 @command{tar} a command such as + +@smallexample +$ @kbd{tar --create --file=foo.tar .} +@end smallexample -<<>>>> +@menu +* list dir:: +@end menu -@node Archive Structure, Reading and Writing, Wizardry, Top -@chapter The Structure of an Archive +@node list dir +@unnumberedsubsec Listing the Contents of a Stored Directory -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. +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. +For example, to find out about files in the directory @file{practice}, in +the archive file @file{music.tar}, type: -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. +@smallexample +$ @kbd{tar --list --verbose --file=music.tar practice} +@end smallexample + +@command{tar} responds: + +@smallexample +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 smallexample + +When you use a directory name as a file name argument, @command{tar} acts on +all the files (including sub-directories) in that directory. + +@node extract +@section How to Extract Members from an Archive +@UNREVISED +@cindex Extraction +@cindex Retrieving files from an archive +@cindex Resurrecting files from an archive -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}. +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. -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. +Using @option{--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 -* Old Style File Information:: Old Style File Information -* Archive Label:: -* Format Variations:: +* extracting archives:: +* extracting files:: +* extract dir:: +* extracting untrusted archives:: +* failing commands:: @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 extracting archives +@subsection Extracting an Entire Archive -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. - -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. +To extract an entire archive, specify the archive file name only, with +no individual file names as arguments. For 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 +@smallexample +$ @kbd{tar -xvf collection.tar} +@end smallexample -@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 +@noindent +produces this: + +@smallexample +-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 smallexample + +@node extracting files +@subsection Extracting Specific Files + +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{At the time of this +writing, atime and ctime are not restored. Since this is a tutorial +for a beginnig user, it should hardly be mentioned here. Maybe in +a footnote? --gray}. + +First, make sure you are in the @file{practice} directory, and list the +files in the directory. Now, delete the file, @samp{blues}, and list +the files in the directory again. + +You can now extract the member @file{blues} from the archive file +@file{collection.tar} like this: + +@smallexample +$ @kbd{tar --extract --file=collection.tar blues} +@end smallexample -@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. +@noindent +If you list the files in the directory again, you will see that the file +@file{blues} has been restored, with its original permissions, creation +times, and owner.@FIXME{This is only accidentally true, but not in +general. In most cases, one has to be root for restoring the owner, and +use a special option for restoring permissions. Here, it just happens +that the restoring user is also the owner of the archived members, and +that the current @code{umask} is compatible with original permissions.} +(These parameters will be identical to those which +the file had when you originally placed it in the archive; any changes +you may have made before deleting the file from the file system, +however, will @emph{not} have been made to the archive member.) The +archive file, @samp{collection.tar}, is the same as it was before you +extracted @samp{blues}. You can confirm this by running @command{tar} with +@value{op-list}. + +@FIXME{we hope this will change:}Remember that as with other operations, +specifying the exact member name is important. @w{@kbd{tar --extract +--file=bfiles.tar birds}} will fail, because there is no member named +@file{birds}. To extract the member named @file{./birds}, you must +specify @w{@kbd{tar --extract --file=bfiles.tar ./birds}}. To find the +exact member names of the members of an archive, use @value{op-list} +(@pxref{list}). + +You can extract a file to standard output by combining the above options +with the @value{op-to-stdout} option (@pxref{Writing to Standard +Output}). + +If you give the @value{op-verbose} option, then @value{op-extract} will +print the names of the archive members as it extracts them. -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. - -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 - -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 -% 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 -@end example +@node extract dir +@subsection Extracting Files that are Directories + +Extracting directories which are members of an archive is similar to +extracting other files. The main difference to be aware of is that if +the extracted directory has the same name as any directory already in +the working directory, then files in the extracted directory will be +placed into the directory of the same name. Likewise, if there are +files in the pre-existing directory with the same names as the members +which you extract, the files from the extracted archive will replace +the files already in the working directory (and possible +subdirectories). This will happen regardless of whether or not the +files in the working directory were more recent than those extracted +(there exist, however, special options that alter this behavior +@pxref{Writing}). + +However, 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, @command{tar} will create the directory. + +We can demonstrate how to use @option{--extract} to extract a directory +file with an example. Change to the @file{practice} directory if you +weren't there, and remove the files @file{folk} and @file{jazz}. Then, +go back to the parent directory and extract the archive +@file{music.tar}. You may either extract the entire archive, or you may +extract only the files you just deleted. To extract the entire archive, +don't give any file names as arguments after the archive name +@file{music.tar}. To extract only the files you deleted, use the +following command: -@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 +@smallexample +$ @kbd{tar -xvf music.tar practice/folk practice/jazz} +practice/folk +practice/jazz +@end smallexample -@node Format Variations, , Archive Label, Archive Structure -@section Format Variations -@cindex Format Parameters -@cindex Format Options -@cindex Options to specify archive format. +@noindent +If you were to specify two @value{op-verbose} options, @command{tar} +would have displayed more detail about the extracted files, as shown +in the example below: -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. +@smallexample +$ @kbd{tar -xvvf music.tar practice/folk practice/jazz} +-rw-rw-rw- me user 28 1996-10-18 16:31 practice/jazz +-rw-rw-rw- me user 20 1996-09-23 16:44 practice/folk +@end smallexample -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. +@noindent +Because you created the directory with @file{practice} as part of the +file names of each of the files by archiving the @file{practice} +directory as @file{practice}, you must give @file{practice} as part +of the file names when you extract those files from the archive. +@FIXME{IMPORTANT! show the final structure, here. figure out what it +will be.} -@menu -* Multi-Volume Archives:: -* Sparse Files:: -* Blocking Factor:: -* Compressed Archives:: -@end menu +@node extracting untrusted archives +@subsection Extracting Archives from Untrusted Sources -@node Multi-Volume Archives, Sparse Files, Format Variations, Format Variations -@subsection Archives Longer than One Tape or Disk -@cindex Multi-volume archives +Extracting files from archives can overwrite files that already exist. +If you receive an archive from an untrusted source, you should make a +new directory and extract into that directory, so that you don't have +to worry about the extraction overwriting one of your existing files. +For example, if @file{untrusted.tar} came from somewhere else on the +Internet, and you don't necessarily trust its contents, you can +extract it as follows: -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. +@smallexample +$ @kbd{mkdir newdir} +$ @kbd{cd newdir} +$ @kbd{tar -xvf ../untrusted.tar} +@end smallexample -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. +It is also a good practice to examine contents of the archive +before extracting it, using @value{op-list} option, possibly combined +with @value{op-verbose}. -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}. +@node failing commands +@subsection Commands That Will Fail -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. +Here are some sample commands you might try which will not work, and why +they won't work. -@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. +If you try to use this command, +@smallexample +$ @kbd{tar -xvf music.tar folk jazz} +@end smallexample -<<< There should be a sample program here, including an exit before -<<< end. +@noindent +you will get the following response: -@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. +@smallexample +tar: folk: Not found in archive +tar: jazz: Not found in archive +$ +@end smallexample -@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 +@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: -@node Sparse Files, Blocking Factor, Multi-Volume Archives, Format Variations -@subsection Archiving Sparse Files -@cindex Sparse Files +@smallexample +$ @kbd{tar -tvf music.tar} +practice/folk +practice/jazz +practice/rock +@end smallexample -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. +@FIXME{make sure the above works when going through the examples in +order...} -@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. +@noindent +Likewise, if you try to use this command, -@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 +@smallexample +$ @kbd{tar -tvf music.tar folk jazz} +@end smallexample -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 +@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 +@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 +@chapter Invoking @GNUTAR{} +@UNREVISED + +This chapter is about how one invokes the @GNUTAR{} +command, from the command synopsis (@pxref{Synopsis}). There are +numerous options, and many styles for writing them. One mandatory +option specifies the operation @command{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 @command{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 +@command{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}). -@code{tar} ignores the @samp{--sparse} option when reading an archive. +@menu +* Synopsis:: +* using tar options:: +* Styles:: +* All Options:: +* help:: +* verbose:: +* interactive:: +@end menu -@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 +@node Synopsis +@section General Synopsis of @command{tar} + +The @GNUTAR{} program is invoked as either one of: + +@smallexample +@kbd{tar @var{option}@dots{} [@var{name}]@dots{}} +@kbd{tar @var{letter}@dots{} [@var{argument}]@dots{} [@var{option}]@dots{} [@var{name}]@dots{}} +@end smallexample + +The second form is for when old options are being used. + +You can use @command{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 @command{tar}, which is called the @dfn{operation}, specifies +which action to take. The other arguments to @command{tar} are either +@dfn{options}, which change the way @command{tar} performs an operation, +or file names or archive members, which specify the files or members +@command{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 @command{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 @command{tar} command. + +@command{tar} interprets relative file names as being relative to the +working directory. @command{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 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. +If you give the name of a directory as either a file name or a member +name, then @command{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 @command{tar}. + +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 @command{tar} itself may glob on archive members, so when +needed, you must ensure that wildcard characters reach @command{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 @command{tar} +execution. The other operations of @command{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, @GNUTAR{} may fail for +many reasons. Some reasons correspond to bad usage, that is, when the +@command{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 @command{tar} has completed all its work. Some +errors are such that it would not meaningful, or at least risky, to +continue processing: @command{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. + +@GNUTAR{} 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 +@section Using @command{tar} Options + +@GNUTAR{} 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 @command{tar} program by +specifying one, and only one operation as an argument to the +@command{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 @command{tar}'s performance by running +@command{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 @command{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}.) + +The @env{TAR_OPTIONS} environment variable specifies default options to +be placed in front of any explicit options. For example, if +@code{TAR_OPTIONS} is @samp{-v --unlink-first}, @command{tar} behaves as +if the two options @option{-v} and @option{--unlink-first} had been +specified before any explicit options. Option specifications are +separated by whitespace. A backslash escapes the next character, so it +can be used to specify an option containing whitespace or a backslash. + +Note that @command{tar} options are case sensitive. For example, the +options @option{-T} and @option{-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 +@command{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 chapter 4 is +incorporated.} + +@node Styles +@section The Three Option Styles + +There are three styles for writing operations and options to the command +line invoking @command{tar}. The different styles were developed at +different times during the history of @command{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, @command{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. + +Some options @emph{may} take an argument (currently, there are +two such options: @value{op-backup} and @value{op-occurrence}). Such +options may have at most long and short forms, they do not have old style +equivalent. The rules for specifying an argument for such options +are stricter than those for specifying mandatory arguments. Please, +pay special attention to them. -@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 +@menu +* Mnemonic Options:: Mnemonic Option Style +* Short Options:: Short Option Style +* Old Options:: Old Option Style +* Mixing:: Mixing Option Styles +@end menu -@node Compressed Archives, , Blocking Factor, Format Variations -@subsection Creating and Reading Compressed Archives -@cindex Compressed archives -@cindex Storing archives in compressed format +@node Mnemonic Options +@subsection Mnemonic Option Style + +@FIXME{have to decide whether or not 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., @option{--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 @option{--compare} and @option{--diff}. In addition, +long option names can be given unique abbreviations. For example, +@option{--cre} can be used in place of @option{--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, @command{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 @command{tar} with a +unique abbreviation for the long name of an option you didn't want to +use, you are stuck; @command{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: + +@smallexample +$ @kbd{tar --create --verbose --blocking-factor=20 --file=/dev/rmt0} +@end smallexample -@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. - -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. - -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. - -If there is no compress utility available, @code{tar} will report an -error. +@noindent +gives a fairly good set of hints about what the command does, even +for those not fully acquainted with @command{tar}. + +Mnemonic options which require arguments take those arguments +immediately following the option name. There are two ways of +specifying a mandatory argument. It can be separated from the +option name either by an equal sign, or by any amount of +white space characters. For example, the @option{--file} option (which +tells the name of the @command{tar} archive) is given a file such as +@file{archive.tar} as argument by using any of the following notations: +@option{--file=archive.tar} or @option{--file archive.tar}. + +In contrast, optional arguments must always be introduced using +an equal sign. For example, the @option{--backup} option takes +an optional argument specifying backup type. It must be used +as @option{--backup=@var{backup-type}}. + +@node Short Options +@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., @option{-t} +(which is equivalent to @option{--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{@option{-f +archive.tar}} or @option{-farchive.tar} instead of using +@option{--file=archive.tar}. Both @option{--file=@var{archive-name}} and +@w{@option{-f @var{archive-name}}} denote the option which indicates a +specific archive, here named @file{archive.tar}. + +Short options which take optional arguments take their arguments +immediately following the option letter, @emph{without any intervening +white space characters}. + +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{@command{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 @acronym{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: + +@smallexample +$ @kbd{tar -c -v -b 20 -f /dev/rmt0} +@end smallexample + +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 +@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 +@command{tar} program name and some white space; 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 @option{-t}, and consequently, the same as the +mnemonic option @option{--list}. So for example, the command @w{@samp{tar +cv}} specifies the option @option{-v} in addition to the operation @option{-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: + +@smallexample +$ @kbd{tar cvbf 20 /dev/rmt0} +@end smallexample -@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. +@noindent +Here, @samp{20} is the argument of @option{-b} and @samp{/dev/rmt0} is +the argument of @option{-f}. -@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 +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 @option{-b}, @samp{/dev/rmt0} is the +argument for @option{-f}, and @option{-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. -@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. +If you want to reorder the letters in the old option argument, be +sure to reorder any corresponding argument appropriately. -@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 +This old way of writing @command{tar} options can surprise even experienced +users. For example, the two commands: -@c >>> MIB -- why not use -Z instead of -z -z ? -ringo +@smallexample +@kbd{tar cfz archive.tar.gz file} +@kbd{tar -cfz archive.tar.gz file} +@end smallexample -@node Reading and Writing, Insuring Accuracy, Archive Structure, Top -@chapter Reading and Writing Archives +@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. + +Old options are kept for compatibility with old versions of @command{tar}. + +This second example could be corrected in many ways, among which the +following are equivalent: + +@smallexample +@kbd{tar -czf archive.tar.gz file} +@kbd{tar -cf archive.tar.gz -z file} +@kbd{tar cf archive.tar.gz -z file} +@end smallexample + +@FIXME{still could explain this better; it's redundant:} + +@cindex option syntax, traditional +As far as we know, all @command{tar} programs, @acronym{GNU} and +non-@acronym{GNU}, support old options. @GNUTAR{} +supports them not only for historical reasons, but also because many +people are used to them. For compatibility with Unix @command{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. + +@node Mixing +@subsection Mixing Option Styles + +All three styles may be intermixed in a single @command{tar} command, +so long as the rules for each style are fully +respected@footnote{Before @GNUTAR{} 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 @command{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 @command{tar} command and some white space). 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. + +For example, all the following commands are wholly equivalent, and +illustrate the many combinations and orderings of option styles. + +@smallexample +@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 smallexample + +On the other hand, the following commands are @emph{not} equivalent to +the previous set: + +@smallexample +@kbd{tar -f -c archive.tar} +@kbd{tar -fc archive.tar} +@kbd{tar -fcarchive.tar} +@kbd{tar -farchive.tarc} +@kbd{tar cfarchive.tar} +@end smallexample -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. +@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 @command{tar} archive would be a file named +@option{-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 +@section All @command{tar} Options + +The coming manual sections contain an alphabetical listing of all +@command{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 @command{tar} commands in scripts. @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 +* Operation Summary:: +* Option Summary:: +* Short Option Summary:: @end menu -@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? +@node Operation Summary +@subsection Operations -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. - -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). - -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. +@table @option -@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 --append +@itemx -r -@cindex Standard input and output -@cindex tar to standard input and output +Appends files to the end of the archive. @xref{append}. -To specify an archive file on a device attached to a remote machine, -use the following: +@item --catenate +@itemx -A -@example ---file=@var{hostname}:/@var{dev}/@var{file name} -@end example +Same as @option{--concatenate}. @xref{concatenate}. -@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 --compare +@itemx -d -@c >>>MIB --- is this clear? +Compares archive members with their counterparts in the file +system, and reports differences in file size, mode, owner, +modification date and contents. @xref{compare}. -@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 +@item --concatenate +@itemx -A -@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 +Appends other @command{tar} archives to the end of the archive. +@xref{concatenate}. -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. +@item --create +@itemx -c -The following example creates an archive named @file{stooges}, -containing the files @file{larry}, @file{moe} and @file{curley}: +Creates a new @command{tar} archive. @xref{create}. -@example -tar --create --file=stooges larry moe curley -@end example +@item --delete -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}: +Deletes members from the archive. Don't try this on a archive on a +tape! @xref{delete}. -@example -tar --create --file=hail/hail/fredonia marx -@end example +@item --diff +@itemx -d -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: +Same @option{--compare}. @xref{compare}. -@example -tar --create --file=home -@end example +@item --extract +@itemx -x -@xref{File Name Lists}, for other ways to specify files to archive. +Extracts members from the archive into the file system. @xref{extract}. -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. +@item --get +@itemx -x -@node Modifying, Listing Contents, Creating in Detail, Reading and Writing -@section Modifying Archives -@cindex Modifying archives +Same as @option{--extract}. @xref{extract}. -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. +@item --list +@itemx -t -To find out what files are already stored in an archive, use @samp{tar ---list --file=@var{archive-name}}. @xref{Listing Contents}. +Lists the members in an archive. @xref{list}. -@menu -* Adding Files:: -* Appending Archives:: -* Deleting Archive Files:: Deleting Files From an Archive -* Matching Format Parameters:: -@end menu +@item --update +@itemx -u -@node Adding Files, Appending Archives, Modifying, Modifying -@subsection Adding Files to an Archive -@cindex Adding files to an archive -@cindex Updating an archive +@FIXME{It was: A combination of the @option{--compare} and +@option{--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}. -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}. - -The following example adds the file @file{shemp} to the archive -@file{stooges} created above: - -@example -tar --add-file --file=stooges shemp -@end example - -You must specify the files to be added; there is no default. - -@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. - -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. - -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. - -@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 +@end table -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. +@node Option Summary +@subsection @command{tar} Options -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): +@table @option -@example -tar --add-archive --file=stooges hail/hail/fredonia -@end example +@item --absolute-names +@itemx -P -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}). +Normally when creating an archive, @command{tar} strips an initial +@samp{/} from member names. This option disables that behavior. +@xref{absolute}. -@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 +@item --after-date -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. +(See @option{--newer}.) @FIXME-pxref{} -The following example removes the file @file{curley} from the archive -@file{stooges}: +@item --anchored +An exclude pattern must match an initial subsequence of the name's components. +@FIXME-xref{} -@example -tar --delete --file=stooges curley -@end example +@item --atime-preserve -You can only use @samp{tar --delete} on an archive if the archive -device allows you to write to any point on the media. +Tells @command{tar} to preserve the access time field in a file's inode when +reading it. Due to limitations in the @code{utimes} system call, the +modification time field is also preserved, which may cause problems if +the file is simultaneously being modified by another program. +This option is incompatible with incremental backups, because +preserving the access time involves updating the last-changed time. +Also, this option does not work on files that you do not own, +unless you're root. +@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 --backup=@var{backup-type} -@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=-). +Rather than deleting files from the file system, @command{tar} will +back them up using simple or numbered backups, depending upon +@var{backup-type}. @FIXME-xref{} -@node Matching Format Parameters, , Deleting Archive Files, Modifying -@subsection Matching the Format Parameters +@item --block-number +@itemx -R -Some format parameters must be taken into consideration when modifying -an archive: +With this option present, @command{tar} prints error messages for read errors +with the block number in the archive file. @FIXME-xref{} -Compressed archives cannot be modified. +@item --blocking-factor=@var{blocking} +@itemx -b @var{blocking} -You have to specify the block size of the archive when modifying an -archive with a non-default block size. +Sets the blocking factor @command{tar} uses to @var{blocking} x 512 bytes per +record. @FIXME-xref{} -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 - -@c <<< xref somewhere, for more information about format parameters. - -@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 - -@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. - -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. - -If the blocking factor of the archive differs from the default, -@code{tar} reports this. @xref{Blocking Factor}. - -@xref{Archive Reading Options} for a list of options which can be used -to modify @samp{--list}'s operation. - -This example prints a list of the archive members of the archive -@file{stooges}: - -@example -tar --list --file=stooges -@end example - -@noindent -@code{tar} responds: - -@example -larry -moe -shemp -marx/julius -marx/alexander -marx/karl -@end example - -This example generates a verbose list of the archive members of the -archive file @file{dwarves}, which has a blocking factor of two: - -@example -tar --list -v --file=blocks -@end example - -@noindent -@code{tar} responds: - -@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 - -@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 --bzip2 +@itemx -j -To read archive members from the archive and write them into the file -system, use @samp{tar --extract}. The archive itself is left -unchanged. +This option tells @command{tar} to read or write archives through +@code{bzip2}. @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 --checkpoint -The following example shows the extraction of the archive -@file{stooges} into an empty directory: +This option directs @command{tar} to print periodic checkpoint messages as it +reads through the archive. Its intended for when you want a visual +indication that @command{tar} is still running, but don't want to see +@option{--verbose} output. @FIXME-xref{} -@example -tar --extract --file=stooges -@end example +@item --check-links +@itemx -l +If this option was given, @command{tar} will check the number of links +dumped for each processed file. If this number does not match the +total number of hard links for the file, a warning message will be +output. -@noindent -Generating a listing of the directory (@samp{ls}) produces: +Future versions will take @option{-l} as a short version of +@option{--check-links}. However, current release still retains the old +semantics for @option{-l}. -@example -larry -moe -shemp -marx -@end example +@xref{Current status}, for more information. -@noindent -The subdirectory @file{marx} contains the files @file{julius}, -@file{alexander} and @file{karl}. +@item --compress +@itemx --uncompress +@itemx -Z -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: +@command{tar} will use the @command{compress} program when reading or +writing the archive. This allows you to directly act on archives +while saving space. @FIXME-xref{} -@example -tar --extract --file=stooges marx -@end example +@item --confirmation -@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 +(See @option{--interactive}.) @FIXME-pxref{} -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 --dereference +@itemx -h -@menu -* Archive Reading Options:: -* File Writing Options:: -* Scarce Disk Space:: Recovering From Scarce Disk Space -@end menu +When creating a @command{tar} archive, @command{tar} will archive the +file that a symbolic link points to, rather than archiving the +symlink. @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 --directory=@var{dir} +@itemx -C @var{dir} -@c <<< each option wants its own node. summary after menu +When this option is specified, @command{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{} -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}. +@item --exclude=@var{pattern} -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. +When performing operations, @command{tar} will skip files that match +@var{pattern}. @FIXME-xref{} -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}. +@item --exclude-from=@var{file} +@itemx -X @var{file} -@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 +Similar to @option{--exclude}, except @command{tar} will use the list of +patterns in the file @var{file}. @FIXME-xref{} -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). +@item --exclude-caches -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. +Automatically excludes all directories +containing a cache directory tag. @FIXME-xref{} -@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 +@item --file=@var{archive} +@itemx -f @var{archive} -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. +@command{tar} will use the file @var{archive} as the @command{tar} archive it +performs operations on, rather than @command{tar}'s compilation dependent +default. @FIXME-xref{} -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. +@item --files-from=@var{file} +@itemx -T @var{file} -This option is probably never needed on modern computer systems. +@command{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{} -@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 +@item --force-local -@c we don't need/want --preserve to exist any more +Forces @command{tar} to interpret the filename given to @option{--file} +as a local file, even if it looks like a remote tape drive name. +@FIXME-xref{} -@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 +@item --format=@var{format} -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. +Selects output archive format. @var{Format} may be one of the +following: @table @samp -@item --keep-old files -@itemx -k -Prevents @code{tar} from overwriting files in the file system during -extraction. -@end table +@item v7 +Creates an archive that is compatible with Unix V7 @command{tar}. -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 oldgnu +Creates an archive that is compatible with GNU @command{tar} version +1.12 or earlier. -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}. +@item gnu +Creates archive in GNU tar 1.13 format. Basically it is the same as +@samp{oldgnu} with the only difference in the way it handles long +numeric fields. -@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 +@item ustar +Creates a @acronym{POSIX.1-1988} compatible archive. -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. +When creating an archive, @command{tar} will only add files that have changed +since @var{date}. If @var{date} begins with @samp{/} or @samp{.}, it +is taken to be the name of a file whose last-modified time specifies +the date. @FIXME-xref{} -@c >>> is write access an issue? +@item --newer-mtime=@var{date} -@table @samp -@item --absolute-paths -Preserves full file names (inclusing superior dirctory names) when -archiving files. Preserves leading slash when extracting files. -@end table +Like @option{--newer}, but add only files whose +contents have changed (as opposed to just @option{--newer}, which will +also back up files for which any status information has changed). -@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 --no-anchored +An exclude pattern can match any subsequence of the name's components. +@FIXME-xref{} -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, +@item --no-ignore-case +Use case-sensitive matching when excluding files. +@FIXME-xref{} -@example -tar --create iggy ziggy --directory=baz melvin -@end example +@item --no-recursion -@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. +With this option, @command{tar} will not recurse into directories. +@FIXME-xref{} -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. +@item --no-same-owner +@itemx -o -Contrast this with the command +When extracting an archive, do not attempt to preserve the owner +specified in the @command{tar} archive. This the default behavior +for ordinary users. -@example -tar -c iggy ziggy bar/melvin -@end example +@item --no-same-permissions -@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}. +When extracting an archive, subtract the user's umask from files from +the permissions specified in the archive. This is the default behavior +for ordinary users. -@table @samp -@item --directory=@file{directory} -@itemx -C @file{directory} -Changes the working directory. -@end table +@item --no-wildcards +Do not use wildcards when excluding files. +@FIXME-xref{} -@c <<>> +Any other @samp{%} characters in @var{string} produce undefined +results. -@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} +If no option @samp{exthdr.name=string} is specified, @command{tar} +will use the following default value: -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. +@smallexample +%d/PaxHeaders.%p/%f +@end smallexample -@menu -* Additional Information:: -* Interactive Operation:: -@end menu +@item globexthdr.name=@var{string} +This keyword allows user control over the name that is written into +the ustar header blocks for global extended header records. The name +shall will be obtained from the contents of @var{string}, after the +following character substitutions have been made: -@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 - -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. +@multitable @columnfractions .30 .70 +@headitem Meta-character @tab Replaced By +@item %n @tab An integer that represents the +sequence number of the global extended header record in the archive, +starting at 1. +@item %p @tab The process ID of the @command{tar} process. +@item %% @tab A @samp{%} character. +@end multitable -@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 +Any other @samp{%} characters in string produce undefined results. -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. +If no option @samp{globexthdr.name=string} is specified, @command{tar} +will use the following default value: -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 +@smallexample +$TMPDIR/GlobalHead.%p.%n +@end smallexample -@table @samp -@item --record-number -@itemx -R -Prints the record number whenever a message is generated by -@code{tar}. Use in conjunction with any operation. +@noindent +where @samp{$TMPDIR} represents the value of the @var{TMPDIR} +environment variable. If @var{TMPDIR} is not set, @command{tar} +uses @samp{/tmp}. + +@item @var{keyword}=@var{value} +When used with one of archive-creation commands, these keyword/value pairs +will be included at the beginning of the archive in a global extended +header record. When used with one of archive-reading commands, +@command{tar} will behave as if it has encountered these keyword/value +pairs at the beginning of the archive in a global extended header +record. + +@item @var{keyword}:=@var{value} +When used with one of archive-creation commands, these keyword/value pairs +will be included as records at the beginning of an extended header for +each file. This is effectively equivalent to @var{keyword}=@var{value} +form except that it creates no global extended header records. + +When used with one of archive-reading commands, @command{tar} will +behave as if these keyword/value pairs were included as records at the +end of each extended header; thus, they will override any global or +file-specific extended header record keywords of the same names. +For example, in the command: + +@smallexample +tar --format=posix --create \ + --file archive --pax-option gname:=user . +@end smallexample + +the group name will be forced to a new value for all files +stored in the archive. @end table -@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: +@item --portability +@itemx --old-archive +Synonym for @option{--format=v7}. -@example -tar --version -@end example +@item --posix +Same as @option{--format=posix}. -@noindent -might return: +@item --preserve -@example -GNU tar version 1.09 -@end example -@c used to be an option. has been fixed. +Synonymous with specifying both @option{--preserve-permissions} and +@option{--same-order}. @FIXME-xref{} -@node Interactive Operation, , Additional Information, User Interaction -@section Asking for Confirmation During Operations -@cindex Interactive operation +@item --preserve-order -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. - -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 +(See @option{--same-order}; @pxref{Reading}.) -@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 --preserve-permissions +@itemx --same-permissions +@itemx -p -@node Backups and Restoration, Media, User Interaction, Top -@chapter Performing Backups and Restoring Files +When @command{tar} is extracting an archive, it normally subtracts the +users' umask from the permissions specified in the archive and uses +that number as the permissions to create the destination file. +Specifying this option instructs @command{tar} that it should use the +permissions directly from the archive. @xref{Writing}. -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 --read-full-records +@itemx -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 +Specifies that @command{tar} should reblock its input, for reading +from pipes on systems with buggy implementations. @xref{Reading}. -@node Backup Levels, Backup Scripts, Backups and Restoration, Backups and Restoration -@section Levels of Backups +@item --record-size=@var{size} -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. +Instructs @command{tar} to use @var{size} bytes per record when accessing the +archive. @FIXME-xref{} -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 --recursion -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). +With this option, @command{tar} recurses into directories. +@FIXME-xref{} -@node Backup Scripts, incremental and listed-incremental, Backup Levels, Backups and Restoration -@section Using Scripts to Perform Backups and Restoration +@item --recursive-unlink -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. +Remove existing +directory hierarchies before extracting directories of the same name +from the archive. @xref{Writing}. -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. - -@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. - -@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 --remove-files -@node Backup Parameters, Scripted Backups, Backup Scripts, Backup Scripts -@subsection Setting Parameters for Backups and Restoration +Directs @command{tar} to remove the source file from the file system after +appending it to an archive. @FIXME-xref{} -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 --rmt-command=@var{cmd} -@c <<< This about backup scripts needs to be written: -@c <<>> +@option{--preserve-permissions} -@item --tape-length=@var{n} (-L) -@c <<>> -@c <<< this needs to be written into main body as well -ringo +@item -r -@item --info-script=@var{program-file} -Create a multi-volume archive via a script. @xref{Multi-Volume Archives}. +@option{--append} -@item --interactive -Ask for confirmation before performing any operation on a file or -archive member. +@item -s -@item --keep-old-files -Prevent overwriting during extraction. @xref{File Writing Options}. +@option{--same-order} -@item --label=@var{archive-label} -Include an archive-label in the archive being created. @xref{Archive -Label}. +@item -t -@item --modification-time -Set the modification time of extracted files to the time they were -extracted. @xref{File Writing Options}. +@option{--list} -@item --multi-volume -Specify a multi-volume archive. @xref{Multi-Volume Archives}. +@item -u -@item --newer=@var{date} -Limit the operation to files changed after the given date. -@xref{File Exclusion}. +@option{--update} -@item --newer-mtime=@var{date} -Limit the operation to files modified after the given date. @xref{File -Exclusion}. +@item -v -@item --old -Create an old format archive. @xref{Old Style File Information}. -@c <<< did we agree this should go away as a synonym? +@option{--verbose} -@item --old-archive -Create an old format archive. @xref{Old Style File Information}. +@item -w -@item --one-file-system -Prevent @code{tar} from crossing file system boundaries when -archiving. @xref{File Exclusion}. +@option{--interactive} -@item --portable -Create an old format archive. @xref{Old Style File Information}. -@c <<< was portability, may still need to be changed +@item -x -@item --preserve-order -Help process large lists of file-names on machines with small amounts of -memory. @xref{Archive Reading Options}. +@option{--extract} -@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}. +@item -z + +@option{--gzip} + +@end table + +@node help +@section @GNUTAR{} documentation + +Being careful, the first thing is really checking that you are using +@GNUTAR{}, indeed. The @value{op-version} option +will generate a message giving confirmation that you are using +@GNUTAR{}, with the precise version of @GNUTAR{} +you are using. @command{tar} identifies itself and +prints the version number to the standard output, then immediately +exits successfully, without doing anything else, ignoring all other +options. For example, @w{@samp{tar --version}} might return: + +@smallexample +tar (@acronym{GNU} tar) @value{VERSION} +@end smallexample + +@noindent +The first occurrence of @samp{tar} in the result above is the program +name in the package (for example, @command{rmt} is another program), +while the second occurrence of @samp{tar} is the name of the package +itself, containing possibly many programs. The package is currently +named @samp{tar}, after the name of the main program it +contains@footnote{There are plans to merge the @command{cpio} and +@command{tar} packages into a single one which would be called +@code{paxutils}. So, who knows if, one of this days, the +@value{op-version} would not yield @w{@samp{tar (@acronym{GNU} +paxutils) 3.2}}}. + +Another thing you might want to do is checking the spelling or meaning +of some particular @command{tar} option, without resorting to this +manual, for once you have carefully read it. @GNUTAR{} +has a short help feature, triggerable through the +@value{op-help} option. By using this option, @command{tar} will +print a usage message listing all available options on standard +output, then exit successfully, without doing anything else and +ignoring all other options. Even if this is only a brief summary, it +may be several screens long. So, if you are not using some kind of +scrollable window, you might prefer to use something like: + +@smallexample +$ @kbd{tar --help | less} +@end smallexample + +@noindent +presuming, here, that you like using @command{less} for a pager. Other +popular pagers are @command{more} and @command{pg}. If you know about some +@var{keyword} which interests you and do not want to read all the +@value{op-help} output, another common idiom is doing: + +@smallexample +tar --help | grep @var{keyword} +@end smallexample + +@noindent +for getting only the pertinent lines. + +The perceptive reader would have noticed some contradiction in the +previous paragraphs. It is written that both @value{op-version} and +@value{op-help} print something, and have all other options ignored. In +fact, they cannot ignore each other, and one of them has to win. We do +not specify which is stronger, here; experiment if you really wonder! + +The short help output is quite succinct, and you might have to get +back to the full documentation for precise points. If you are reading +this paragraph, you already have the @command{tar} manual in some +form. This manual is available in printed form, as a kind of small +book. It may printed out of the @GNUTAR{} +distribution, provided you have @TeX{} already installed somewhere, +and a laser printer around. Just configure the distribution, execute +the command @w{@samp{make dvi}}, then print @file{doc/tar.dvi} the +usual way (contact your local guru to know how). If @GNUTAR{} +has been conveniently installed at your place, this +manual is also available in interactive, hypertextual form as an Info +file. Just call @w{@samp{info tar}} or, if you do not have the +@command{info} program handy, use the Info reader provided within +@acronym{GNU} Emacs, calling @samp{tar} from the main Info menu. + +There is currently no @code{man} page for @GNUTAR{}. +If you observe such a @code{man} page on the system you are running, +either it does not long to @GNUTAR{}, or it has not +been produced by @acronym{GNU}. Currently, @GNUTAR{} +documentation is provided in Texinfo format only, if we +except, of course, the short result of @kbd{tar --help}. + +@node verbose +@section Checking @command{tar} progress + +@cindex Progress information +@cindex Status information +@cindex Information on progress and status of operations +@cindex Verbose operation +@cindex Block number where error occurred +@cindex Error message, block number of +@cindex Version of the @command{tar} program + +@cindex Getting more information during the operation +@cindex Information during operation +@cindex Feedback from @command{tar} + +Typically, @command{tar} performs most operations without reporting any +information to the user except error messages. When using @command{tar} +with many options, particularly ones with complicated or +difficult-to-predict behavior, it is possible to make serious mistakes. +@command{tar} provides several options that make observing @command{tar} +easier. These options cause @command{tar} to print information as it +progresses in its job, and you might want to use them just for being +more careful about what is going on, or merely for entertaining +yourself. 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. + +Normally, the @value{op-list} command to list an archive prints just +the file names (one per line) and the other commands are silent. +When used with most operations, the @value{op-verbose} option causes +@command{tar} to print the name of each file or archive member as it +is processed. This and the other options which make @command{tar} print +status information can be useful in monitoring @command{tar}. + +With @value{op-create} or @value{op-extract}, @value{op-verbose} used once +just prints the names of the files or members as they are processed. +Using it twice causes @command{tar} to print a longer listing (reminiscent +of @samp{ls -l}) for each member. Since @value{op-list} already prints +the names of the members, @value{op-verbose} used once with @value{op-list} +causes @command{tar} to print an @samp{ls -l} type listing of the files +in the archive. The following examples both extract members with +long list output: + +@smallexample +$ @kbd{tar --extract --file=archive.tar --verbose --verbose} +$ @kbd{tar xvvf archive.tar} +@end smallexample + +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} (@samp{tar cfv -}, or even @samp{tar cv}---if the +installer let standard output be the default archive). In that case +@command{tar} writes verbose output to the standard error stream. + +If @option{--index-file=@var{file}} is specified, @command{tar} sends +verbose output to @var{file} rather than to standard output or standard +error. + +The @value{op-totals} option---which is only meaningful when used with +@value{op-create}---causes @command{tar} to print the total +amount written to the archive, after it has been fully created. + +The @value{op-checkpoint} option prints an occasional message +as @command{tar} reads or writes the archive. In fact, it prints +a message each 10 records read or written. It is designed for +those who don't need the more detailed (and voluminous) output of +@value{op-block-number}, but do want visual confirmation that @command{tar} +is actually making forward progress. + +@FIXME{There is some confusion here. It seems that -R once wrote a +message at @samp{every} record read or written.} + +The @value{op-show-omitted-dirs} option, when reading an archive---with +@value{op-list} or @value{op-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 @value{op-exclude} option, or +some other reason. + +If @value{op-block-number} is used, @command{tar} prints, along with +every message it would normally produce, the block number within the +archive where the message was triggered. Also, supplementary messages +are triggered when reading blocks full of NULs, or when hitting end of +file on the archive. As of now, if the archive if properly terminated +with a NUL block, the reading of the file may stop before end of file +is met, so the position of end of file will not usually show when +@value{op-block-number} is used. Note that @GNUTAR{} +drains the archive before exiting when reading the +archive from a pipe. + +This option is especially useful when reading damaged archives, since +it helps pinpoint the damaged sections. It can also be used with +@value{op-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). @FIXME-xref{when the node name is set and the +backup section written.} + +@node interactive +@section Asking for Confirmation During Operations +@cindex Interactive operation + +Typically, @command{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{Choosing}), or by performing +an operation interactively, using the @value{op-interactive} option. +@command{tar} also accepts @option{--confirmation} for this option. + +When the @value{op-interactive} option is specified, before +reading, writing, or deleting files, @command{tar} first prints a message +for each such file, telling what operation it intends to take, then asks +for confirmation on the terminal. The actions which require +confirmation include adding a file to the archive, extracting a file +from the archive, deleting a file from the archive, and deleting a file +from disk. 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}, @command{tar} skips that file. + +If @command{tar} is reading the archive from the standard input, +@command{tar} opens the file @file{/dev/tty} to support the interactive +communications. + +Verbose output is normally sent to standard output, separate from +other error messages. However, if the archive is produced directly +on standard output, then verbose output is mixed with errors on +@code{stderr}. Producing the archive on standard output may be used +as a way to avoid using disk space, when the archive is soon to be +consumed by another process reading it, say. Some people felt the need +of producing an archive on stdout, still willing to segregate between +verbose output and error output. A possible approach would be using a +named pipe to receive the archive, and having the consumer process to +read from that named pipe. This has the advantage of letting standard +output free to receive verbose output, all separate from errors. + +@node operations +@chapter @GNUTAR{} Operations + +@menu +* Basic tar:: +* Advanced tar:: +* create options:: +* extract options:: +* backup:: +* Applications:: +* looking ahead:: +@end menu + +@node Basic tar +@section Basic @GNUTAR{} Operations + +The basic @command{tar} operations, @value{op-create}, @value{op-list} and +@value{op-extract}, are currently presented and described in the tutorial +chapter of this manual. This section provides some complementary notes +for these operations. + +@table @asis +@item @value{op-create} + +Creating an empty archive would have some kind of elegance. One can +initialize an empty archive and later use @value{op-append} for adding +all members. Some applications would not welcome making an exception +in the way of adding the first archive member. On the other hand, +many people reported that it is dangerously too easy for @command{tar} +to destroy a magnetic tape with an empty archive@footnote{This is well +described in @cite{Unix-haters Handbook}, by Simson Garfinkel, Daniel +Weise & Steven Strassmann, IDG Books, ISBN 1-56884-203-1.}. The two most +common errors are: + +@enumerate +@item +Mistakingly using @code{create} instead of @code{extract}, when the +intent was to extract the full contents of an archive. This error +is likely: keys @kbd{c} and @kbd{x} are right next to each other on +the QWERTY keyboard. Instead of being unpacked, the archive then +gets wholly destroyed. When users speak about @dfn{exploding} an +archive, they usually mean something else :-). + +@item +Forgetting the argument to @code{file}, when the intent was to create +an archive with a single file in it. This error is likely because a +tired user can easily add the @kbd{f} key to the cluster of option +letters, by the mere force of habit, without realizing the full +consequence of doing so. The usual consequence is that the single +file, which was meant to be saved, is rather destroyed. +@end enumerate + +So, recognizing the likelihood and the catastrophical nature of these +errors, @GNUTAR{} now takes some distance from elegance, and +cowardly refuses to create an archive when @value{op-create} option is +given, there are no arguments besides options, and @value{op-files-from} +option is @emph{not} used. To get around the cautiousness of @GNUTAR{} +and nevertheless create an archive with nothing in it, +one may still use, as the value for the @value{op-files-from} option, +a file with no names in it, as shown in the following commands: + +@smallexample +@kbd{tar --create --file=empty-archive.tar --files-from=/dev/null} +@kbd{tar cfT empty-archive.tar /dev/null} +@end smallexample + +@item @value{op-extract} + +A socket is stored, within a @GNUTAR{} archive, as a pipe. + +@item @value{op-list} + +@GNUTAR{} now shows dates as @samp{1996-08-30}, +while it used to show them as @samp{Aug 30 1996}. (One can revert to +the old behavior by defining @code{USE_OLD_CTIME} in @file{src/list.c} +before reinstalling.) But preferably, people should get used to ISO +8601 dates. Local American dates should be made available again with +full date localization support, once ready. In the meantime, programs +not being localizable for dates should prefer international dates, +that's really the way to go. + +Look up @url{http://www.ft.uni-erlangen.de/~mskuhn/iso-time.html} if you +are curious, it contains a detailed explanation of the ISO 8601 standard. + +@end table + +@node Advanced tar +@section Advanced @GNUTAR{} Operations + +Now that you have learned the basics of using @GNUTAR{}, you may want +to learn about further ways in which @command{tar} can help you. + +This chapter presents five, more advanced operations which you probably +won't use on a daily basis, but which serve more specialized functions. +We also explain the different styles of options and why you might want +to use one or another, or a combination of them in your @command{tar} +commands. Additionally, this chapter includes options which allow you to +define the output from @command{tar} more carefully, and provide help and +error correction in special circumstances. + +@FIXME{check this after the chapter is actually revised to make sure +it still introduces the info in the chapter correctly : ).} + +@menu +* Operations:: +* append:: +* update:: +* concatenate:: +* delete:: +* compare:: +@end menu + +@node Operations +@subsection The Five Advanced @command{tar} Operations +@UNREVISED + +In the last chapter, you learned about the first three operations to +@command{tar}. This chapter presents the remaining five operations to +@command{tar}: @option{--append}, @option{--update}, @option{--concatenate}, +@option{--delete}, and @option{--compare}. + +You are not likely to use these operations as frequently as those +covered in the last chapter; however, since they perform specialized +functions, they are quite useful when you do need to use them. We +will give examples using the same directory and files that you created +in the last chapter. As you may recall, the directory is called +@file{practice}, the files are @samp{jazz}, @samp{blues}, @samp{folk}, +@samp{rock}, and the two archive files you created are +@samp{collection.tar} and @samp{music.tar}. + +We will also use the archive files @samp{afiles.tar} and +@samp{bfiles.tar}. @samp{afiles.tar} contains the members @samp{apple}, +@samp{angst}, and @samp{aspic}. @samp{bfiles.tar} contains the members +@samp{./birds}, @samp{baboon}, and @samp{./box}. + +Unless we state otherwise, all practicing you do and examples you follow +in this chapter will take place in the @file{practice} directory that +you created in the previous chapter; see @ref{prepare for examples}. +(Below in this section, we will remind you of the state of the examples +where the last chapter left them.) + +The five operations that we will cover in this chapter are: + +@table @option +@item --append +@itemx -r +Add new entries to an archive that already exists. +@item --update +@itemx -r +Add more recent copies of archive members to the end of an archive, if +they exist. +@item --concatenate +@itemx --catenate +@itemx -A +Add one or more pre-existing archives to the end of another archive. +@item --delete +Delete items from an archive (does not work on tapes). +@item --compare +@itemx --diff +@itemx -d +Compare archive members to their counterparts in the file system. +@end table + +@node append +@subsection How to Add Files to Existing Archives: @option{--append} +@UNREVISED + +If you want to add files to an existing archive, you don't need to +create a new archive; you can use @value{op-append}. The archive must +already exist in order to use @option{--append}. (A related operation +is the @option{--update} operation; you can use this to add newer +versions of archive members to an existing archive. To learn how to +do this with @option{--update}, @pxref{update}.) + +If you use @value{op-append} to add a file that has the same name as an +archive member to an archive containing that archive member, then the +old member is not deleted. What does happen, however, is somewhat +complex. @command{tar} @emph{allows} you to have infinite number of files +with the same name. Some operations treat these same-named members no +differently than any other set of archive members: for example, if you +view an archive with @value{op-list}, you will see all of those members +listed, with their modification times, owners, etc. + +Other operations don't deal with these members as perfectly as you might +prefer; if you were to use @value{op-extract} to extract the archive, +only the most recently added copy of a member with the same name as four +other members would end up in the working directory. This is because +@option{--extract} extracts an archive in the order the members appeared +in the archive; the most recently archived members will be extracted +last. Additionally, an extracted member will @emph{replace} a file of +the same name which existed in the directory already, and @command{tar} +will not prompt you about this@footnote{Unless you give it +@option{--keep-old-files} option, or the disk copy is newer than the +the one in the archive and you invoke @command{tar} with +@option{--keep-newer-files} option}. Thus, only the most recently archived +member will end up being extracted, as it will replace the one +extracted before it, and so on. + +There exists a special option that allows you to get around this +behavior and extract (or list) only a particular copy of the file. +This is @option{--occurrence} option. If you run @command{tar} with +this option, it will extract only the first copy of the file. You +may also give this option an argument specifying the number of +copy to be extracted. Thus, for example if the archive +@file{archive.tar} contained three copies of file @file{myfile}, then +the command + +@smallexample +tar --extract --file archive.tar --occurrence=2 myfile +@end smallexample + +@noindent +would extract only the second copy. @xref{Option Summary,---occurrence}, for the description of @value{op-occurrence} option. + +@FIXME{ hag -- you might want to incorporate some of the above into the +MMwtSN node; not sure. i didn't know how to make it simpler... + +There are a few ways to get around this. (maybe xref Multiple Members +with the Same Name.} + +@cindex Members, replacing with other members +@cindex Replacing members with other members +If you want to replace an archive member, use @value{op-delete} to +delete the member you want to remove from the archive, , and then use +@option{--append} to add the member you want to be in the archive. Note +that you can not change the order of the archive; the most recently +added member will still appear last. In this sense, you cannot truly +``replace'' one member with another. (Replacing one member with another +will not work on certain types of media, such as tapes; see @ref{delete} +and @ref{Media}, for more information.) + +@menu +* appending files:: Appending Files to an Archive +* multiple:: +@end menu + +@node appending files +@subsubsection Appending Files to an Archive +@UNREVISED +@cindex Adding files to an Archive +@cindex Appending files to an Archive +@cindex Archives, Appending files to + +The simplest way to add a file to an already existing archive is the +@value{op-append} operation, which writes specified files into the +archive whether or not they are already among the archived files. +When you use @option{--append}, you @emph{must} specify file name +arguments, as there is no default. If you specify a file that already +exists in the archive, another copy of the file will be added to the +end of the archive. As with other operations, the member names of the +newly added files will be exactly the same as their names given on the +command line. The @value{op-verbose} option will print out the names +of the files as they are written into the archive. + +@option{--append} cannot be performed on some tape drives, unfortunately, +due to deficiencies in the formats those tape drives use. The archive +must be a valid @command{tar} archive, or else the results of using this +operation will be unpredictable. @xref{Media}. + +To demonstrate using @option{--append} to add a file to an archive, +create a file called @file{rock} in the @file{practice} directory. +Make sure you are in the @file{practice} directory. Then, run the +following @command{tar} command to add @file{rock} to +@file{collection.tar}: + +@smallexample +$ @kbd{tar --append --file=collection.tar rock} +@end smallexample + +@noindent +If you now use the @value{op-list} operation, you will see that +@file{rock} has been added to the archive: + +@smallexample +$ @kbd{tar --list --file=collection.tar} +-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 +-rw-rw-rw- me user 20 1996-09-23 16:44 rock +@end smallexample + +@FIXME{in theory, dan will (soon) try to turn this node into what it's +title claims it will become...} + +@node multiple +@subsubsection Multiple Files with the Same Name + +You can use @value{op-append} to add copies of files which have been +updated since the archive was created. (However, we do not recommend +doing this since there is another @command{tar} option called +@option{--update}; @pxref{update} for more information. We describe this +use of @option{--append} here for the sake of completeness.) @FIXME{is +this really a good idea, to give this whole description for something +which i believe is basically a Stupid way of doing something? certain +aspects of it show ways in which tar is more broken than i'd personally +like to admit to, specifically the last sentence. On the other hand, i +don't think it's a good idea to be saying that we explicitly don't +recommend using something, but i can't see any better way to deal with +the situation.}When you extract the archive, the older version will be +effectively lost. This works because files are extracted from an +archive in the order in which they were archived. Thus, when the +archive is extracted, a file archived later in time will replace a +file of the same name which was archived earlier, even though the older +version of the file will remain in the archive unless you delete all +versions of the file. + +Supposing you change the file @file{blues} and then append the changed +version to @file{collection.tar}. As you saw above, the original +@file{blues} is in the archive @file{collection.tar}. If you change the +file and append the new version of the file to the archive, there will +be two copies in the archive. When you extract the archive, the older +version of the file will be extracted first, and then replaced by the +newer version when it is extracted. + +You can append the new, changed copy of the file @file{blues} to the +archive in this way: + +@smallexample +$ @kbd{tar --append --verbose --file=collection.tar blues} +blues +@end smallexample + +@noindent +Because you specified the @option{--verbose} option, @command{tar} has +printed the name of the file being appended as it was acted on. Now +list the contents of the archive: + +@smallexample +$ @kbd{tar --list --verbose --file=collection.tar} +-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 +-rw-rw-rw- me user 20 1996-09-23 16:44 rock +-rw-rw-rw- me user 58 1996-10-24 18:30 blues +@end smallexample + +@noindent +The newest version of @file{blues} is now at the end of the archive +(note the different creation dates and file sizes). If you extract +the archive, the older version of the file @file{blues} will be +replaced by the newer version. You can confirm this by extracting +the archive and running @samp{ls} on the directory. + +If you wish to extract the first occurrence of the file @file{blues} +from the archive, use @value{op-occurrence} option, as shown in +the following example: + +@smallexample +$ @kbd{tar --extract -vv --occurrence --file=collection.tar blues} +-rw-rw-rw- me user 21 1996-09-23 16:44 blues +@end smallexample + +@xref{Writing}, for more information on @value{op-extract} and +@xref{Option Summary, --occurrence}, for the description of +@value{op-occurrence} option. + +@node update +@subsection Updating an Archive +@UNREVISED +@cindex Updating an archive + +In the previous section, you learned how to use @value{op-append} to add +a file to an existing archive. A related operation is +@value{op-update}. The @option{--update} operation updates a @command{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 newer version of the +file is added to the archive (as with @value{op-append}). + +Unfortunately, you cannot use @option{--update} with magnetic tape drives. +The operation will fail. + +@FIXME{other examples of media on which --update will fail? need to ask +charles and/or mib/thomas/dave shevett..} + +Both @option{--update} and @option{--append} 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, unless you use +the @value{op-backup} option. @FIXME-ref{Multiple Members with the +Same Name} + +@menu +* how to update:: +@end menu + +@node how to update +@subsubsection How to Update an Archive Using @option{--update} + +You must use file name arguments with the @value{op-update} operation. +If you don't specify any files, @command{tar} won't act on any files and +won't tell you that it didn't do anything (which may end up confusing +you). + +@FIXME{note: the above parenthetical added because in fact, this +behavior just confused the author. :-) } + +To see the @option{--update} option at work, create a new file, +@file{classical}, in your practice directory, and some extra text to the +file @file{blues}, using any text editor. Then invoke @command{tar} with +the @samp{update} operation and the @value{op-verbose} option specified, +using the names of all the files in the practice directory as file name +arguments: + +@smallexample +$ @kbd{tar --update -v -f collection.tar blues folk rock classical} +blues +classical +$ +@end smallexample + +@noindent +Because we have specified verbose mode, @command{tar} prints out the names +of the files it is working on, which in this case are the names of the +files that needed to be updated. If you run @samp{tar --list} and look +at the archive, you will see @file{blues} and @file{classical} at its +end. There will be a total of two versions of the member @samp{blues}; +the one at the end will be newer and larger, since you added text before +updating it. + +(The reason @command{tar} does not overwrite the older file when updating +it is because writing to the middle of a section of tape is a difficult +process. Tapes are not designed to go backward. @xref{Media}, for more +information about tapes. + +@value{op-update} is not suitable for performing backups for two +reasons: it does not change directory content entries, and it +lengthens the archive every time it is used. The @GNUTAR{} +options intended specifically for backups are more +efficient. If you need to run backups, please consult @ref{Backups}. + +@node concatenate +@subsection Combining Archives with @option{--concatenate} + +@cindex Adding archives to an archive +@cindex Concatenating Archives +Sometimes it may be convenient to add a second archive onto the end of +an archive rather than adding individual files to the archive. To add +one or more archives to the end of another archive, you should use the +@value{op-concatenate} operation. + +To use @option{--concatenate}, name the archives to be concatenated 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 +any members; all the members with the same name coexist. @FIXME-ref{For +information on how this affects reading the archive, Multiple +Members with the Same Name.} + +To demonstrate how @option{--concatenate} works, create two small archives +called @file{bluesrock.tar} and @file{folkjazz.tar}, using the relevant +files from @file{practice}: + +@smallexample +$ @kbd{tar -cvf bluesrock.tar blues rock} +blues +classical +$ @kbd{tar -cvf folkjazz.tar folk jazz} +folk +jazz +@end smallexample + +@noindent +If you like, You can run @samp{tar --list} to make sure the archives +contain what they are supposed to: + +@smallexample +$ @kbd{tar -tvf bluesrock.tar} +-rw-rw-rw- melissa user 105 1997-01-21 19:42 blues +-rw-rw-rw- melissa user 33 1997-01-20 15:34 rock +$ @kbd{tar -tvf folkjazz.tar} +-rw-rw-rw- melissa user 20 1996-09-23 16:44 folk +-rw-rw-rw- melissa user 65 1997-01-30 14:15 jazz +@end smallexample + +We can concatenate these two archives with @command{tar}: + +@smallexample +$ @kbd{cd ..} +$ @kbd{tar --concatenate --file=bluesrock.tar jazzfolk.tar} +@end smallexample + +If you now list the contents of the @file{bluesclass.tar}, you will see +that now it also contains the archive members of @file{jazzfolk.tar}: + +@smallexample +$ @kbd{tar --list --file=bluesrock.tar} +blues +rock +jazz +folk +@end smallexample + +When you use @option{--concatenate}, the source and target archives must +already exist and must have been created using compatible format +parameters. @FIXME-pxref{Matching Format Parameters}The new, +concatenated archive will be called by the same name as the first +archive listed on the command line. @FIXME{is there a way to specify a +new name?} + +Like @value{op-append}, this operation cannot be performed on some +tape drives, due to deficiencies in the formats those tape drives use. + +@cindex @code{concatenate} vs @command{cat} +@cindex @command{cat} vs @code{concatenate} +It may seem more intuitive to you to want or try to use @command{cat} to +concatenate two archives instead of using the @option{--concatenate} +operation; after all, @command{cat} is the utility for combining files. + +However, @command{tar} archives incorporate an end-of-file marker which +must be removed if the concatenated archives are to be read properly as +one archive. @option{--concatenate} removes the end-of-archive marker +from the target archive before each new archive is appended. If you use +@command{cat} to combine the archives, the result will not be a valid +@command{tar} format archive. If you need to retrieve files from an +archive that was added to using the @command{cat} utility, use the +@value{op-ignore-zeros} option. @xref{Ignore Zeros}, for further +information on dealing with archives improperly combined using the +@command{cat} shell utility. + +@FIXME{this shouldn't go here. where should it go?} You must specify +the source archives using @value{op-file} (@value{pxref-file}). If you +do not specify the target archive, @command{tar} uses the value of the +environment variable @env{TAPE}, or, if this has not been set, the +default archive name. + +@node delete +@subsection Removing Archive Members Using @option{--delete} +@UNREVISED +@cindex Deleting files from an archive +@cindex Removing files from an archive + +You can remove members from an archive by using the @value{op-delete} +option. Specify the name of the archive with @value{op-file} and then +specify the names of the members to be deleted; if you list no member +names, nothing will be deleted. The @value{op-verbose} option will +cause @command{tar} to print the names of the members as they are deleted. +As with @value{op-extract}, you must give the exact member names when +using @samp{tar --delete}. @option{--delete} will remove all versions of +the named file from the archive. The @option{--delete} operation can run +very slowly. + +Unlike other operations, @option{--delete} has no short form. + +@cindex Tapes, using @option{--delete} and +@cindex Deleting from tape archives +This operation will rewrite the archive. You can only use +@option{--delete} on an archive if the archive device allows you to +write to any point on the media, such as a disk; because of this, it +does not work on magnetic tapes. Do not try to delete an archive member +from a magnetic tape; the action will not succeed, and you will be +likely to scramble the archive and damage your tape. There is no safe +way (except by completely re-writing the archive) to delete files from +most kinds of magnetic tape. @xref{Media}. + +To delete all versions of the file @file{blues} from the archive +@file{collection.tar} in the @file{practice} directory, make sure you +are in that directory, and then, + +@smallexample +$ @kbd{tar --list --file=collection.tar} +blues +folk +jazz +rock +practice/blues +practice/folk +practice/jazz +practice/rock +practice/blues +$ @kbd{tar --delete --file=collection.tar blues} +$ @kbd{tar --list --file=collection.tar} +folk +jazz +rock +$ +@end smallexample + +@FIXME{I changed the order of these nodes around and haven't had a chance +to fix the above example's results, yet. I have to play with this and +follow it and see what it actually does!} + +The @value{op-delete} option has been reported to work properly when +@command{tar} acts as a filter from @code{stdin} to @code{stdout}. + +@node compare +@subsection Comparing Archive Members with the File System +@cindex Verifying the currency of an archive +@UNREVISED + +The @option{--compare} (@option{-d}), or @option{--diff} operation compares +specified archive members against files with the same names, and then +reports differences in file size, mode, owner, modification date and +contents. You should @emph{only} specify archive member names, not file +names. If you do not name any members, then @command{tar} will compare the +entire archive. If a file is represented in the archive but does not +exist in the file system, @command{tar} reports a difference. + +You have to specify the record size of the archive when modifying an +archive with a non-default record size. + +@command{tar} ignores files in the file system that do not have +corresponding members in the archive. + +The following example compares the archive members @file{rock}, +@file{blues} and @file{funk} in the archive @file{bluesrock.tar} with +files of the same name in the file system. (Note that there is no file, +@file{funk}; @command{tar} will report an error message.) + +@smallexample +$ @kbd{tar --compare --file=bluesrock.tar rock blues funk} +rock +blues +tar: funk not found in archive +@end smallexample + +@noindent +@FIXME{what does this actually depend on? i'm making a guess, +here.}Depending on the system where you are running @command{tar} and the +version you are running, @command{tar} may have a different error message, +such as: + +@smallexample +funk: does not exist +@end smallexample + +@FIXME-xref{somewhere, for more information about format parameters. +Melissa says: such as "format variations"? But why? Clearly I don't +get it yet; I'll deal when I get to that section.} + +The spirit behind the @value{op-compare} option is to check whether the +archive represents the current state of files on disk, more than validating +the integrity of the archive media. For this later goal, @xref{verify}. + +@node create options +@section Options Used by @option{--create} + +The previous chapter described the basics of how to use +@value{op-create} to create an archive from a set of files. +@xref{create}. This section described advanced options to be used with +@option{--create}. + +@menu +* Ignore Failed Read:: +@end menu + +@node Ignore Failed Read +@subsection Ignore Fail Read + +@table @option +@item --ignore-failed-read +Do not exit with nonzero on unreadable files or directories. +@end table + +@node extract options +@section Options Used by @option{--extract} +@UNREVISED + +@FIXME{i need to get dan to go over these options with me and see if +there's a better way of organizing them.} + +The previous chapter showed how to use @value{op-extract} to extract +an archive into the filesystem. Various options cause @command{tar} to +extract more information than just file contents, such as the owner, +the permissions, the modification date, and so forth. This section +presents options to be used with @option{--extract} when certain special +considerations arise. You may review the information presented in +@ref{extract} for more basic information about the +@option{--extract} operation. + +@menu +* Reading:: Options to Help Read Archives +* Writing:: Changing How @command{tar} Writes Files +* Scarce:: Coping with Scarce Resources +@end menu + +@node Reading +@subsection Options to Help Read Archives +@cindex Options when reading archives +@cindex Reading incomplete records +@cindex Records, incomplete +@cindex End-of-archive entries, ignoring +@cindex Ignoring end-of-archive entries +@cindex Large lists of file names on small machines +@cindex Small memory +@cindex Running out of space +@UNREVISED + +Normally, @command{tar} will request data in full record increments from +an archive storage device. If the device cannot return a full record, +@command{tar} will report an error. However, some devices do not always +return full records, or do not require the last record of an archive to +be padded out to the next record boundary. To keep reading until you +obtain a full record, or to accept an incomplete record if it contains +an end-of-archive marker, specify the @value{op-read-full-records} option +in conjunction with the @value{op-extract} or @value{op-list} operations. +@value{xref-read-full-records}. + +The @value{op-read-full-records} option is turned on by default when +@command{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, @command{tar} +would fail as soon as it read an incomplete record from the pipe. + +If you're not sure of the blocking factor of an archive, you can +read the archive by specifying @value{op-read-full-records} and +@value{op-blocking-factor}, using a blocking factor larger than what the +archive uses. This lets you avoid having to determine the blocking factor +of an archive. @value{xref-blocking-factor}. + +@menu +* read full records:: +* Ignore Zeros:: +@end menu + +@node read full records +@unnumberedsubsubsec Reading Full Records + +@FIXME{need sentence or so of intro here} + +@table @option +@item --read-full-records +@item -B +Use in conjunction with @value{op-extract} to read an archive which +contains incomplete records, or one which has a blocking factor less +than the one specified. +@end table + +@node Ignore Zeros +@unnumberedsubsubsec Ignoring Blocks of Zeros + +Normally, @command{tar} stops reading when it encounters a block of zeros +between file entries (which usually indicates the end of the archive). +@value{op-ignore-zeros} allows @command{tar} to completely read an archive +which contains a block of zeros before the end (i.e., a damaged +archive, or one that was created by concatenating several archives +together). + +The @value{op-ignore-zeros} option is turned off by default because many +versions of @command{tar} write garbage after the end-of-archive entry, +since that part of the media is never supposed to be read. @GNUTAR{} +does not write after the end of an archive, but seeks to +maintain compatiblity among archiving utilities. + +@table @option +@item --ignore-zeros +@itemx -i +To ignore blocks of zeros (i.e., end-of-archive entries) which may be +encountered while reading an archive. Use in conjunction with +@value{op-extract} or @value{op-list}. +@end table + +@node Writing +@subsection Changing How @command{tar} Writes Files +@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 +@UNREVISED + +@FIXME{need to mention the brand new option, --backup} + +@menu +* Dealing with Old Files:: +* Overwrite Old Files:: +* Keep Old Files:: +* Keep Newer Files:: +* Unlink First:: +* Recursive Unlink:: +* Modification Times:: +* Setting Access Permissions:: +* Writing to Standard Output:: +* remove files:: +@end menu + +@node Dealing with Old Files +@unnumberedsubsubsec Options Controlling the Overwriting of Existing Files + +When extracting files, if @command{tar} discovers that the extracted +file already exists, it normally replaces the file by removing it before +extracting it, to prevent confusion in the presence of hard or symbolic +links. (If the existing file is a symbolic link, it is removed, not +followed.) However, if a directory cannot be removed because it is +nonempty, @command{tar} normally overwrites its metadata (ownership, +permission, etc.). The @option{--overwrite-dir} option enables this +default behavior. To be more cautious and preserve the metadata of +such a directory, use the @option{--no-overwrite-dir} option. + +To be even more cautious and prevent existing files from being replaced, use +the @value{op-keep-old-files} option. It causes @command{tar} to refuse +to replace or update a file that already exists, i.e., a file with the +same name as an archive member prevents extraction of that archive +member. Instead, it reports an error. + +To be more aggressive about altering existing files, use the +@value{op-overwrite} option. It causes @command{tar} to overwrite +existing files and to follow existing symbolic links when extracting. + +Some people argue that @GNUTAR{} should not hesitate +to overwrite files with other files when extracting. When extracting +a @command{tar} archive, they expect to see a faithful copy of the +state of the filesystem when the archive was created. It is debatable +that this would always be a proper behavior. For example, suppose one +has an archive in which @file{usr/local} is a link to +@file{usr/local2}. Since then, maybe the site removed the link and +renamed the whole hierarchy from @file{/usr/local2} to +@file{/usr/local}. Such things happen all the time. I guess it would +not be welcome at all that @GNUTAR{} removes the +whole hierarchy just to make room for the link to be reinstated +(unless it @emph{also} simultaneously restores the full +@file{/usr/local2}, of course!) @GNUTAR{} is indeed +able to remove a whole hierarchy to reestablish a symbolic link, for +example, but @emph{only if} @value{op-recursive-unlink} is specified +to allow this behavior. In any case, single files are silently +removed. + +Finally, the @value{op-unlink-first} option can improve performance in +some cases by causing @command{tar} to remove files unconditionally +before extracting them. + +@node Overwrite Old Files +@unnumberedsubsubsec Overwrite Old Files + +@table @option +@item --overwrite +Overwrite existing files and directory metadata when extracting files +from an archive. + +This +causes @command{tar} to write extracted files into the file system without +regard to the files already on the system; i.e., files with the same +names as archive members are overwritten when the archive is extracted. +It also causes @command{tar} to extract the ownership, permissions, +and time stamps onto any preexisting files or directories. +If the name of a corresponding file name is a symbolic link, the file +pointed to by the symbolic link will be overwritten instead of the +symbolic link itself (if this is possible). Moreover, special devices, +empty directories and even symbolic links are automatically removed if +they are in the way of extraction. + +Be careful when using the @value{op-overwrite} option, particularly when +combined with the @value{op-absolute-names} option, as this combination +can change the contents, ownership or permissions of any file on your +system. Also, many systems do not take kindly to overwriting files that +are currently being executed. + +@item --overwrite-dir +Overwrite the metadata of directories when extracting files from an +archive, but remove other files before extracting. +@end table + +@node Keep Old Files +@unnumberedsubsubsec Keep Old Files + +@table @option +@item --keep-old-files +@itemx -k +Do not replace existing files from archive. The +@value{op-keep-old-files} option prevents @command{tar} from replacing +existing files with files with the same name from the archive. +The @value{op-keep-old-files} option is meaningless with @value{op-list}. +Prevents @command{tar} from replacing files in the file system during +extraction. +@end table + +@node Keep Newer Files +@unnumberedsubsubsec Keep Newer Files + +@table @option +@item --keep-newer-files +Do not replace existing files that are newer than their archive +copies. This option is meaningless with @value{op-list}. +@end table + +@node Unlink First +@unnumberedsubsubsec Unlink First + +@table @option +@item --unlink-first +@itemx -U +Remove files before extracting over them. +This can make @command{tar} run a bit faster if you know in advance +that the extracted files all need to be removed. Normally this option +slows @command{tar} down slightly, so it is disabled by default. +@end table + +@node Recursive Unlink +@unnumberedsubsubsec Recursive Unlink + +@table @option +@item --recursive-unlink +When this option is specified, try removing files and directory hierarchies +before extracting over them. @emph{This is a dangerous option!} +@end table + +If you specify the @value{op-recursive-unlink} option, +@command{tar} removes @emph{anything} that keeps you from extracting a file +as far as current permissions will allow it. This could include removal +of the contents of a full directory hierarchy. + +@node Modification Times +@unnumberedsubsubsec Setting Modification Times + +Normally, @command{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. + +To set the modification times of extracted files to the time when +the files were extracted, use the @value{op-touch} option in +conjunction with @value{op-extract}. + +@table @option +@item --touch +@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 @value{op-extract}. +@end table + +@node Setting Access Permissions +@unnumberedsubsubsec Setting Access Permissions + +To set the modes (access permissions) of extracted files to those +recorded for those files in the archive, use @option{--same-permissions} +in conjunction with the @value{op-extract} operation. @FIXME{Should be +aliased to ignore-umask.} + +@table @option +@item --preserve-permission +@itemx --same-permission +@itemx --ignore-umask +@itemx -p +Set modes of extracted archive members to those recorded in the +archive, instead of current umask settings. Use in conjunction with +@value{op-extract}. +@end table + +@FIXME{Following paragraph needs to be rewritten: why doesn't this cat +files together, why is this useful. is it really useful with +more than one file?} + +@node Writing to Standard Output +@unnumberedsubsubsec Writing to Standard Output + +To write the extracted files to the standard output, instead of +creating the files on the file system, use @value{op-to-stdout} in +conjunction with @value{op-extract}. This option is useful if you are +extracting files to send them through a pipe, and do not need to +preserve them in the file system. If you extract multiple members, +they appear on standard output concatenated, in the order they are +found in the archive. + +@table @option +@item --to-stdout +@itemx -O +Writes files to the standard output. Used in conjunction with +@value{op-extract}. Extract files to standard output. When this option +is used, instead of creating the files specified, @command{tar} writes +the contents of the files extracted to its standard output. This may +be useful if you are only extracting the files in order to send them +through a pipe. This option is meaningless with @value{op-list}. +@end table + +This can be useful, for example, if you have a tar archive containing +a big file and don't want to store the file on disk before processing +it. You can use a command like this: + +@smallexample +tar -xOzf foo.tgz bigfile | process +@end smallexample + +or even like this if you want to process the concatenation of the files: + +@smallexample +tar -xOzf foo.tgz bigfile1 bigfile2 | process +@end smallexample + +@node remove files +@unnumberedsubsubsec Removing Files + +@FIXME{the various macros in the front of the manual think that this +option goes in this section. i have no idea; i only know it's nowhere +else in the book...} + +@table @option +@item --remove-files +Remove files after adding them to the archive. +@end table + +@node Scarce +@subsection Coping with Scarce Resources +@cindex Middle of the archive, starting in the +@cindex Running out of space during extraction +@cindex Disk space, running out of +@cindex Space on the disk, recovering from lack of +@UNREVISED + +@menu +* Starting File:: +* Same Order:: +@end menu + +@node Starting File +@unnumberedsubsubsec Starting File + +@table @option +@item --starting-file=@var{name} +@itemx -K @var{name} +Starts an operation in the middle of an archive. Use in conjunction +with @value{op-extract} or @value{op-list}. +@end table + +If a previous attempt to extract files failed due to lack of disk +space, you can use @value{op-starting-file} to start extracting only +after member @var{name} of the archive. This assumes, of course, that +there is now free space, or that you are now extracting into a +different file system. (You could also choose to suspend @command{tar}, +remove unnecessary files from the file system, and then restart the +same @command{tar} operation. In this case, @value{op-starting-file} is +not necessary. @value{xref-incremental}, @value{xref-interactive}, +and @value{ref-exclude}.) + +@node Same Order +@unnumberedsubsubsec Same Order + +@table @option +@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 @value{op-compare}, +@value{op-list} +or @value{op-extract}. +@end table + +@FIXME{we don't need/want --preserve to exist any more (from melissa: +ie, don't want that *version* of the option to exist, or don't want +the option to exist in either version?} + +@FIXME{i think this explanation is lacking.} + +The @value{op-same-order} option tells @command{tar} that the list of file +names to be listed or extracted is sorted in the same order as the +files in the archive. This allows a large list of names to be used, +even on a small machine that would not otherwise be able to hold all +the names in memory at the same time. Such a sorted list can easily be +created by running @samp{tar -t} on the archive and editing its output. + +This option is probably never needed on modern computer systems. + +@node backup +@section Backup options + +@cindex backup options + +@GNUTAR{} offers options for making backups of files +before writing new versions. These options control the details of +these backups. They may apply to the archive itself before it is +created or rewritten, as well as individual extracted members. Other +@acronym{GNU} programs (@command{cp}, @command{install}, @command{ln}, +and @command{mv}, for example) offer similar options. + +Backup options may prove unexpectedly useful when extracting archives +containing many members having identical name, or when extracting archives +on systems having file name limitations, making different members appear +has having similar names through the side-effect of name truncation. +(This is true only if we have a good scheme for truncated backup names, +which I'm not sure at all: I suspect work is needed in this area.) +When any existing file is backed up before being overwritten by extraction, +then clashing files are automatically be renamed to be unique, and the +true name is kept for only the last file of a series of clashing files. +By using verbose mode, users may track exactly what happens. + +At the detail level, some decisions are still experimental, and may +change in the future, we are waiting comments from our users. So, please +do not learn to depend blindly on the details of the backup features. +For example, currently, directories themselves are never renamed through +using these options, so, extracting a file over a directory still has +good chances to fail. Also, backup options apply to created archives, +not only to extracted members. For created archives, backups will not +be attempted when the archive is a block or character device, or when it +refers to a remote file. + +For the sake of simplicity and efficiency, backups are made by renaming old +files prior to creation or extraction, and not by copying. The original +name is restored if the file creation fails. If a failure occurs after a +partial extraction of a file, both the backup and the partially extracted +file are kept. + +@table @samp + +@item --backup[=@var{method}] +@opindex --backup +@vindex VERSION_CONTROL +@cindex backups +Back up files that are about to be overwritten or removed. +Without this option, the original versions are destroyed. + +Use @var{method} to determine the type of backups made. +If @var{method} is not specified, use the value of the @env{VERSION_CONTROL} +environment variable. And if @env{VERSION_CONTROL} is not set, +use the @samp{existing} method. + +@vindex version-control @r{Emacs variable} +This option corresponds to the Emacs variable @samp{version-control}; +the same values for @var{method} are accepted as in Emacs. This option +also allows more descriptive names. The valid @var{method}s are: + +@table @samp +@item t +@itemx numbered +@opindex numbered @r{backup method} +Always make numbered backups. + +@item nil +@itemx existing +@opindex existing @r{backup method} +Make numbered backups of files that already have them, simple backups +of the others. + +@item never +@itemx simple +@opindex simple @r{backup method} +Always make simple backups. + +@end table + +@item --suffix=@var{suffix} +@opindex --suffix +@cindex backup suffix +@vindex SIMPLE_BACKUP_SUFFIX +Append @var{suffix} to each backup file made with @option{--backup}. If this +option is not specified, the value of the @env{SIMPLE_BACKUP_SUFFIX} +environment variable is used. And if @env{SIMPLE_BACKUP_SUFFIX} is not +set, the default is @samp{~}, just as in Emacs. + +@end table + +Some people express the desire to @emph{always} use the @value{op-backup} +option, by defining some kind of alias or script. This is not as easy +as one may think, due to the fact that old style options should appear first +and consume arguments a bit unpredictably for an alias or script. But, +if you are ready to give up using old style options, you may resort to +using something like (a Bourne shell function here): + +@smallexample +tar () @{ /usr/local/bin/tar --backup $*; @} +@end smallexample + +@node Applications +@section Notable @command{tar} Usages +@UNREVISED + +@FIXME{Using Unix file linking capability to recreate directory +structures---linking files into one subdirectory and then +@command{tar}ring that directory.} + +@FIXME{Nice hairy example using absolute-names, newer, etc.} + +@findex uuencode +You can easily use archive files to transport a group of files from +one system to another: put all relevant files into an archive on one +computer system, transfer the archive to another system, and extract +the contents there. The basic transfer medium might be magnetic tape, +Internet FTP, or even electronic mail (though you must encode the +archive with @command{uuencode} in order to transport it properly by +mail). Both machines do not have to use the same operating system, as +long as they both support the @command{tar} program. + +For example, here is how you might copy a directory's contents from +one disk to another, while preserving the dates, modes, owners and +link-structure of all the files therein. In this case, the transfer +medium is a @dfn{pipe}, which is one a Unix redirection mechanism: + +@smallexample +$ @kbd{cd sourcedir; tar -cf - . | (cd targetdir; tar -xf -)} +@end smallexample + +@noindent +The command also works using short option forms: + +@smallexample +$ @w{@kbd{cd sourcedir; tar --create --file=- . | (cd targetdir; tar --extract --file=-)}} +@end smallexample + +@noindent +This is one of the easiest methods to transfer a @command{tar} archive. + +@node looking ahead +@section Looking Ahead: The Rest of this Manual + +You have now seen how to use all eight of the operations available to +@command{tar}, and a number of the possible options. The next chapter +explains how to choose and change file and archive names, how to use +files to store names of other files which you can then call as +arguments to @command{tar} (this can help you save time if you expect to +archive the same list of files a number of times), and so forth. +@FIXME{in case it's not obvious, i'm making this up in some sense +based on my limited memory of what the next chapter *really* does. i +just wanted to flesh out this final section a little bit so i'd +remember to stick it in here. :-)} + +If there are too many files to conveniently list on the command line, +you can list the names in a file, and @command{tar} will read that file. +@value{xref-files-from}. + +There are various ways of causing @command{tar} to skip over some files, +and not archive them. @xref{Choosing}. + +@node Backups +@chapter Performing Backups and Restoring Files +@UNREVISED + +@GNUTAR{} is distributed along with the scripts +which the Free Software Foundation uses for performing backups. There +is no corresponding scripts available yet for doing restoration of +files. Even if there is a good chance those scripts may be satisfying +to you, they are not the only scripts or methods available for doing +backups and restore. You may well create your own, or use more +sophisticated packages dedicated to that purpose. + +Some users are enthusiastic about @code{Amanda} (The Advanced Maryland +Automatic Network Disk Archiver), a backup system developed by James +da Silva @file{jds@@cs.umd.edu} and available on many Unix systems. +This is free software, and it is available at these places: + +@smallexample +http://www.cs.umd.edu/projects/amanda/amanda.html +ftp://ftp.cs.umd.edu/pub/amanda +@end smallexample + +@FIXME{ + +Here is a possible plan for a future documentation about the backuping +scripts which are provided within the @GNUTAR{} +distribution. + +@itemize @bullet +@item dumps + @itemize @minus + @item what are dumps + @item different levels of dumps + @itemize + + @item full dump = dump everything + @item level 1, level 2 dumps etc + A level @var{n} dump dumps everything changed since the last level + @var{n}-1 dump (?) + @end itemize + @item how to use scripts for dumps (ie, the concept) + @itemize + + @item scripts to run after editing backup specs (details) + @end itemize + @item Backup Specs, what is it. + @itemize + + @item how to customize + @item actual text of script [/sp/dump/backup-specs] + @end itemize + @item Problems + @itemize + + @item rsh doesn't work + @item rtape isn't installed + @item (others?) + @end itemize + @item the @option{--incremental} option of tar + @item tapes + @itemize + + @item write protection + @item types of media, different sizes and types, useful for different things + @item files and tape marks + one tape mark between files, two at end. + @item positioning the tape + MT writes two at end of write, + backspaces over one when writing again. + @end itemize + @end itemize +@end itemize +} + +This chapter documents both the provided shell scripts and @command{tar} +options which are more specific to usage as a backup tool. + +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 accidentally deleted). File system @dfn{backups} are also +called @dfn{dumps}. + +@menu +* Full Dumps:: Using @command{tar} to Perform Full Dumps +* Incremental Dumps:: Using @command{tar} to Perform Incremental Dumps +* 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 +@end menu + +@node Full Dumps +@section Using @command{tar} to Perform Full Dumps +@UNREVISED + +@cindex full dumps +@cindex dumps, full + +@cindex corrupted archives +Full dumps should only be made when no other people or programs +are modifying files in the filesystem. If files are modified while +@command{tar} is making the backup, they may not be stored properly in +the archive, in which case you won't be able to restore them if you +have to. (Files not being modified are written with no trouble, and do +not corrupt the entire archive.) + +You will want to use the @value{op-label} option to give the archive a +volume label, so you can tell what this archive is even if the label +falls off the tape, or anything like that. + +Unless the filesystem you are dumping is guaranteed to fit on +one volume, you will need to use the @value{op-multi-volume} option. +Make sure you have enough tapes on hand to complete the backup. + +If you want to dump each filesystem separately you will need to use +the @value{op-one-file-system} option to prevent @command{tar} from crossing +filesystem boundaries when storing (sub)directories. + +The @value{op-incremental} (@FIXME-pxref{}) option is not needed, +since this is a complete copy of everything in the filesystem, and a +full restore from this backup would only be done onto a completely +empty disk. + +Unless you are in a hurry, and trust the @command{tar} program (and your +tapes), it is a good idea to use the @value{op-verify} option, to make +sure your files really made it onto the dump properly. This will +also detect cases where the file was modified while (or just after) +it was being archived. Not all media (notably cartridge tapes) are +capable of being verified, unfortunately. + +@node Incremental Dumps +@section Using @command{tar} to Perform Incremental Dumps + +@dfn{Incremental backup} is a special form of @GNUTAR{} archive that +stores additional metadata so that exact state of the filesystem +can be restored when extracting the archive. + +@GNUTAR{} currently offers two options for handling incremental +backups: @value{op-listed-incremental} and @value{op-incremental}. + +The option @option{--listed-incremental} instructs tar to operate on +an incremental archive with additional metadata stored in a standalone +file, called @dfn{snapshot file}. The purpose of this file is to help +determine what files have been changed, added or deleted since the +last backup, so that the next incremental backup will contain only +modified files. The name of the snapshot file is given as an argument +to the option: + +@table @option +@item --listed-incremental=@var{file} +@itemx -g @var{file} + Handle incremental backups with snapshot data in @var{file}. +@end table + +To create an incremental backup, you would use +@option{--listed-incremental} together with @option{--create} +(@pxref{create}). For example: + +@smallexample +$ @kbd{tar --create \ + --file=archive.1.tar \ + --listed-incremental=/var/log/usr.snar \ + /usr} +@end smallexample + +This will create in @file{archive.1.tar} an incremental backup of +@file{/usr} filesystem, storing additional metadata in the file +@file{/var/log/usr.snar}. If this file does not exist, it will be +created. The created archive will then be called @dfn{level 0 backup} +(see the next section for more info on backup levels). + +Otherwise, if the file @file{/var/log/usr.snar} exists, it is used to +determine the modified files. In this case only these files will be +stored in the archive. Suppose, for example, that after running the +above command, you delete file @file{/usr/doc/old} and create +directory @file{/usr/local/db} with the following contents: + +@smallexample +$ @kbd{ls /usr/local/db} +/usr/local/db/data +/usr/local/db/index +@end smallexample + +Some time later you create another incremental backup. You will +then see: + +@smallexample +$ @kbd{tar --create \ + --file=archive.2.tar \ + --listed-incremental=/var/log/usr.snar \ + /usr} +tar: usr/local/db: Directory is new +usr/local/db/ +usr/local/db/data +usr/local/db/index +@end smallexample + +@noindent +The created archive @file{archive.2.tar} will contain only these +three members. This archive is called @dfn{level 1 backup}. Notice, +that @file{/var/log/usr.snar} will be updated with the new data, so if +you plan to create more @samp{level 1} backups, it is necessary to +create a working copy of the snapshot file before running +@command{tar}. The above example will then be modified as follows: + +@smallexample +$ @kbd{cp /var/log/usr.snar /var/log/usr.snar-1} +$ @kbd{tar --create \ + --file=archive.2.tar \ + --listed-incremental=/var/log/usr.snar-1 \ + /usr} +@end smallexample + +Incremental dumps depend crucially on time stamps, so the results are +unreliable if you modify a file's time stamps during dumping (e.g., +with the @option{--atime-preserve} option), or if you set the clock +backwards. + +Metadata stored in snapshot files include device numbers, which, +obviously is supposed to be a non-volatile value. However, it turns +out that NFS devices have non-dependable values when an automounter +gets in the picture. This can lead to a great deal of spurious +redumping in incremental dumps, so it is somewhat useless to compare +two NFS devices numbers over time. The solution implemented currently +is to 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. + +Note that incremental archives use @command{tar} extensions and may +not be readable by non-@acronym{GNU} versions of the @command{tar} program. + +To extract from the incremental dumps, use +@option{--listed-incremental} together with @option{--extract} +option (@pxref{extracting files}). In this case, @command{tar} does +not need to access snapshot file, since all the data necessary for +extraction are stored in the archive itself. So, when extracting, you +can give whatever argument to @option{--listed-incremental}, the usual +practice is to use @option{--listed-incremental=/dev/null}. +Alternatively, you can use @option{--incremental}, which needs no +arguments. In general, @option{--incremental} (@option{-G}) can be +used as a shortcut for @option{--listed-incremental} when listing or +extracting incremental backups (for more information, regarding this +option, @pxref{incremental-op}). + +When extracting from the incremental backup @GNUTAR{} attempts to +restore the exact state the file system had when the archive was +created. In particular, it will @emph{delete} those files in the file +system that did not exist in their directories when the archive was +created. If you have created several levels of incremental files, +then in order to restore the exact contents the file system had when +the last level was created, you will need to restore from all backups +in turn. Continuing our example, to restore the state of @file{/usr} +file system, one would do@footnote{Notice, that since both archives +were created withouth @option{-P} option (@pxref{absolute}), these +commands should be run from the root filesystem.}: + +@smallexample +$ @kbd{tar --extract \ + --listed-incremental=/dev/null \ + --file archive.1.tar} +$ @kbd{tar --extract \ + --listed-incremental=/dev/null \ + --file archive.2.tar} +@end smallexample + +To list the contents of an incremental archive, use @option{--list} +(@pxref{list}), as usual. To obtain more information about the +archive, use @option{--listed-incremental} or @option{--incremental} +combined with two @option{--verbose} options@footnote{Two +@option{--verbose} options were selected to avoid breaking usual +verbose listing output (@option{--list --verbose}) when using in +scripts. + +Versions of @GNUTAR{} up to 1.15.1 used to dump verbatim binary +contents of the DUMPDIR header (with terminating nulls) when +@option{--incremental} or @option{--listed-incremental} option was +given, no matter what the verbosity level. This behavior, and, +especially, the binary output it produced were considered incovenient +and were changed in version 1.16}: + +@smallexample +@kbd{tar --list --incremental --verbose --verbose archive.tar} +@end smallexample + +This command will 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 which is both human-readable and +unambiguous for a program: each file name is printed as + +@smallexample +@var{x} @var{file} +@end smallexample + +@noindent +where @var{x} is a letter describing the status of the file: @samp{Y} +if the file is present in the archive, @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).@FIXME-xref{dumpdir format}. Each such +line is terminated by a newline character. The last line is followed +by an additional newline to indicate the end of the data. + +@anchor{incremental-op}The option @option{--incremental} (@option{-G}) +gives the same behavior as @option{--listed-incremental} when used +with @option{--list} and @option{--extract} options. When used with +@option{--create} option, it creates an incremental archive without +creating snapshot file. Thus, it is impossible to create several +levels of incremental backups with @option{--incremental} option. + +@node Backup Levels +@section Levels of Backups + +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 use @dfn{incremental dumps}. 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). + +@GNUTAR{} comes with scripts you can use to do full +and level-one (actually, even level-two and so on) dumps. Using +scripts (shell programs) to perform backups and restoration is a +convenient and reliable alternative to typing out file name lists +and @command{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. This file is usually located +in @file{/etc/backup} directory. @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 backup script is @code{backup}. The name of the +restore script is @code{restore}. The following sections describe +their use in detail. + +@emph{Please Note:} The backup and 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. @value{xref-incremental}, before +making such an attempt. + +@node Backup Parameters +@section Setting Parameters for Backups and Restoration + +The file @file{backup-specs} specifies backup parameters for the +backup and restoration scripts provided with @command{tar}. You must +edit @file{backup-specs} to fit your system configuration and schedule +before using these scripts. + +Syntactically, @file{backup-specs} is a shell script, containing +mainly variable assignments. However, any valid shell construct +is allowed in this file. Particularly, you may wish to define +functions within that script (e.g., see @code{RESTORE_BEGIN} below). +For more information about shell script syntax, please refer to +@url{http://www.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html#ta +g_02, the definition of the Shell Command Language}. See also +@ref{Top,,Bash Features,bashref,Bash Reference Manual}. + +The shell variables controlling behavior of @code{backup} and +@code{restore} are described in the following subsections. + +@menu +* General-Purpose Variables:: +* Magnetic Tape Control:: +* User Hooks:: +* backup-specs example:: An Example Text of @file{Backup-specs} +@end menu + +@node General-Purpose Variables +@subsection General-Purpose Variables + +@defvr {Backup variable} ADMINISTRATOR +The user name of the backup administrator. @code{Backup} scripts +sends a backup report to this address. +@end defvr + +@defvr {Backup variable} BACKUP_HOUR +The hour at which the backups are done. This can be a number from 0 +to 23, or the time specification in form @var{hours}:@var{minutes}, +or the string @samp{now}. + +This variable is used by @code{backup}. Its value may be overridden +using @option{--time} option (@pxref{Scripted Backups}). +@end defvr + +@defvr {Backup variable} TAPE_FILE + +The device @command{tar} writes the archive to. If @var{TAPE_FILE} +is a remote archive (@pxref{remote-dev}), backup script will suppose +that your @command{mt} is able to access remote devices. If @var{RSH} +(@pxref{RSH}) is set, @option{--rsh-command} option will be added to +invocations of @command{mt}. +@end defvr + +@defvr {Backup variable} BLOCKING + +The blocking factor @command{tar} will use when writing the dump archive. +@value{xref-blocking-factor}. +@end defvr + +@defvr {Backup variable} BACKUP_DIRS + +A list of file systems to be dumped (for @code{backup}), or restored +(for @code{restore}). 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 @command{tar} on, and should +normally be the host that actually contains the file system. However, +the host machine must have @GNUTAR{} 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 @command{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. + +If the list of file systems is very long you may wish to put it +in a separate file. This file is usually named +@file{/etc/backup/dirs}, but this name may be overridden in +@file{backup-specs} using @code{DIRLIST} variable. +@end defvr + +@defvr {Backup variable} DIRLIST + +A path to the file containing the list of the filesystems to backup +or restore. By default it is @file{/etc/backup/dirs}. +@end defvr + +@defvr {Backup variable} BACKUP_FILES + +A list of individual files to be dumped (for @code{backup}), or restored +(for @code{restore}). These should be accessible from the machine on +which the backup script is run. + +If the list of file systems is very long you may wish to store it +in a separate file. This file is usually named +@file{/etc/backup/files}, but this name may be overridden in +@file{backup-specs} using @code{FILELIST} variable. +@end defvr + +@defvr {Backup variable} FILELIST + +A path to the file containing the list of the individual files to backup +or restore. By default it is @file{/etc/backup/files}. +@end defvr + +@defvr {Backup variable} MT + +Full file name of @command{mt} binary. +@end defvr + +@defvr {Backup variable} RSH +@anchor{RSH} +Full file name of @command{rsh} binary or its equivalent. You may wish to +set it to @code{ssh}, to improve security. In this case you will have +to use public key authentication. +@end defvr + +@defvr {Backup variable} RSH_COMMAND + +Full file name of @command{rsh} binary on remote mashines. This will +be passed via @option{--rsh-command} option to the remote invocation +of @GNUTAR{}. +@end defvr + +@defvr {Backup variable} VOLNO_FILE + +Name of temporary file to hold volume numbers. This needs to be accessible +by all the machines which have filesystems to be dumped. +@end defvr + +@defvr {Backup variable} XLIST + +Name of @dfn{exclude file list}. An @dfn{exclude file list} is a file +located on the remote machine and containing the list of files to +be excluded from the backup. Exclude file lists are searched in +/etc/tar-backup directory. A common use for exclude file lists +is to exclude files containing security-sensitive information +(e.g., @file{/etc/shadow} from backups). + +This variable affects only @code{backup}. +@end defvr + +@defvr {Backup variable} SLEEP_TIME + +Time to sleep between dumps of any two successive filesystems + +This variable affects only @code{backup}. +@end defvr + +@defvr {Backup variable} DUMP_REMIND_SCRIPT + +Script to be run when it's time to insert a new tape in for the next +volume. Administrators may want to tailor this script for their site. +If this variable isn't set, @GNUTAR{} will display its built-in prompt +@FIXME-xref{describe it somewhere!}, and will expect confirmation from +the console. +@end defvr + +@defvr {Backup variable} SLEEP_MESSAGE + +Message to display on the terminal while waiting for dump time. Usually +this will just be some literal text. +@end defvr + +@defvr {Backup variable} TAR + +Full file name of the @GNUTAR{} executable. If this is not set, backup +scripts will search @command{tar} in the current shell path. +@end defvr + +@node Magnetic Tape Control +@subsection Magnetic Tape Control + +Backup scripts access tape device using special @dfn{hook functions}. +These functions take a single argument -- the name of the tape +device. Their names are kept in the following variables: + +@defvr {Backup variable} MT_BEGIN +The name of @dfn{begin} function. This function is called before +accessing the drive. By default it retensions the tape: + +@smallexample +MT_BEGIN=mt_begin + +mt_begin() @{ + mt -f "$1" retension +@} +@end smallexample +@end defvr + +@defvr {Backup variable} MT_REWIND +The name of @dfn{rewind} function. The default definition is as +follows: + +@smallexample +MT_REWIND=mt_rewind + +mt_rewind() @{ + mt -f "$1" rewind +@} +@end smallexample + +@end defvr + +@defvr {Backup variable} MT_OFFLINE +The name of the function switching the tape off line. By default +it is defined as follows: + +@smallexample +MT_OFFLINE=mt_offline + +mt_offline() @{ + mt -f "$1" offl +@} +@end smallexample +@end defvr + +@defvr {Backup variable} MT_STATUS +The name of the function used to obtain the status of the archive device, +including error count. Default definition: + +@smallexample +MT_STATUS=mt_status + +mt_status() @{ + mt -f "$1" status +@} +@end smallexample +@end defvr + +@node User Hooks +@subsection User Hooks + +@dfn{User hooks} are shell functions executed before and after +each @command{tar} invocation. Thus, there are @dfn{backup +hooks}, which are executed before and after dumping each file +system, and @dfn{restore hooks}, executed before and +after restoring a file system. Each user hook is a shell function +taking four arguments: + +@deffn {User Hook Function} hook @var{level} @var{host} @var{fs} @var{fsname} +Its arguments are: + +@table @var +@item level +Current backup or restore level. + +@item host +Name or IP address of the host machine being dumped or restored. + +@item fs +Full path name to the filesystem being dumped or restored. + +@item fsname +Filesystem name with directory separators replaced with colons. This +is useful, e.g., for creating unique files. +@end table +@end deffn + +Following variables keep the names of user hook functions + +@defvr {Backup variable} DUMP_BEGIN +Dump begin function. It is executed before dumping the filesystem. +@end defvr + +@defvr {Backup variable} DUMP_END +Executed after dumping the filesystem. +@end defvr + +@defvr {Backup variable} RESTORE_BEGIN +Executed before restoring the filesystem. +@end defvr + +@defvr {Backup variable} RESTORE_END +Executed after restoring the filesystem. +@end defvr + +@node backup-specs example +@subsection An Example Text of @file{Backup-specs} + +The following is an example of @file{backup-specs}: + +@smallexample +# site-specific parameters for file system backup. + +ADMINISTRATOR=friedman +BACKUP_HOUR=1 +TAPE_FILE=/dev/nrsmt0 + +# Use @code{ssh} instead of the less secure @code{rsh} +RSH=/usr/bin/ssh +RSH_COMMAND=/usr/bin/ssh + +# Override MT_STATUS function: +my_status() @{ + mts -t $TAPE_FILE +@} +MT_STATUS=my_status + +# Disable MT_OFFLINE function +MT_OFFLINE=: + +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 smallexample + +@node Scripted Backups +@section Using the Backup Scripts + +The syntax for running a backup script is: + +@smallexample +backup --level=@var{level} --time=@var{time} +@end smallexample + +The @option{level} option requests the dump level. Thus, to produce +a full dump, specify @code{--level=0} (this is the default, so +@option{--level} may be omitted if its value is @code{0}). +@footnote{For backward compatibility, the @code{backup} will also +try to deduce the requested dump level from the name of the +script itself. If the name consists of a string @samp{level-} +followed by a single decimal digit, that digit is taken as +the dump level number. Thus, you may create a link from @code{backup} +to @code{level-1} and then run @code{level-1} whenever you need to +create a level one dump.} + +The @option{--time} option determines when should the backup be +run. @var{Time} may take three forms: + +@table @asis +@item @var{hh}:@var{mm} + +The dump must be run at @var{hh} hours @var{mm} minutes. + +@item @var{hh} + +The dump must be run at @var{hh} hours + +@item now + +The dump must be run immediately. +@end table + +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 (@pxref{Scripted +Restoration}). + +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{mm-dd-yyyy}-level-@var{n}}, where @var{mm-dd-yyyy} +represents current date, and @var{n} represents current dump level number. + +The script also prints the name of each system being dumped to the +standard output. + +Following is the full list of options accepted by @code{backup} +script: + +@table @option +@item -l @var{level} +@itemx --level=@var{level} +Do backup level @var{level} (default 0). + +@item -f +@itemx --force +Force backup even if today's log file already exists. + +@item -v[@var{level}] +@itemx --verbose[=@var{level}] +Set verbosity level. The higher the level is, the more debugging +information will be output during execution. Devault @var{level} +is 100, which means the highest debugging level. + +@item -t @var{start-time} +@itemx --time=@var{start-time} +Wait till @var{time}, then do backup. + +@item -h +@itemx --help +Display short help message and exit. + +@item -L +@itemx --license +Display program license and exit. + +@item -V +@itemx --version +Display program version and exit. +@end table + + +@node Scripted Restoration +@section Using the Restore Script + +To restore files that were archived using a scripted backup, use the +@code{restore} script. Its usage is quite straightforward. In the +simplest form, invoke @code{restore --all}, it will +then restore all the filesystems and files specified in +@file{backup-specs} (@pxref{General-Purpose Variables,BACKUP_DIRS}). + +You may select the filesystems (and/or files) to restore by +giving @code{restore} list of @dfn{patterns} in its command +line. For example, running + +@smallexample +restore 'albert:*' +@end smallexample + +@noindent +will restore all filesystems on the machine @samp{albert}. A more +complicated example: + +@smallexample +restore 'albert:*' '*:/var' +@end smallexample + +@noindent +This command will restore all filesystems on the machine @samp{albert} +as well as @file{/var} filesystem on all machines. + +By default @code{restore} will start restoring files from the lowest +available dump level (usually zero) and will continue through +all available dump levels. There may be situations where such a +thorough restore is not necessary. For example, you may wish to +restore only files from the recent level one backup. To do so, +use @option{--level} option, as shown in the example below: + +@smallexample +restore --level=1 +@end smallexample + +The full list of options accepted by @code{restore} follows: + +@table @option +@item -a +@itemx --all +Restore all filesystems and files specified in @file{backup-specs} + +@item -l @var{level} +@itemx --level=@var{level} +Start restoring from the given backup level, instead of the default 0. + +@item -v[@var{level}] +@itemx --verbose[=@var{level}] +Set verbosity level. The higher the level is, the more debugging +information will be output during execution. Devault @var{level} +is 100, which means the highest debugging level. + +@item -h +@itemx --help +Display short help message and exit. + +@item -L +@itemx --license +Display program license and exit. + +@item -V +@itemx --version +Display program version and exit. +@end table + +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.} + +@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}, for an explanation of how the script makes +that determination. + +@node Choosing +@chapter Choosing Files and Names for @command{tar} +@UNREVISED + +@FIXME{Melissa (still) Doesn't Really Like This ``Intro'' Paragraph!!!} + +Certain options to @command{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 +@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, @command{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 @command{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 +@command{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 @option +@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 @command{tar} command, + +@smallexample +$ @kbd{tar -cvf collection.tar blues folk jazz} +@end smallexample + +@noindent +@file{collection.tar} is the name of the archive. It must directly +follow the @option{-f} option, since whatever directly follows @option{-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 @command{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, @command{tar} uses the value of the +environment variable @env{TAPE} as the file name for the archive. If +that is not available, @command{tar} uses a default, compiled-in archive +name, usually that for tape unit zero (ie. @file{/dev/tu00}). +@command{tar} always needs an archive name. + +If you use @file{-} as an @var{archive-name}, @command{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, +@command{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".} + +@smallexample +$ @kbd{cd sourcedir; tar -cf - . | (cd targetdir; tar -xf -)} +@end smallexample + +@FIXME{help!} + +@cindex Standard input and output +@cindex tar to standard input and output +@anchor{remote-dev} +To specify an archive file on a device attached to a remote machine, +use the following: + +@smallexample +@kbd{--file=@var{hostname}:/@var{dev}/@var{file name}} +@end smallexample + +@noindent +@command{tar} will complete the remote connection, if possible, and +prompt you for a username and password. If you use +@option{--file=@@@var{hostname}:/@var{dev}/@var{file name}}, @command{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 @command{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 @command{rsh} behavior.) It is necessary for the +remote machine, in addition to permitting your @command{rsh} access, to +have the @file{rmt} program installed (This command is included in +the @GNUTAR{} distribution and by default is installed under +@file{@var{prefix}/libexec/rmt}, were @var{prefix} means your +installation prefix). 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}, @GNUTAR{} +tries to minimize input and output operations. The +Amanda backup system, when used with @GNUTAR{}, has +an initial sizing pass which uses this feature. + +@node Selecting Archive Members +@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 +@command{tar} operates on, when creating or adding to an archive, or which +archive members @command{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 a file name begins with dash (@samp{-}), preceede it with +@option{--add-file} option to preventit from being treated as an +option. + +If you specify a directory name as a file name argument, all the files +in that directory are operated on by @command{tar}. + +If you do not specify files when @command{tar} is invoked with +@value{op-create}, @command{tar} operates on all the non-directory files in +the working directory. If you specify either @value{op-list} or +@value{op-extract}, @command{tar} operates on all the archive members in the +archive. If you specify any operation other than one of these three, +@command{tar} does nothing. + +By default, @command{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 @command{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 +@section Reading Names from a File + +@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 @command{tar}. Give the name of the file +which contains the list of files to include as the argument to +@option{--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 @command{find} utility. + +@table @option +@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 @option{--files-from}, (i.e., +you specify either @code{--files-from=-} or @code{-T -}), then the file +names are read from standard input. + +Unless you are running @command{tar} with @option{--create}, you can not use +both @code{--files-from=-} and @code{--file=-} (@code{-f -}) in the same +command. + +Any number of @option{-T} options can be given in the command line. + +@FIXME{add bob's example, from his message on 2-10-97} + +The following example shows how to use @command{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 @option{-T} option to +@command{tar} to specify the files from that file, @file{small-files}, to +create the archive @file{little.tgz}. (The @option{-z} option to +@command{tar} compresses the archive with @command{gzip}; @pxref{gzip} for +more information.) + +@smallexample +$ @kbd{find . -size -400 -print > small-files} +$ @kbd{tar -c -v -z -T small-files -f little.tgz} +@end smallexample + +@noindent +In the file list given by @option{-T} option, any file name beginning +with @samp{-} character is considered a @command{tar} option and is +processed accordingly.@footnote{Versions of @GNUTAR{} up to 1.15.1 +recognized only @option{-C} option in file lists, and only if the +option and its argument occupied two consecutive lines.} For example, +the common use of this feature is to change to another directory by +specifying @option{-C} option: + +@smallexample +@group +$ @kbd{cat list} +-C/etc +passwd +hosts +-C/lib +libc.a +$ @kbd{tar -c -f foo.tar --files-from list} +@end group +@end smallexample + +@noindent +In this example, @command{tar} will first switch to @file{/etc} +directory and add files @file{passwd} and @file{hosts} to the +archive. Then it will change to @file{/lib} directory and will archive +the file @file{libc.a}. Thus, the resulting archive @file{foo.tar} will +contain: + +@smallexample +@group +$ @kbd{tar tf foo.tar} +passwd +hosts +libc.a +@end group +@end smallexample + +@noindent +Notice that the option parsing algorithm used with @option{-T} is +stricter than the one used by shell. Namely, when specifying option +arguments, you should observe the following rules: + +@itemize @bullet +@item +When using short (single-letter) option form, its argument must +immediately follow the option letter, without any intervening +whitespace. For example: @code{-Cdir}. + +@item +When using long option form, the option argument must be separated +from the option by a single equal sign. No whitespace is allowed on +any side of the equal sign. For example: @code{--directory=dir}. + +@item +For both short and long option forms, the option argument can be given +on the next line after the option name, e.g.: + +@smallexample +@group +--directory +dir +@end group +@end smallexample + +@noindent +and + +@smallexample +@group +-C +dir +@end group +@end smallexample +@end itemize + +@cindex @option{--add-file} +If you happen to have a file whose name starts with @samp{-}, +precede it with @option{--add-file} option to prevent it from +being recognized as an option. For example: @code{--add-file --my-file}. + +@menu +* nul:: +@end menu + +@node nul +@subsection @code{NUL} Terminated File Names + +@cindex File names, terminated by @code{NUL} +@cindex @code{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 @option{--files-from}. + +@table @option +@item --null +Only consider @code{NUL} terminated file names, instead of files that +terminate in a newline. +@end table + +The @value{op-null} option is just like the one in @acronym{GNU} +@command{xargs} and @command{cpio}, and is useful with the +@option{-print0} predicate of @acronym{GNU} @command{find}. In +@command{tar}, @value{op-null} also disables special handling for +file names that begin with dash. + +This example shows how to use @command{find} to generate a list of files +larger than 800K in length and put that list into a file called +@file{long-files}. The @option{-print0} option to @command{find} is just +like @option{-print}, except that it separates files with a @code{NUL} +rather than with a newline. You can then run @command{tar} with both the +@option{--null} and @option{-T} options to specify that @command{tar} get the +files from that file, @file{long-files}, to create the archive +@file{big.tgz}. The @option{--null} option to @command{tar} will cause +@command{tar} to recognize the @code{NUL} separator between files. + +@smallexample +$ @kbd{find . -size +800 -print0 > long-files} +$ @kbd{tar -c -v --null --files-from=long-files --file=big.tar} +@end smallexample + +@FIXME{say anything else here to conclude the section?} + +@node exclude +@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 @option +@item --exclude=@var{pattern} +Causes @command{tar} to ignore files that match the @var{pattern}. +@end table + +@findex exclude +The @value{op-exclude} option prevents any file or member whose name +matches the shell wildcard (@var{pattern}) from being operated on. +For example, to create an archive with all the contents of the directory +@file{src} except for files whose names end in @file{.o}, use the +command @samp{tar -cf src.tar --exclude='*.o' src}. + +You may give multiple @option{--exclude} options. + +@table @option +@item --exclude-from=@var{file} +@itemx -X @var{file} +Causes @command{tar} to ignore files that match the patterns listed in +@var{file}. +@end table + +@findex exclude-from +Use the @option{--exclude-from=@var{file-of-patterns}} option to read a +list of patterns, one per line, from @var{file}; @command{tar} will +ignore files matching those patterns. Thus if @command{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?} + +@table @option +@item --exclude-caches +Causes @command{tar} to ignore directories containing a cache directory tag. +@end table + +@findex exclude-caches +When creating an archive, +the @option{--exclude-caches} option +causes @command{tar} to exclude all directories +that contain a @dfn{cache directory tag}. +A cache directory tag is a short file +with the well-known name @file{CACHEDIR.TAG} +and having a standard header +specified in @url{http://www.brynosaurus.com/cachedir/spec.html}. +Various applications write cache directory tags +into directories they use to hold regenerable, non-precious data, +so that such data can be more easily excluded from backups. + +@menu +* controlling pattern-patching with exclude:: +* problems with exclude:: +@end menu + +@node controlling pattern-patching with exclude +@unnumberedsubsec Controlling Pattern-Matching with the @code{exclude} Options + +Normally, a pattern matches a name if an initial subsequence of the +name's components matches the pattern, where @samp{*}, @samp{?}, and +@samp{[...]} are the usual shell wildcards, @samp{\} escapes wildcards, +and wildcards can match @samp{/}. + +Other than optionally stripping leading @samp{/} from names +(@pxref{absolute}), patterns and names are used as-is. For +example, trailing @samp{/} is not trimmed from a user-specified name +before deciding whether to exclude it. + +However, this matching procedure can be altered by the options listed +below. These options accumulate. For example: + +@smallexample +--ignore-case --exclude='makefile' --no-ignore-case ---exclude='readme' +@end smallexample + +ignores case when excluding @samp{makefile}, but not when excluding +@samp{readme}. + +@table @option +@item --anchored +@itemx --no-anchored +If anchored, a pattern must match an initial subsequence +of the name's components. Otherwise, the pattern can match any +subsequence. Default is @option{--no-anchored}. + +@item --ignore-case +@itemx --no-ignore-case +When ignoring case, upper-case patterns match lower-case names and vice versa. +When not ignoring case (the default), matching is case-sensitive. + +@item --wildcards +@itemx --no-wildcards +When using wildcards (the default), @samp{*}, @samp{?}, and @samp{[...]} +are the usual shell wildcards, and @samp{\} escapes wildcards. +Otherwise, none of these characters are special, and patterns must match +names literally. + +@item --wildcards-match-slash +@itemx --no-wildcards-match-slash +When wildcards match slash (the default), a wildcard like @samp{*} in +the pattern can match a @samp{/} in the name. Otherwise, @samp{/} is +matched only by @samp{/}. + +@end table + +The @option{--recursion} and @option{--no-recursion} options +(@pxref{recurse}) also affect how exclude patterns are interpreted. If +recursion is in effect, a pattern excludes a name if it matches any of +the name's parent directories. + +@node problems with exclude +@unnumberedsubsec Problems with Using the @code{exclude} Options + +Some users find @samp{exclude} options confusing. Here are some common +pitfalls: + +@itemize @bullet +@item +The main operating mode of @command{tar} does not act on a path name +explicitly listed on the command line if one of its file name +components is excluded. In the example above, if +you create an archive and exclude files that end with @samp{*.o}, but +explicitly name the file @samp{dir.o/foo} after all the options have been +listed, @samp{dir.o/foo} will be excluded from 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 +@option{--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 @GNUTAR{} sees wildcard characters +like @samp{*}. If you do not do this, the shell might expand the +@samp{*} itself using files at hand, so @command{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: + +@smallexample +$ @kbd{tar -c -f @var{archive.tar} --exclude '*.o' @var{directory}} +@end smallexample + +@noindent +rather than: + +@smallexample +$ @kbd{tar -c -f @var{archive.tar} --exclude *.o @var{directory}} +@end smallexample + +@item +You must use use shell syntax, or globbing, rather than @code{regexp} +syntax, when using exclude options in @command{tar}. If you try to use +@code{regexp} syntax to describe files to be excluded, your command +might fail. + +@item +In earlier versions of @command{tar}, what is now the +@option{--exclude-from=@var{file-of-patterns}} option was called +@option{--exclude=@var{pattern}} instead. Now, +@option{--exclude=@var{pattern}} applies to patterns listed on the command +line and @option{--exclude-from=@var{file-of-patterns}} applies to +patterns listed in a file. + +@end itemize + +@node Wildcards +@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, @command{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 @command{tar} archives. This section has the +purpose of explaining wildcard syntax for @command{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. + +@node after +@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 @command{tar} to only work on files +whose modification or inode-changed times are newer than the @var{date} +given. If @var{date} starts with @samp{/} or @samp{.}, it is taken to +be a file name; the last-modified time of that file is used as the date. +If you use this option when creating or appending to an archive, +the archive will only include new files. If you use @option{--after-date} +when extracting an archive, @command{tar} will only extract files newer +than the @var{date} you specify. + +If you only want @command{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 @command{tar} can compare when +deciding whether or not to archive the files. + +@table @option +@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. + +If @var{date} starts with @samp{/} or @samp{.}, it is taken to be a file +name; the last-modified time of that file is used as the date. + +@item --newer-mtime=@var{date} +Acts like @value{op-after-date}, but only looks at modification times. +@end table + +These options limit @command{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 Dumps}. +@end quotation + +@noindent +@FIXME{which tells -- need to fill this in!} + +@node recurse +@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, @command{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 @command{tar} to act this way. + +The @value{op-no-recursion} option inhibits @command{tar}'s recursive descent +into specified directories. If you specify @option{--no-recursion}, you can +use the @command{find} utility for hunting through levels of directories to +construct a list of file names which you could then pass to @command{tar}. +@command{find} allows you to be more selective when choosing which files to +archive; see @ref{files} for more information on using @command{find} with +@command{tar}, or look. + +@table @option +@item --no-recursion +Prevents @command{tar} from recursively descending directories. + +@item --recursion +Requires @command{tar} to recursively descend directories. +This is the default. +@end table + +When you use @option{--no-recursion}, @GNUTAR{} grabs +directory entries themselves, but does not descend on them +recursively. Many people use @command{find} for locating files they +want to back up, and since @command{tar} @emph{usually} recursively +descends on directories, they have to use the @samp{@w{! -d}} option +to @command{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-files-from} option to archive the files +located via @command{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 @command{tar} to grab only the directory entries given to it, adding +no new files on its own. + +The @value{op-no-recursion} option also applies when extracting: it +causes @command{tar} to extract only the matched directory entries, not +the files under those directories. + +The @value{op-no-recursion} option also affects how exclude patterns +are interpreted (@pxref{controlling pattern-patching with exclude}). + +The @option{--no-recursion} and @option{--recursion} options apply to +later options and operands, and can be overridden by later occurrences +of @option{--no-recursion} and @option{--recursion}. For example: + +@smallexample +$ @kbd{tar -cf jams.tar --norecursion grape --recursion grape/concord} +@end smallexample + +@noindent +creates an archive with one entry for @file{grape}, and the recursive +contents of @file{grape/concord}, but no entries under @file{grape} +other than @file{grape/concord}. + +@node one +@section Crossing Filesystem Boundaries +@cindex File system boundaries, not crossing +@UNREVISED + +@command{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 @command{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; +@command{tar} will still archive files explicitly named on the command line +or through @value{op-files-from}, regardless of where they reside. + +@table @option +@item --one-file-system +@itemx -l +Prevents @command{tar} from crossing file system boundaries when +archiving. Use in conjunction with any write operation. +@end table + +The @option{--one-file-system} option causes @command{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 +@command{tar} will not archive that file. If the file is a directory +itself, @command{tar} will not archive anything beneath it; in other words, +@command{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 +@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 @option +@item --directory=@var{directory} +@itemx -C @var{directory} +Changes the working directory in the middle of a command line. +@end table + +For example, + +@smallexample +$ @kbd{tar -c -f jams.tar grape prune -C food cherry} +@end smallexample + +@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, + +@smallexample +$ @kbd{tar -c -f jams.tar grape prune -C food red/cherry} +@end smallexample + +@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 @option{--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}: + +@smallexample +$ @kbd{tar -c -f foo.tar -C /etc passwd hosts -C /lib libc.a} +@end smallexample + +@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 @option{--directory} options are interpreted consecutively. If +@option{--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 @command{tar}, due to a previous +@option{--directory} option. + +When using @option{--files-from} (@pxref{files}), you can put various +@command{tar} options (including @option{-C}) in the file list. Notice, +however, that in this case the option and its argument may not be +separated by whitespace. If you use short option, its argument must +either follow the option letter immediately, without any intervening +whitespace, or occupy the next line. Otherwise, if you use long +option, separate its argument by an equal sign. + +For instance, the file list for the above example will be: + +@smallexample +@group +-C +/etc +passwd +hosts +-C +/lib +libc.a +@end group +@end smallexample + +@noindent +To use it, you would invoke @command{tar} as follows: + +@smallexample +$ @kbd{tar -c -f foo.tar --files-from list} +@end smallexample + +Notice also that you can only use the short option variant in the file +list, i.e., always use @option{-C}, not @option{--directory}. + +The interpretation of @value{op-directory} is disabled by +@value{op-null} option. + +@node absolute +@subsection Absolute File Names +@UNREVISED + +@table @option +@item -P +@itemx --absolute-names +Do not strip leading slashes from file names, and permit file names +containing a @file{..} file name component. +@end table + +By default, @GNUTAR{} drops a leading @samp{/} on +input or output, and complains about file names containing a @file{..} +component. This option turns off this behavior. + +When @command{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}, @command{tar} will extract it as if the name were +really @file{etc/passwd}. + +File names containing @file{..} can cause problems when extracting, so +@command{tar} normally warns you about such files when creating an +archive, and rejects attempts to extracts such files. + +Other @command{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-@GNUTAR{} +program to use. Therefore, @GNUTAR{} also strips +leading slashes from member names when putting members into the +archive. For example, if you ask @command{tar} to add the file +@file{/bin/ls} to an archive, it will do so, but the member name will +be @file{bin/ls}.@footnote{A side effect of this is that when +@option{--create} is used with @option{--verbose} the resulting output +is not, generally speaking, the same as the one you'd get running +@kbd{tar --list} command. This may be important if you use some +scripts for comparing both outputs. @xref{listing member and file names}, +for the information on how to handle this case.} + +If you use the @value{op-absolute-names} option, @command{tar} will do +none of these transformations. + +To archive or extract files relative to the root directory, specify +the @value{op-absolute-names} option. + +Normally, @command{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}, @command{tar} stores file names +including all superior directory names, and preserves leading slashes. +If you only invoked @command{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 @option +@item --absolute-names +Preserves full file names (including superior directory names) when +archiving files. Preserves leading slash when extracting files. + +@end table + +@FIXME{this is still horrible; need to talk with dan on monday.} + +@command{tar} prints out a message about removing the @samp{/} from +file names. This message appears once per @GNUTAR{} +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 @command{tar} standard +error to the sink. For example, under @command{sh}: + +@smallexample +$ @kbd{tar -c -f archive.tar /home 2> /dev/null} +@end smallexample + +@noindent +Another solution, both nicer and simpler, would be to change to +the @file{/} directory first, and then avoid absolute notation. +For example: + +@smallexample +$ @kbd{(cd / && tar -c -f archive.tar home)} +$ @kbd{tar -c -f archive.tar -C / home} +@end smallexample + +@include getdate.texi + +@node Formats +@chapter Controlling the Archive Format + +Due to historical reasons, there are several formats of tar archives. +All of them are based on the same principles, but have some subtle +differences that often make them incompatible with each other. + +GNU tar is able to create and handle archives in a variety of formats. +The most frequently used formats are (in alphabetical order): + +@table @asis +@item gnu +Format used by @GNUTAR{} versions up to 1.13.25. This format derived +from an early @acronym{POSIX} standard, adding some improvements such as +sparse file handling and incremental archives. Unfortunately these +features were implemented in a way incompatible with other archive +formats. + +Archives in @samp{gnu} format are able to hold pathnames of unlimited +length. + +@item oldgnu +Format used by @GNUTAR{} of versions prior to 1.12. + +@item v7 +Archive format, compatible with the V7 implementation of tar. This +format imposes a number of limitations. The most important of them +are: + +@enumerate +@item The maximum length of a file name is limited to 99 characters. +@item The maximum length of a symbolic link is limited to 99 characters. +@item It is impossible to store special files (block and character +devices, fifos etc.) +@item Maximum value of user or group ID is limited to 2097151 (7777777 +octal) +@item V7 archives do not contain symbolic ownership information (user +and group name of the file owner). +@end enumerate + +This format has traditionally been used by Automake when producing +Makefiles. This practice will change in the future, in the meantime, +however this means that projects containing filenames more than 99 +characters long will not be able to use @GNUTAR{} @value{VERSION} and +Automake prior to 1.9. + +@item ustar +Archive format defined by @acronym{POSIX.1-1988} specification. It stores +symbolic ownership information. It is also able to store +special files. However, it imposes several restrictions as well: + +@enumerate +@item The maximum length of a file name is limited to 256 characters, +provided that the filename can be split at directory separator in +two parts, first of them being at most 155 bytes long. So, in most +cases the maximum file name length will be shorter than 256 +characters. +@item The maximum length of a symbolic link name is limited to +100 characters. +@item Maximum size of a file the archive is able to accomodate +is 8GB +@item Maximum value of UID/GID is 2097151. +@item Maximum number of bits in device major and minor numbers is 21. +@end enumerate + +@item star +Format used by J@"org Schilling @command{star} +implementation. @GNUTAR{} is able to read @samp{star} archives but +currently does not produce them. + +@item posix +Archive format defined by @acronym{POSIX.1-2001} specification. This is the +most flexible and feature-rich format. It does not impose any +restrictions on file sizes or filename lengths. This format is quite +recent, so not all tar implementations are able to handle it properly. +However, this format is designed in such a way that any tar +implementation able to read @samp{ustar} archives will be able to read +most @samp{posix} archives as well, with the only exception that any +additional information (such as long file names etc.) will in such +case be extracted as plain text files along with the files it refers to. + +This archive format will be the default format for future versions +of @GNUTAR{}. + +@end table + +The following table summarizes the limitations of each of these +formats: + +@multitable @columnfractions .10 .20 .20 .20 .20 +@headitem Format @tab UID @tab File Size @tab Path Name @tab Devn +@item gnu @tab 1.8e19 @tab Unlimited @tab Unlimited @tab 63 +@item oldgnu @tab 1.8e19 @tab Unlimited @tab Unlimited @tab 63 +@item v7 @tab 2097151 @tab 8GB @tab 99 @tab n/a +@item ustar @tab 2097151 @tab 8GB @tab 256 @tab 21 +@item posix @tab Unlimited @tab Unlimited @tab Unlimited @tab Unlimited +@end multitable + +The default format for @GNUTAR{} is defined at compilation +time. You may check it by running @command{tar --help}, and examining +the last lines of its output. Usually, @GNUTAR{} is configured +to create archives in @samp{gnu} format, however, future version will +switch to @samp{posix}. + +@menu +* Portability:: Making @command{tar} Archives More Portable +* Compression:: Using Less Space through Compression +* Attributes:: Handling File Attributes +* Standard:: The Standard Format +* Extensions:: @acronym{GNU} Extensions to the Archive Format +* cpio:: Comparison of @command{tar} and @command{cpio} +@end menu + +@node Portability +@section Making @command{tar} Archives More Portable + +Creating a @command{tar} archive on a particular system that is meant to be +useful later on many other machines and with other versions of @command{tar} +is more challenging than you might think. @command{tar} archive formats +have been evolving since the first versions of Unix. Many such formats +are around, and are not always compatible with each other. This section +discusses a few problems, and gives some advice about making @command{tar} +archives more portable. + +One golden rule is simplicity. For example, limit your @command{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 +* ustar:: Ustar Archives +* gnu:: GNU and old GNU format archives. +* posix:: @acronym{POSIX} archives +* Checksumming:: Checksumming Problems +* Large or Negative Values:: Large files, negative time stamps, etc. +@end menu + +@node Portable Names +@subsection Portable Names + +Use portable file and member names. A name is portable if it contains +only ASCII letters and digits, @samp{/}, @samp{.}, @samp{_}, and +@samp{-}; it cannot be empty, start with @samp{-} or @samp{//}, or +contain @samp{/-}. Avoid deep directory nesting. For portability to +old Unix hosts, limit your file name components to 14 characters or +less. + +If you intend to have your @command{tar} archives to be read under +MSDOS, you should not rely on case distinction for file names, and you +might use the @acronym{GNU} @command{doschk} program for helping you +further diagnosing illegal MSDOS names, which are even more limited +than System V's. + +@node dereference +@subsection Symbolic Links +@cindex File names, using symbolic links +@cindex Symbolic link as file name + +Normally, when @command{tar} archives a symbolic link, it writes a +block to the archive naming the target of the link. In that way, the +@command{tar} archive is a faithful record of the filesystem contents. +@value{op-dereference} is used with @value{op-create}, and causes +@command{tar} to archive the files symbolic links point to, instead of +the links themselves. When this option is used, when @command{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 @command{tar}, an extracted file +might be linked to a file name that no longer exists in the file +system. + +If a linked-to file is encountered again by @command{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 +@subsection Old V7 Archives +@cindex Format, old style +@cindex Old style format +@cindex Old style archives + +Certain old versions of @command{tar} cannot handle additional +information recorded by newer @command{tar} programs. To create an +archive in V7 format (not ANSI), which can be read by these old +versions, specify the @value{op-format-v7} option in +conjunction with the @value{op-create} (@command{tar} also +accepts @option{--portability} or @samp{op-old-archive} for this +option). When you specify it, +@command{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-format-v7} +unless the archive was created using this option. + +In most cases, a @emph{new} format archive can be read by an @emph{old} +@command{tar} program without serious trouble, so this option should +seldom be needed. On the other hand, most modern @command{tar}s are +able to read old format archives, so it might be safer for you to +always use @value{op-format-v7} for your distributions. + +@node ustar +@subsection Ustar Archive Format + +Archive format defined by @acronym{POSIX}.1-1988 specification is called +@code{ustar}. Although it is more flexible than the V7 format, it +still has many restrictions (@xref{Formats,ustar}, for the detailed +description of @code{ustar} format). Along with V7 format, +@code{ustar} format is a good choice for archives intended to be read +with other implementations of @command{tar}. + +To create archive in @code{ustar} format, use @value{op-format-ustar} +option in conjunction with the @value{op-create}. + +@node gnu +@subsection @acronym{GNU} and old @GNUTAR{} format + +@GNUTAR{} was based on an early draft of the +@acronym{POSIX} 1003.1 @code{ustar} standard. @acronym{GNU} extensions to +@command{tar}, such as the support for file names longer than 100 +characters, use portions of the @command{tar} header record which were +specified in that @acronym{POSIX} draft as unused. Subsequent changes in +@acronym{POSIX} have allocated the same parts of the header record for +other purposes. As a result, @GNUTAR{} format is +incompatible with the current @acronym{POSIX} specification, and with +@command{tar} programs that follow it. + +In the majority of cases, @command{tar} will be configured to create +this format by default. This will change in the future releases, since +we plan to make @samp{posix} format the default. + +To force creation a @GNUTAR{} archive, use option +@value{op-format-gnu}. + +Some @command{tar} options are currently basing on @GNUTAR{} +format, and can therefore be used only with @samp{gnu} +or @samp{oldgnu} archive formats. The list of such options follows: + +@itemize @bullet +@item @value{op-label}, when used with @value{op-create}. +@item @value{op-incremental} +@item @value{op-multi-volume} +@end itemize + +These options will be re-implemented for the @samp{posix} archive +format in the future. + +@node posix +@subsection @GNUTAR{} and @acronym{POSIX} @command{tar} + +The version @value{VERSION} of @GNUTAR{} is able +to read and create archives conforming to @acronym{POSIX.1-2001} standard. + +A @acronym{POSIX} conformant archive will be created if @command{tar} +was given @value{op-format-posix} option. +Notice, that currently @acronym{GNU} extensions are not +allowed with this format. Following is the list of options that +cannot be used with @value{op-format-posix}: + +@itemize @bullet +@item @value{op-label}, when used with @value{op-create}. +@item @value{op-incremental} +@item @value{op-multi-volume} +@end itemize + +This restriction will disappear in the future versions. + +@node Checksumming +@subsection Checksumming Problems + +SunOS and HP-UX @command{tar} fail to accept archives created using +@GNUTAR{} and containing non-ASCII file names, that +is, file names having characters with the eight bit set, because they +use signed checksums, while @GNUTAR{} uses unsigned +checksums while creating archives, as per @acronym{POSIX} standards. On +reading, @GNUTAR{} 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. + +@GNUTAR{} compute checksums both ways, and accept +any on read, so @acronym{GNU} tar can read Sun tapes even with their +wrong checksums. @GNUTAR{} produces the standard +checksum, however, raising incompatibilities with Sun. That is to +say, @GNUTAR{} has not been modified to +@emph{produce} incorrect archives to be read by buggy @command{tar}'s. +I've been told that more recent Sun @command{tar} now read standard +archives, so maybe Sun did a similar patch, after all? + +The story seems to be that when Sun first imported @command{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 @command{tar} archives to be compatible with Sun's. +The current standards do not favor Sun @command{tar} format. In any +case, it now falls on the shoulders of SunOS and HP-UX users to get +a @command{tar} able to read the good archives they receive. + +@node Large or Negative Values +@subsection Large or Negative Values +@cindex large values +@cindex future time stamps +@cindex negative time stamps + +@acronym{POSIX} @command{tar} format uses fixed-sized unsigned octal strings +to represent numeric values. User and group IDs and device major and +minor numbers have unsigned 21-bit representations, and file sizes and +times have unsigned 33-bit representations. @GNUTAR{} +generates @acronym{POSIX} representations when possible, but for values +outside the @acronym{POSIX} range it generates two's-complement base-256 +strings: uids, gids, and device numbers have signed 57-bit +representations, and file sizes and times have signed 89-bit +representations. These representations are an extension to @acronym{POSIX} +@command{tar} format, so they are not universally portable. + +The most common portability problems with out-of-range numeric values +are large files and future or negative time stamps. + +Portable archives should avoid members of 8 GB or larger, as @acronym{POSIX} +@command{tar} format cannot represent them. + +Portable archives should avoid time stamps from the future. @acronym{POSIX} +@command{tar} format can represent time stamps in the range 1970-01-01 +00:00:00 through 2242-03-16 12:56:31 @sc{utc}. However, many current +hosts use a signed 32-bit @code{time_t}, or internal time stamp format, +and cannot represent time stamps after 2038-01-19 03:14:07 @sc{utc}; so +portable archives must avoid these time stamps for many years to come. + +Portable archives should also avoid time stamps before 1970. These time +stamps are a common @acronym{POSIX} extension but their @code{time_t} +representations are negative. Many traditional @command{tar} +implementations generate a two's complement representation for negative +time stamps that assumes a signed 32-bit @code{time_t}; hence they +generate archives that are not portable to hosts with differing +@code{time_t} representations. @GNUTAR{} recognizes this +situation when it is run on host with a signed 32-bit @code{time_t}, but +it issues a warning, as these time stamps are nonstandard and unportable. + +@node Compression +@section Using Less Space through Compression + +@menu +* gzip:: Creating and Reading Compressed Archives +* sparse:: Archiving Sparse Files +@end menu + +@node gzip +@subsection Creating and Reading Compressed Archives +@cindex Compressed archives +@cindex Storing archives in compressed format + +@GNUTAR{} is able to create and read compressed archives. It supports +@command{gzip} and @command{bzip2} compression programs. For backward +compatibilty, it also supports @command{compress} command, although +we strongly recommend against using it, since there is a patent +covering the algorithm it uses and you could be sued for patent +infringement merely by running @command{compress}! Besides, it is less +effective than @command{gzip} and @command{bzip2}. + +Creating a compressed archive is simple: you just specify a +@dfn{compression option} along with the usual archive creation +commands. The compression option is @option{-z} (@option{--gzip}) to +create a @command{gzip} compressed archive, @option{-j} +(@option{--bzip2}) to create a @command{bzip2} compressed archive, and +@option{-Z} (@option{--compress}) to use @command{compress} program. +For example: + +@smallexample +$ @kbd{tar cfz archive.tar.gz .} +@end smallexample + +Reading compressed archive is even simpler: you don't need to specify +any additional options as @GNUTAR{} recognizes its format +automatically. Thus, the following commands will list and extract the +archive created in previous example: + +@smallexample +# List the compressed archive +$ @kbd{tar tf archive.tar.gz} +# Extract the compressed archive +$ @kbd{tar xf archive.tar.gz} +@end smallexample + +The only case when you have to specify a decompression option while +reading the archive is when reading from a pipe or from a tape drive +that does not support random access. However, in this case @GNUTAR{} +will indicate which option you should use. For example: + +@smallexample +$ @kbd{cat archive.tar.gz | tar tf -} +tar: Archive is compressed. Use -z option +tar: Error is not recoverable: exiting now +@end smallexample + +If you see such diagnostics, just add the suggested option to the +invocation of @GNUTAR{}: + +@smallexample +$ @kbd{cat archive.tar.gz | tar tfz -} +@end smallexample + +Notice also, that there are several restrictions on operations on +compressed archives. First of all, compressed archives cannot be +modified, i.e., you cannot update (@value{op-update}) them or delete +(@value{op-delete}) members from them. Likewise, you cannot append +another @command{tar} archive to a compressed archive using +@value{op-append}). Secondly, multi-volume archives cannot be +compressed. + +The following table summarizes compression options used by @GNUTAR{}. + +@table @option +@item -z +@itemx --gzip +@itemx --ungzip +Filter the archive through @command{gzip}. + +You can use @option{--gzip} and @option{--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 @command{tar} program to enforce the specified (or default) record +size. The default compression parameters are used; if you need to +override them, set @env{GZIP} environment variable, e.g.: + +@smallexample +$ @kbd{GZIP=--best tar cfz archive.tar.gz subdir} +@end smallexample + +@noindent +Another way would be to avoid the @value{op-gzip} option and run +@command{gzip} explicitly: + +@smallexample +$ @kbd{tar cf - subdir | gzip --best -c - > archive.tar.gz} +@end smallexample + +@cindex corrupted archives +About corrupted compressed archives: @command{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 unsynchronized, 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 @GNUTAR{}. 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 +lose some compressibility. But this would have make recovering easier. +So, there are pros and cons. We'll see! + +@item -j +@itemx --bzip2 +Filter the archive through @code{bzip2}. Otherwise like @value{op-gzip}. + +@item -Z +@itemx --compress +@itemx --uncompress +Filter the archive through @command{compress}. Otherwise like +@value{op-gzip}. + +The @acronym{GNU} Project recommends you not use +@command{compress}, because there is a patent covering the algorithm it +uses. You could be sued for patent infringement merely by running +@command{compress}. + +@item --use-compress-program=@var{prog} +Use external compression program @var{prog}. Use this option if you +have a compression program that @GNUTAR{} does not support. There +are two requirements to which @var{prog} should comply: + +First, when called without options, it should read data from standard +input, compress it and output it on standard output. + +Secondly, if called with @option{-d} argument, it should do exactly +the opposite, i.e., read the compressed data from the standard input +and produce uncompressed data on the standard output. +@end table + +@FIXME{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 @acronym{GNU} +@command{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 @command{tar} through @command{ecc}, but then I lose (though I +haven't started using it yet, I confess) the ability to have +@command{tar} use @command{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 @command{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 (apparently) 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 @option{-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 +@subsection Archiving Sparse Files +@cindex Sparse Files +@UNREVISED + +@table @option +@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 +@acronym{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, @command{tar} +could create an archive longer than the original. To have @command{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, @command{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 +holes created wherever the continuous stretches of zeros were found. +Thus, if you use @value{op-sparse}, @command{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, @command{tar} tests all files for sparseness while archiving. +If @command{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 + +@command{tar} ignores the @value{op-sparse} option when reading an archive. + +@table @option +@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, +@GNUTAR{} 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 Sparcstation +ELC, with full CPU utilization.) + +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 @command{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 @command{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. +@GNUTAR{} 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 +@section Handling File Attributes +@UNREVISED + +When @command{tar} reads files, this causes them to have the access +times updated. To have @command{tar} attempt to set the access times +back to what they were before they were read, use the +@value{op-atime-preserve} option. + +Handling of file attributes + +@table @option +@item --atime-preserve +Preserve access times on files that are read. +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}), and it can set access or +modification times incorrectly if other programs access the file while +@command{tar} is running; but it is good enough for some purposes. + +@item -m +@itemx --touch +Do not extract file modified time. + +When this option is used, @command{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. + +This is the default behavior for the superuser, +so this option is meaningful only for non-root users, when @command{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, @command{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 --no-same-owner +@itemx -o +Do not attempt to restore ownership when extracting. This is the +default behavior for ordinary users, so this option has an effect +only for the superuser. + +@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 @command{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 @command{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 +@command{tar} archive out of this directory, before cleaning +everything out. Of course, we could add a lot of options to +@GNUTAR{} for fine tuning permissions and ownership. +This is not the good way, I think. @GNUTAR{} 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 @command{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 by default enabled when +@command{tar} is executed by a superuser. + + +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 +@section Basic Tar 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 @command{tar} utility or Tar mode in +@acronym{GNU} Emacs. + +Physically, an archive consists of a series of file entries terminated +by an end-of-archive entry, which consists of two 512 blocks of 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 @command{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}. +@FIXME-xref{To learn more about having more than one archive member with the +same name, see -backup node, when it's written.} + +In addition to entries describing archive members, an archive may +contain entries which @command{tar} itself uses to store information. +@value{xref-label}, for an example of such an archive entry. + +A @command{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 are two 512-byte blocks +filled with binary zeros as an end-of-file marker. A reasonable system +should write such end-of-file marker at the end of an archive, but +must not assume that such a block exists when reading an archive. In +particular @GNUTAR{} always issues a warning if it does not encounter it. + +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 @command{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 @GNUTAR{} +distribution, this is part of file @file{src/tar.h}: + +@smallexample +@include header.texi +@end smallexample + +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 @command{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 fields +are zero-filled octal numbers in ASCII. Each numeric field of width +@var{w} contains @var{w} minus 1 digits, and a 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, @command{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 @command{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 @command{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 +allotted 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 @command{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, @command{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 --same-order -Help process large lists of file-names on machines with small amounts of -memory. @xref{Archive Reading Options}. +@item @code{A} @dots{} @code{Z} +These are reserved for custom implementations. Some of these are +used in the @acronym{GNU} modified format, as described below. -@item --same-permission -Set the modes of extracted files to those recorded in the archive. -@xref{File Writing Options}. +@end table -@item --sparse -Archive sparse files sparsely. @xref{Sparse Files}. +Other values are reserved for specification in future revisions of +the P1003 standard, and should not be used by any @command{tar} program. -@item --starting-file=@var{file-name} -Begin reading in the middle of an archive. @xref{Scarce Disk Space}. +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. -@item --to-stdout -Write files to the standard output. @xref{File Writing Options}. +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}. -@item --uncompress -Specifdo a compressed archive. @xref{Compressed Archives}. +@node Extensions +@section @acronym{GNU} Extensions to the Archive Format +@UNREVISED -@item -V @var{archive-label} -Include an archive-label in the archive being created. @xref{Archive -Label}. -@c was --volume +The @acronym{GNU} format uses additional file types to describe new types of +files in an archive. These are listed below. -@item --verbose -Print the names of files or archive members as they are being -operated on. @xref{Additional Information}. +@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 --verify -Check for discrepancies in the archive immediately after it is -written. @xref{Write Verification}. +@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. -@item -B -Read an archive with a smaller than specified block size or which -contains incomplete blocks. @xref{Archive Reading Options}). +@end table -@item -K @var{file-name} -Begin reading in the middle of an archive. @xref{Scarce Disk Space}. +You may have trouble reading a @acronym{GNU} format archive on a +non-@acronym{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 @command{tar} does not +use the @acronym{GNU}-added fields of the header, other versions of +@command{tar} should be able to read the archive. Otherwise, the +@command{tar} program will give an error, the most likely one being a +checksum error. + +@node cpio +@section Comparison of @command{tar} and @command{cpio} +@UNREVISED + +@FIXME{Reorganize the following material} + +The @command{cpio} archive formats, like @command{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. @acronym{GNU} @command{cpio} can read and write archives +with arbitrary pathname lengths, but other @command{cpio} implementations +may crash unexplainedly trying to read them. + +@command{tar} handles symbolic links in the form in which it comes in BSD; +@command{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 @command{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 @command{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 @command{tar}; basically, its @command{cpio} +can handle @command{tar} format input, and write it on output, and it +probably handles symbolic links. They may not have bothered doing +anything to enhance @command{tar} as a result.) + +@command{cpio} handles special files; traditional @command{tar} doesn't. + +@command{tar} comes with V7, System III, System V, and BSD source; +@command{cpio} comes only with System III, System V, and later BSD +(4.3-tahoe and later). + +@command{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); +@command{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 +@command{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. + +@command{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; @command{cpio}s +way puts one copy for every link, but you can retrieve it using any +of the names. -@item -M -Specify a multi-volume archive. @xref{Multi-Volume Archives}. +@quotation +What type of check sum (if any) is used, and how is this calculated. +@end quotation -@item -N @var{date} -Limit operation to files changed after the given date. @xref{File Exclusion}. +See the attached manual pages for @command{tar} and @command{cpio} format. +@command{tar} uses a checksum which is the sum of all the bytes in the +@command{tar} header for a file; @command{cpio} uses no checksum. -@item -O -Write files to the standard output. @xref{File Writing Options}. +@quotation +If anyone knows why @command{cpio} was made when @command{tar} was present +at the unix scene, +@end quotation -@c <<<<- P is absolute paths, add when resolved. -ringo>>> +It wasn't. @command{cpio} first showed up in PWB/UNIX 1.0; no +generally-available version of UNIX had @command{tar} at the time. I don't +know whether any version that was generally available @emph{within AT&T} +had @command{tar}, or, if so, whether the people within AT&T who did +@command{cpio} knew about it. -@item -R -Print the record number where a message is generated. -@xref{Additional Information}. +On restore, if there is a corruption on a tape @command{tar} will stop at +that point, while @command{cpio} will skip over it and try to restore the +rest of the files. -@item -S -Archive sparse files sparsely. @xref{Sparse Files}. +The main difference is just in the command syntax and header format. -@item -T @var{file} -Read file-name arguments from a file on the file system. -@xref{File Name Lists}. +@command{tar} is a little more tape-oriented in that everything is blocked +to start on a record boundary. -@item -W -Check for discrepancies in the archive immediately after it is -written. @xref{Write Verification}. +@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 -@item -Z -Specify a compressed archive. @xref{Compressed Archives}. +Theoretically it should be easier under @command{tar} since the blocking +lets you find a header with some variation of @samp{dd skip=@var{nn}}. +However, modern @command{cpio}'s and variations have an option to just +search for the next file header after an error with a reasonable chance +of resyncing. 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. -@item -b @var{number} -Specify the blocking factor of an archive. @xref{Blocking Factor}. +@quotation +If anyone knows why @command{cpio} was made when @command{tar} was present +at the unix scene, please tell me about this too. +@end quotation -@item -f @var{archive-name} -Name the archive. @xref{Archive Name}). +Probably because it is more media efficient (by not blocking everything +and using only the space needed for the headers where @command{tar} +always uses 512 bytes per file header) and it knows how to archive +special files. -@item -h -Treat a symbolic link as an alternate name for the file the link -points to. @xref{Symbolic Links}. +You might want to look at the freely available alternatives. The +major ones are @command{afio}, @GNUTAR{}, and +@command{pax}, each of which have their own extensions with some +backwards compatibility. -@item -i -Ignore end-of-archive entries. @xref{Archive Reading Options}. +Sparse files were @command{tar}red as sparse files (which you can +easily test, because the resulting archive gets smaller, and +@acronym{GNU} @command{cpio} can no longer read it). -@item -k -Prevent overwriting during extraction. @xref{File Writing Options}. +@node Media +@chapter Tapes and Other Archive Media +@UNREVISED -@item -l -Prevent @code{tar} from crossing file system boundaries when -archiving. @xref{File Exclusion}. +A few special cases about tape handling warrant more detailed +description. These special cases are discussed below. -@item -m -Set the modification time of extracted files to the time they were -extracted. @xref{File Writing Options}. +Many complexities surround the use of @command{tar} on tape drives. Since +the creation and manipulation of archives located on magnetic tape was +the original purpose of @command{tar}, it contains many features making +such manipulation easier. -@item -o -Create an old format archive. @xref{Old Style File Information}. +Archives are usually written on dismountable media---tape cartridges, +mag tapes, or floppy disks. -@item -p -Set the modes of extracted files to those recorded in the archive. -@xref{File Writing Options}. +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 formatted at 1600 bits per inch. The +physically smaller EXABYTE tape cartridge holds 2.3 gigabytes. -@item -s -Help process large lists of file-names on machines with small amounts of -memory. @xref{Archive Reading Options}. +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 discarded when they begin to produce data errors. EXABYTE +tape cartridges should be discarded when they generate an @dfn{error +count} (number of non-usable bits) of more than 10k. -@item -v -Print the names of files or archive members they are being operated -on. @xref{Additional Information}. +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. -@item -w -@c <<>> +@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 -@item -z -Specify a compressed archive. @xref{Compressed Archives}. +@node Device +@section Device Selection and Switching +@UNREVISED + +@table @option +@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 @command{tar} +works on. + +If the file name is @samp{-}, @command{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, @command{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 (@samp{@@}), it is treated as @samp{user@@hostname:file name}. In +either case, @command{tar} will invoke the command @command{rsh} (or +@command{remsh}) to start up an @command{/usr/libexec/rmt} on the remote +machine. If you give an alternate login name, it will be given to the +@command{rsh}. +Naturally, the remote machine must have an executable +@command{/usr/libexec/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 @command{tar}; it's compiled and installed by default. +The exact path to this utility is determined when configuring the package. +It is @file{@var{prefix}/libexec/rmt}, where @var{prefix} stands for +your installation prefix. This location may also be overridden at +runtime by using @value{op-rmt-command} option (@xref{Option Summary, +---rmt-command}, for detailed description of this option. @xref{Remote +Tape Server}, for the description of @command{rmt} command). + +If this option is not given, but the environment variable @env{TAPE} +is set, its value is used; otherwise, old versions of @command{tar} +used a default archive name (which was picked when @command{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, @GNUTAR{} 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 +preferable. Further, I think @emph{most} actual usages of +@command{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. + +@GNUTAR{} 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 @option +@item --force-local +Archive file is local even if it contains a colon. + +@item --rsh-command=@var{command} +Use remote @var{command} instead of @command{rsh}. This option exists +so that people who use something other than the standard @command{rsh} +(e.g., a Kerberized @command{rsh}) can access a remote device. + +When this command is not used, the shell command found when +the @command{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 overridden this by defining the environment +variable @env{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 @command{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. If @file{file} exits with +nonzero status, exit. This implies @value{op-multi-volume}. +@end table + +@node Remote Tape Server +@section The Remote Tape Server + +@cindex remote tape drive +@pindex rmt +In order to access the tape drive on a remote machine, @command{tar} +uses the remote tape server written at the University of California at +Berkeley. The remote tape server must be installed as +@file{@var{prefix}/libexec/rmt} on any machine whose tape drive you +want to use. @command{tar} calls @command{rmt} by running an +@command{rsh} or @command{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. It is compiled and +installed by default. + +@cindex absolute file names +Unless you use the @value{op-absolute-names} option, @GNUTAR{} +will not allow you to create an archive that contains +absolute file names (a file name beginning with @samp{/}.) If you try, +@command{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 +@command{tar} program, @GNUTAR{} 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 @command{tar} tape to an operator to restore; +the operator used Sun @command{tar} instead of @GNUTAR{}, +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}, +@GNUTAR{} 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 known 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, @command{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 @command{tar} will work on them +once @command{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 @command{tar}. In particular, restoring a file that was split over +a volume boundary will require some careful work with @command{dd}, if +it can be done at all. Other versions of @command{tar} may also create +an empty file whose name is that of the volume header. Some versions +of @command{tar} may create normal files instead of directories archived +with the @value{op-incremental} option. + +@node Common Problems and Solutions +@section Some Common Problems and their Solutions + +@ifclear PUBLISH -@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. +@format +errors from system: +permission denied +no such file or directory +not owner + +errors from @command{tar}: +directory checksum error +header format error + +errors from media/system: +i/o error +device busy +@end format + +@end ifclear + +@node Blocking +@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 @command{tar} from which +@GNUTAR{} 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 @command{tar} was totally confused about this. +When I wrote @code{PD TAR}, I used the historically correct terminology +(@command{tar} writes data records, which are grouped into blocks). +It appears that the bogus terminology made it into @acronym{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 @command{tar} ignore these differences in its own +format, which is meant to be portable, so a @command{tar} block is always +512 bytes in length, and @dfn{block} always mean a @command{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 @GNUTAR{}. + +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 @command{tar} archive is kept on magnetic tape. +It is true that archives may be put on disk or used with pipes, +but nevertheless, @command{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 @GNUTAR{}. + +When writing to tapes, @command{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 @command{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, @command{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, @command{tar} will +print a message about a non-standard blocking factor, and then operate +normally. On some tape devices, however, @command{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 @command{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 @command{tar} cannot +figure it out. In any case, use @value{op-list} before doing any +extractions to see whether @command{tar} is reading the archive +correctly. + +@command{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. @command{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 @command{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 @command{tar} +programs might not be able to read the archive, so we recommend this +as a limit to use in practice. @GNUTAR{}, 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 +@subsection Format Variations +@cindex Format Parameters +@cindex Format Options +@cindex Options, archive format specifying +@cindex Options, format specifying +@UNREVISED -@item -C @file{directory} -Change the working directory. @xref{Changing Working Directory}. +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. + +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, @command{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 +@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 (i.e., +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 useful when archiving small files, to avoid archiving lots +of nulls as @command{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 @command{tar}, or by some newer versions +of @command{tar} running on old machines with small address spaces. +With @GNUTAR{}, 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: + +@smallexample +Cannot write to /dev/dlt: Invalid argument +@end smallexample + +@noindent +In such cases, it sometimes happen that the @command{tar} bundled by +the system is aware of block size idiosyncrasies, while @GNUTAR{} +requires an explicit specification for the block size, +which it cannot guess. This yields some people to consider +@GNUTAR{} is misbehaving, because by comparison, +@cite{the bundle @command{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---@command{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 @option +@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 + +Device blocking + +@table @option +@item -b @var{blocks} +@itemx --blocking-factor=@var{blocks} +Set record size to @math{@var{blocks} * 512} bytes. + +This option is used to specify a @dfn{blocking factor} for the archive. +When reading or writing the archive, @command{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, @command{tar} +pads the archive out to the next record boundary. + +The default blocking factor is set when @command{tar} is compiled, and is +typically 20. Blocking factors larger than 20 cannot be read by very +old versions of @command{tar}, or by some newer versions of @command{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 disappear. Id est, we are using block size of 112 right +now, and we haven't had the problem since we switched@dots{} + +With @GNUTAR{} 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 explicitly specified on the @command{tar} +invocation. +@end itemize -@item -F @var{program-file} -Create a multi-volume archive via a script. @xref{Multi-Volume Archives}. +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 +@command{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 +@command{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. +@command{tar} might ignore the exit status returned, but I hate doing +that, as it weakens the protection @command{tar} offers users against +other possible problems at decompression time. If @command{gzip} was +silently skipping trailing zeros @emph{and} also avoiding setting the +exit status in this innocuous case, that would solve this situation. + +@item +@command{tar} should become more solid at not stopping to read a pipe at +the first null block encountered. This inelegantly breaks the pipe. +@command{tar} should rather drain the pipe out before exiting itself. +@end itemize + +@item -i +@itemx --ignore-zeros +Ignore blocks of zeros in archive (means EOF). + +The @value{op-ignore-zeros} option causes @command{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 concatenating several archives together, this option +allows @command{tar} to read the entire archive. This option is not on +by default because many versions of @command{tar} write garbage after +the zeroed blocks. + +Note that this option causes @command{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, @command{tar} will not panic if an +attempt to read a record from the archive does not return a full record. +Instead, @command{tar} will keep reading until it has obtained a full +record. + +This option is turned on by default when @command{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 @command{tar} +requested. If this option was not used, @command{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 -X @file{file} -Exclude files which match any of the regular expressions listed in -the file @file{file}. @xref{File Exclusion}. @end table -@node Data Format Details, Concept Index, Quick Reference, Top -@appendix Details of the Archive Data Format +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. @command{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 accommodate 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 @option{--number-blocks} instead of +@option{--block-number}, so @option{--block} will then expand to +@option{--blocking-factor} unambiguously. + +@node Many +@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 @command{tar} +opens the archive file before using it and closes it afterwards, this +means that a simple: + +@smallexample +$ @kbd{tar cf /dev/tape @var{directory}} +@end smallexample + +@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 @command{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 +positioning 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: + +@smallexample +$ @kbd{mt -f /dev/nrtape rewind} +$ @kbd{tar cf /dev/nrtape @var{directory-1}} +@end smallexample + +@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 @command{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: + +@smallexample +$ @kbd{tar cf /dev/nrtape @var{directory-2}} +@end smallexample + +@noindent +and so on for all the archives you want to put on the same tape. -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. +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: -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. +@smallexample +$ @kbd{mt -f /dev/nrtape rewind} +$ @kbd{mt -f /dev/nrtape fsf 16} +$ @kbd{tar cf /dev/nrtape @var{directory-17}} +@end smallexample -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. +In all the previous examples, we put aside blocking considerations, but +you should do the proper things for that as well. @xref{Blocking}. @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 +* Tape Positioning:: Tape Positions and Tape Marks +* mt:: The @command{mt} Utility @end menu -@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. +@node Tape Positioning +@subsection Tape Positions and Tape Marks +@UNREVISED -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. +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. -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. +If you think of data as a series of records "rrrr"'s, and tape marks as +"*"'s, a tape might look like the following: -The @code{longnames} field supports a feature that is not yet -implemented. This field should be empty. +@smallexample +rrrr*rrrrrr*rrrrr*rr*rrrrr**------------------------- +@end smallexample -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. +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 @command{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-xref{Scripted Restoration}@xref{mt}, for +an explanation of the tape moving utility. + +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: + +@smallexample +rrrr*rrrrrr*rrrrr*rr*rrrrr*rrr*rrrr**---------------- +@end smallexample + +@node mt +@subsection The @command{mt} Utility +@UNREVISED + +@FIXME{Is it true that this only works on non-block devices? +should explain the difference, (fixed or variable).} +@value{xref-blocking-factor}. + +You can use the @command{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"?} + +The syntax of the @command{mt} command is: + +@smallexample +@kbd{mt [-f @var{tapename}] @var{operation} [@var{number}]} +@end smallexample + +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: + +@FIXME{is there any use for record operations?} + +@table @option +@item eof +@itemx weof +Writes @var{number} tape marks at the current position on the tape. + +@item fsf +Moves tape position forward @var{number} files. + +@item bsf +Moves tape position back @var{number} files. + +@item rewind +Rewinds the tape. (Ignores @var{number}). + +@item offline +@itemx rewoff1 +Rewinds the tape and takes the tape device off-line. (Ignores @var{number}). + +@item status +Prints status information about the tape unit. + +@end table + +@FIXME{Is there a better way to frob the spacing on the list?} -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. +If you don't specify a @var{tapename}, @command{mt} uses the environment +variable @env{TAPE}; if @env{TAPE} is not set, @command{mt} uses the device +@file{/dev/rmt12}. + +@command{mt} returns a 0 exit status when the operation(s) were +successful, 1 if the command was unrecognized, and 2 if an operation +failed. -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}. +@node Using Multiple Tapes +@section Using Multiple Tapes +@UNREVISED + +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 +@command{tar} commands, but this can be inconvenient, particularly if you +are using options like @value{op-exclude} or dumping entire filesystems. +Therefore, @command{tar} supports multiple tapes automatically. + +Use @value{op-multi-volume} on the command line, and then @command{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 @command{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 @command{tar} can restore both halves of the +file.) + +@GNUTAR{} multi-volume archives do not use a truly +portable format. You need @GNUTAR{} at both end to +process them properly. + +When prompting for a new tape, @command{tar} accepts any of the following +responses: + +@table @kbd +@item ? +Request @command{tar} to explain possible responses +@item q +Request @command{tar} to exit immediately. +@item n @var{file name} +Request @command{tar} to write the next volume on the file @var{file name}. +@item ! +Request @command{tar} to run a subshell. +@item y +Request @command{tar} to begin writing the next volume. +@end table + +(You should only type @samp{y} after you have changed the tape; +otherwise @command{tar} will write over the volume it just finished.) + +If you want more elaborate behavior than this, give @command{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. If the program fails, @command{tar} exits; +otherwise, @command{tar} begins 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}. + +The method @command{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 @command{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 @command{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 @command{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.) + +If you want @command{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 @command{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 @command{tar} prompt for a tape change (or run the info +script). + +Multi-volume archives + +With @value{op-multi-volume}, @command{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. + +Each volume of a multi-volume archive is an independent @command{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 @option{--extract +--multi-volume} (@option{-xM}) starting on or before the volume where +the file begins. + +For example, let's presume someone has two tape drives on a system +named @file{/dev/tape0} and @file{/dev/tape1}. For having @GNUTAR{} +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: + +@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 @menu -* File Types:: File Types -* GNU File Types:: Additional File Types Supported by GNU +* Multi-Volume Archives:: Archives Longer than One Tape or Disk +* Tape Files:: Tape Files @end menu -@node File Types, GNU File Types, Header Fields, Header Fields -@appendixsubsec File Types +@node Multi-Volume Archives +@subsection Archives Longer than One Tape or Disk +@cindex Multi-volume archives +@UNREVISED + +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. + +When you specify @value{op-multi-volume}, @command{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. -The following flags are used to describe file types: +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}. -@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. +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}---@command{tar} will prompt for later +volumes as it needs them. @xref{extracting archives}, for more +information about extracting archives. + +@value{op-info-script} is like @value{op-multi-volume}, except that +@command{tar} does not prompt you directly to change media volumes when +a volume is full---instead, @command{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, @command{tar} will assume that the media has been changed. + +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 @value{op-label} +(@value{pxref-label}) when it was created, @command{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 LF_FIFO -Indicates a FIFO special file. Note that archiving a FIFO file -archives the existence of the file and not its contents. +@cindex Labeling multi-volume archives +@FIXME{example} -@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. +@FIXME{There should be a sample program here, including an exit +before end. Is the exit status even checked in tar? :-(} -@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 @option +@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. + +@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 -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. +Beware that there is @emph{no} real standard about the proper way, for +a @command{tar} archive, to span volume boundaries. If you have a +multi-volume created by some vendor's @command{tar}, there is almost +no chance you could read all the volumes with @GNUTAR{}. +The converse is also true: you may not expect +multi-volume archives created by @GNUTAR{} to be +fully recovered by vendor's @command{tar}. Since there is little +chance that, in mixed system configurations, some vendor's +@command{tar} will work on another vendor's machine, and there is a +great chance that @GNUTAR{} will work on most of +them, your best bet is to install @GNUTAR{} on all +machines between which you know exchange of files is possible. + +@node Tape Files +@subsection Tape Files +@UNREVISED + +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 @command{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 @command{tar}. To do this, use the @command{mt} command. +For more information on the @command{mt} command and on the organization +of tapes into a sequence of tape files, see @ref{mt}. + +People seem to often do: + +@smallexample +@kbd{--label="@var{some-prefix} `date +@var{some-format}`"} +@end smallexample + +or such, for pushing a common date in all volumes or an archive set. + +@node label +@section Including a Label in the Archive +@cindex Labeling an archive +@cindex Labels on the archive media +@UNREVISED -@node GNU File Types, , File Types, Header Fields -@appendixsubsec Additional File Types Supported by GNU +@cindex @option{--label} option introduced +@cindex @option{-V} option introduced + 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. -GNU @code{tar} uses additional file types to describe new types of -files in an archive. These are listed below. +@table @option +@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} operation. Checks to make sure the archive label +matches the one specified (when used in conjunction with any other +operation. +@end table -@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. + 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.} + +@cindex Volume label, listing +@cindex Listing volume label + The volume label will be displayed by @option{--list} along with +the file contents. If verbose display is requested, it will also be +explicitely marked as in the example below: + +@smallexample +@group +$ @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 group +@end smallexample + +@cindex @option{--test-label} option introduced +@anchor{--test-label option} + However, @option{--list} option will cause listing entire +contents of the archive, which may be undesirable (for example, if the +archive is stored on a tape). You can request checking only the volume +by specifying @option{--test-label} option. This option reads only the +first block of an archive, so it can be used with slow storage +devices. For example: + +@smallexample +@group +$ @kbd{tar --test-label --file=iamanarchive} +iamalabel +@end group +@end smallexample + + If @option{--test-label} is used with a single command line +argument, @command{tar} compares the volume label with the +argument. It exits with code 0 if the two strings match, and with code +2 otherwise. In this case no output is displayed. For example: + +@smallexample +@group +$ @kbd{tar --test-label --file=iamanarchive 'iamalable'} +@result{} 0 +$ @kbd{tar --test-label --file=iamanarchive 'iamalable' alabel} +@result{} 1 +@end group +@end smallexample + + If you request any operation, other than @option{--create}, along +with using @option{--label} option, @command{tar} will first check if +the archive label matches the one specified and will refuse to proceed +if it does not. Use this as a safety precaution to avoid accidentally +overwriting existing archives. For example, if you wish to add files +to @file{archive}, presumably labelled with string @samp{My volume}, +you will get: + +@smallexample +@group +$ @kbd{tar -rf archive --label 'My volume' .} +tar: Archive not labeled to match `My volume' +@end group +@end smallexample -@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. +@noindent +in case its label does not match. This will work even if +@file{archive} is not labelled at all. + + Similarly, @command{tar} will refuse to list or extract the +archive if its label doesn't match the @var{archive-label} +specified. 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 @command{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 +@command{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 @option{--volume}, but is not available +under that name anymore. + + You can also use @option{--label} to get a common information on +all tapes of a series. 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: + +@smallexample +@group +$ @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 group +@end smallexample + + Also note that each label has its own date and time, which corresponds +to when @GNUTAR{} initially attempted to write it, +often soon after the operator launches @command{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 usually not the case. + +@node verify +@section Verifying Data as It is Stored +@cindex Verifying a write operation +@cindex Double-checking a write operation + +@table @option +@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 @command{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. + +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. @command{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. + +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, @command{tar} checks archive members against their counterparts +in the file system, and reports discrepancies on the standard error. + +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. + +One can explicitly 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, @command{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. + +The @value{op-verify} option will not work in conjunction with the +@value{op-multi-volume} option or the @value{op-append}, +@value{op-update} and @value{op-delete} operations. @xref{Operations}, +for more information on these operations. + +Also, since @command{tar} normally strips leading @samp{/} from file +names (@pxref{absolute}), a command like @samp{tar --verify -cf +/tmp/foo.tar /etc} will work as desired only if the working directory is +@file{/}, as @command{tar} uses the archive's relative member names +(e.g., @file{etc/motd}) when verifying the archive. + +@node Write Protection +@section Write Protection + +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 accidentally 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). + +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. -The following header information was added to deal with sparse files -(@pxref{Sparse Files}): +@node Free Software Needs Free Documentation +@appendix Free Software Needs Free Documentation +@include freemanuals.texi -@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. +@node Genfile +@appendix Genfile +@include genfile.texi -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. +@node Snapshot Files +@appendix Format of the Incremental Snapshot Files +@include snapshot.texi -The @code{isextended} flag is only set for sparse files, and then only -if extended header records are needed when archiving the file. +@node Copying This Manual +@appendix Copying This Manual -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. +@menu +* GNU Free Documentation License:: License for copying this manual +@end menu -@c so is @code{extended_header} the right way to write this? +@include fdl.texi -@node Concept Index, , Data Format Details, Top -@unnumbered Concept Index +@node Index +@appendix Index @printindex cp @@ -3677,4 +8916,6 @@ 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: