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[chaz/tar] / doc / tar.texi

\input texinfo    @c -*-texinfo-*-
@c %**start of header
@setfilename tar.info
@settitle The Tar Manual: DRAFT
@setchapternewpage odd
@c %**end of header

@c Note: the edition number and date is listed in *two* places; please update.
@c subtitle and top node; search for !!set

@c Search for comments marked with !! or <<<  (or >>>)

@c <<< CONVENTIONS: this manual refers to "ordinary files" , "directory
files" (or "directories"), "archive files", "archive members", and
various I/O devices (which have names and file names).>>>

@c <<< it's "file name" (not filename) unless we are talking about an
argument, ie. @var{file-name}.  also, you "use" a "file-name argument"
to "specify" a "file".>>>

@c <<<  @code{tar} is always lower case, in bold. >>>

@c <<< it's "operations of tar", "options to tar" also, it's " @samp{tar
--foo}" or "the @samp{--foo} operation".  MIB doesn't like using
operations and options as separate concepts. I disagree --- would be a
mess to explain otherwise

@c <<< (don't forget to comment these out in final draft)  -ringo

@c <<< please dont' change this without sending me e-mail.  some things
@c are in progress or waiting to be edited in hardcopy.  -ringo
@c smallbook

@iftex
@c finalout
@end iftex

@ifinfo
This file documents @code{tar}, a utility used to store, backup, and
transport files.

Copyright (C) 1992 Free Software Foundation, Inc.  DRAFT!
@c Need to put distribution information here when ready.
@end ifinfo

@c !!set    edition number and date here
@titlepage
@title @code{tar}
@subtitle The GNU Tape Archiver
@subtitle Edition 0.01, for @code{tar} Version 1.10
@subtitle @today{}   
@c remove preceding today line when ready
@sp 1
@subtitle DRAFT
@c subtitle   insert month here when ready

@author Amy Gorin, Michael I. Bushnell, and Jay Fenlason
@c <<<best to have hack read this over and see if anything is left he
@c wrote.  I don't think so.  -ringo>>>>

@page
@vskip 0pt plus 1filll
Copyright @copyright{} 1992 Free Software Foundation, Inc.

@sp 2
This draft is not yet ready for distribution.
@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.

@c !!set    edition number and date here
This is DRAFT Edition 0.01 of the @code{tar} documentation, @today{}, for @code{tar}
version 1.12.
@end ifinfo

@c <<< The menus need to be gone over, and node names fixed.
@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

@node Introduction, Invoking @code{tar}, Top, Top
@chapter @code{tar}: The GNU Tape Archiver

You can use @code{tar} to create an @dfn{archive}---a single file
which contains other files' contents as well as a listing of those
files' characteristics.  You can also use @code{tar} to read, add to,
or manipulate already existing archives.  Because an archive created
by @code{tar} is capable of preserving file information and directory
structure, @code{tar} is ideal for performing full and incremental
backups, as well as for transferring groups of files between disks and
over networks.

The name @code{tar} comes from the words ``Tape ARchiver'', but
@code{tar} can actually process archives wherever they are stored; on
tapes and disk files, for example.  In addition, tar can read archives
from standard input or write them to standard output.  (This is often
useful if redirected another program with a pipe.)

@c <<< this menu will conflict with menu above in info mode.  -ringo
@menu
* Invoking @code{tar}::         How to invoke @code{tar} and specify arguments.
* Tutorial::                    An introduction to @code{tar}.
* Operations::                  What you can use @code{tar} to do.
* Options::                     How to change the way @code{tar} behaves.
* Problems::                    Common problems with @code{tar}.
@end menu

@node Invoking @code{tar}, Tutorial, Introduction, Top
@chapter How To Invoke @code{tar}

The usual way to invoke tar is

@example
@code{tar} @var{options}... [@var{file-names}...]
@end example

All the options start with @samp{-}.  The arguments which do not start
with @samp{-} are taken to be file-name arguments.  (But
@xref{Argument Form}.)  You can actually type in arguments in any
order.  In this manual the options always precede the file-name
arguments, to make examples easier to understand.

@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

@node Option Form, Old Syntax for Commands, Argument Functions, Invoking @code{tar}
@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.  (This
is an exception to the rule that @code{tar} arguments which are not
options are file-name arguments.)  Mnemonic options are separated from
their arguments by an @samp{=} sign.  For example, to create an an
archive file named @file{george}, use either @samp{tar --create
--file=george} or @samp{tar --create -f george}.  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 @samp{george}.  

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!

@node Argument Functions, Argument Form, Invoking @code{tar}, Invoking @code{tar}
@section The Functions of Arguments

You must give exactly one option from the following list to tar.  This
option specifies the basic operation for tar to perform.

@itemize
@item
Add files to an existing archive (@samp{--add-file}, @samp{--append} or
@samp{-r})

@item
Compare files in an archive with files in the file system
(@samp{--compare}, @samp{--diff} or @samp{-d})

@item
Add archives to another archive (@samp{--add-archive}, @samp{--catenate}
or @samp{-A})
@c was --concatenate.   -ringo

@item
Create a new archive (@samp{--create} or @samp{-c})

@item
Delete files from an archive (@samp{--delete})

@item
Extract files from an archive (@samp{--extract}, @samp{--get} or @samp{-x})

@item
List the files in an archive (@samp{--list} or @samp{-t})

@item
Update an archive by appending newer versions of already stored files
(@samp{--update} or @samp{-u})
@end itemize

@xref{Reading and Writing}, for more information about these
operations. 

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 file-name arguments.  For --add-file and
--create these arguments specify the names of files (which must
already exist) to place in the archive.  For the remaining operation
types, the file-name 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.  When naming
files, the normal file name rules apply.

If you don't use any file-name arguments, @samp{--add-file},
@samp{--update} and @samp{--delete} will do nothing.  Naturally,
@samp{--create} will make an empty archive if given no file-name
arguments.  The other operations of @code{tar} will act on defaults.

Anytime you use a file-name argument to specify a directory file,
@code{tar} acts recursively on all the files and directories beneath
that directory.

@node Old Syntax for Commands,  , Argument Form, Invoking @code{tar}
@section An Old, but Still Supported, Syntax for @code{tar} Commands

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:

@example
@code{tar} @var{option-letters}... [@var{option-arguments}...] [@var{file-names}...]@refill
@end example

@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.

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}.

On the other hand, this old style syntax makes it difficult to match
option letters with their corresponding arguments, and is often
confusing.  In the command @samp{tar cvbf 20 /dev/rmt0}, for example,
@samp{20} is the argument for @samp{-b}, @samp{/dev/rmt0} is the
argument for @samp{-f}, and @samp{-v} does not have a corresponding
argument.  The modern syntax---@samp{tar -c -v -b 20 -f
/dev/rmt0}---is clearer.

@node Tutorial, Wizardry, Invoking @code{tar}, Top
@chapter Getting Started With @code{tar}

This chapter guides you through some basic examples of @code{tar}
operations.  If you already know how to use some other version of tar,
then you probably don't need to read this chapter.  In the examples,
the lines you should type are preceded by a @samp{%}, which is a
typical shell prompt.  

@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

@node Creating Archives, Listing Archive Contents, Tutorial, Tutorial
@section Creating Archives

To create a new archive, use @code{tar --create} (or @code{tar -c}).
You can use options to specify the name and format of the archive (as
well as other characteristics), and you can use file-name arguments to
specify which files to put in the archive.  If you don't use any
options or file-name arguments, @code{tar} will use default values.
@xref{Creating Example}, for more information about the
@samp{--create} operation.

@menu
* Creating Example::            Creating Archives of Files
* Subdirectory::                Creating an Archive of a Subdirectory
@end menu

@node Creating Example, Subdirectory, Creating Archives, Creating Archives
@subsection Creating Archives of Files

This example shows you how to create an archive file in the working
directory containing other files in the working directory.  The three
files you archive in this example are called @file{blues},
@file{folk}, and @file{jazz}.  The archive file is called
@file{records}.  While the archive in this example is written to the
file system, it could also be written to any other device.

(If you want to follow along with this and future examples, create a
directory called @file{practice} containing files called @file{blues},
@file{folk} and @file{jazz}.  To create the directory, type
@samp{mkdir practice} at the system prompt.  It will probably be
easiest to create the files using a text editor, such as Emacs.)

First, change into the directory containing the files you want to
archive:

@example
% cd practice
@end example

@noindent
@file{~/practice} is now your working directory.

Then, check that the files to be archived do in fact exist in the
working directory, and make sure there isn't already a file in the
working directory with the archive name you intend to use.  If you
specify an archive file name that is already in use, @code{tar} will
overwrite the old file and its contents will be lost.

To list the names of files in the working directory, type:

@example
% ls
@end example

The system responds:

@example
blues   folk    jazz
%
@end example

@noindent
Then,
@itemize @bullet
@item
Create a new archive (@samp{tar -c} or @samp{tar --create})

@item
Explicitly name the archive file being created (@samp{-f
@var{archive-name}} or @samp{--file=@var{archive-name}}).  If you don't
use this option @code{tar} will write the archive to the default
storage device, which varies from system to system. 
@c <<< this syntax may change.  OK now---check before printing  -ringo

@code{tar} interprets archive file names relative to the working
directory.  Make sure you have write access to the working directory
before using @code{tar}.  

@item
Specify which files to put into the archive (@code{tar} interprets
file names relative to the working directory).  If you don't use any
@var{file-name} arguments, @code{tar} will archive everything in the
working directory.
@end itemize

@noindent
Type:
@example
% tar --create --file=records blues folk jazz 
@end example

@noindent
If you now list the contents of the working directory (@samp{ls}), you
will find the archive file listed as well as the files you saw
previously.

@example
% ls
blues folk jazz records
%
@end example

@menu
* Listing Files::               Listing files in an archive
* Verbose::                     Using @code{tar} in Verbose Mode
@end menu

@node Listing Files, Verbose, Creating Example, Creating Example
@subsubsection Listing files in an archive

You can list the contents of an archive with another operation of
@code{tar}---@samp{--list} or @samp{-l}.  To list the contents of the
archive you just created, type:

@example
% tar --list --file=records
@end example

@noindent
@code{tar} will respond:

@example
blues folk jazz
@end example

@xref{Listing Archive Contents}, for a more detailed tutorial of the
@samp{--list} operation.  @xref{Listing Contents}, for more information
about the @samp{--list} operation.

@node Verbose,  , Listing Files, Creating Example
@subsubsection Using @code{tar} in Verbose Mode

If you include the @samp{--verbose} or @samp{-v} option on the command
line, @code{tar} will list the files it is acting on as it is working.
In verbose mode, the creation example above would appear as:
@cindex Verbose mode example
@findex -v (verbose mode example)

@example
% tar --create --file=records --verbose blues folk jazz
blues
folk
jazz
@end example

@noindent
The first line is the command typed in by the user.  The remaining
lines are generated by @code{tar}.  In the following examples we
usually use verbose mode, though it is almost never required.

@node Subdirectory, Changing, Creating Example, Creating Archives
@subsection Creating an Archive of a Subdirectory

You can store a directory in an archive by using the directory name as
a file-name argument to @code{tar}.  When you specify a directory
file, @code{tar} archives the directory file and all the files it
contains.  The names of the directory and the files it contains are
stored in the archive relative to the current working directory---when
the directory is extracted they will be written into the file system
relative to the working directory at that time.
@c <<< add an xref to --absolute-paths   -ringo

To archive a directory, first move to its superior directory.  If you
have been following the tutorial, you should type:

@example
% cd ..
%
@end example

Once in the superior directory, specify the subdirectory using a
file-name argument.  To store the directory file @file{~/practice} in
the archive file @file{music}, type:

@example
% tar --create --verbose --file=music practice
@end example

@noindent
@code{tar} should respond:

@example
practice/
practice/blues
practice/folk
practice/jazz
practice/records
@end example

Note that @file{~/practice/records}, another archive file, has
itself been archived.  @code{tar} will accept any file as a file to be
archived, even an archive file.

@c >>> symbolic links and changing directories are now in main body, not in
@c >>> tutorial. -ringo

@node Extracting Files
@section Extracting Files from an Archive

Creating an archive is only half the job---there would be no point in
storing files in an archive if you couldn't retrieve them.  To extract
files from an archive, use the @samp{--extract} or @samp{-x} operation.

To extract specific files, use their names as file-name arguments.  If
you use a directory name as a file-name argument, @code{tar} extracts
all the files (including subdirectories) in that directory.  If you
don't use any file-name arguments, @code{tar} extracts all the files
in the archive.

Note: @code{tar} will extract an archive member into the file system
without checking to see if there is already a file with the archive
member's file name.  If there is a file with that name, @code{tar}
will @strong{overwrite} that file and its contents will be lost.
@c <<<xref keep-old   -ringo

@node Extracting Specific Files, , ,
@subsection Extracting Specific Files

To extract specific files, specify them using file-name arguments.

In an example above, you created the archive file
@file{~/practice/records}, which contained the files @file{blues},
@file{folk} and @file{jazz} in the @file{practice} directory.  If, for
some reason, you were to lose one of those text files (@samp{rm
~/practice/blues}), you could extract it from the archive file.

First, change into the @file{practice} directory.  Then, 

@itemize @bullet
@item
Invoke @code{tar} and specify the @samp{--extract} operation
(@samp{--extract}, @samp{--get} or @samp{-x})

@item
Specify the archive that the files will be extracted from
(@samp{--file=@var{archive-name}} or @samp{-f @var{archive-name}})

@item
Specify the files to extract, using file-name arguments (if you don't
specify any files, @code{tar} extracts all the archive members)
@end itemize

@example
% tar --extract --file=records blues
@end example

If you list the contents of the directory, you will see that
@file{blues} is back:

@example
% ls
folk
jazz
records
blues
@end example

@node Extracting Directories, , ,
@subsection Extracting Directories

To extract a directory and all the files it contains, use the
directory's name as a file-name argument in conjunction with @samp{tar
--extract}.  Remember---@code{tar} stores and extracts file names
relative to the working directory.

In a previous example you stored the directory @file{~/practice} in
the archive file @file{~/music}.  If you delete the contents of
@file{practice}, you can restore them using @code{tar}.

First, change into the @file{practice} subdirectory (@samp{cd
~/practice}).  Then, remove all the files in @samp{~/practice}
(@samp{rm *}).  If you list the contents of the directory, you should
now see that it is empty:

@example
%ls
%
@end example

@noindent Let's try to restore the contents of @file{practice} by extracting
them from the archive file @file{~/music}:

@example
tar --extract --file=~/music practice
@end example

@noindent Now, list the contents of @file{practice} again:

@example
%ls 
practice
@end example

What happened to the files?  When you created @file{~/music}, your
working directory was your home directory.  When you extracted
@file{~/music}, your working directory was @file{~/practice}.
@code{tar} stored the files in @file{practice} relative to your home
directory, and then extracted them relative to @file{~/practice}.  The
files are now in a new subdirectory, called
@file{~/practice/practice}.

To restore your files to their old positions, delete the new directory
and its contents, and then redo the example above with your home
directory as the working directory:

@example
% rm ~/practice/practice/*
% rmdir practice
% cd ..
% tar --extract --file=music practice
@end example

@noindent (@code{tar} will report that it is unable to create the
directory @file{~/practice} because it already exists.  This will not
effect the extraction of the other archive members.)

@node Listing Archive Contents, Adding to Archives, Creating Archives, Tutorial
@section Listing the Contents of an Archive

Use @samp{--list} or @samp{-t} to print the names of files stored in an
archive.  If you use file-name arguments with this operation,
@code{tar} prints the names of the specified files if they are stored
in the archive.  If you use a directory name as a file-name argument,
@code{tar} also prints the names of all underlying files, including
sub-directories.  If you use no file-name arguments, @code{tar} prints
the names of all the archive members.

You can use @samp{--list} with the @samp{--verbose} option to print
archive members' attributes (owner, size, etc.).

@menu
* Listing names::               Listing the names of stored files
* Additional File Info::        Getting Additional File Information
* Specific File::               List A Specific File in an Archive
* Listing Directories::         Listing the Contents of a Stored Directory
@end menu

@node Listing names, Additional File Info, Listing Archive Contents, Listing Archive Contents
@subsection Listing the names of stored files

To list the names of files stored in an archive, use the @samp{--list}
operation of @code{tar}.  

In a previous example, you created the archive @file{~/music}.  To
list the contents of @file{music}, while in your home directory:

@itemize @bullet
@item
List the contents of an archive (@samp{tar -t} or @samp{tar --list})

@item
Specify the archive to be listed (@samp{-f @var{archive-name}} or
@samp{--file=@var{archive-name}}) @refill
@end itemize

Thus:

@example
% tar --list --file=music
practice/
practice/blues
practice/folk
practice/jazz
practice/records
@end example

@node Additional File Info, Specific File, Listing names, Listing Archive Contents
@subsection Listing Additional File Information

To get more information when you list the names of files stored in an
archive, specify the @samp{--verbose} option in conjunction with
@samp{tar --list}.  @code{tar} will print archive member's file
protection, owner and group ID, size, and date and time of creation.

For example:

@example
% tar --list --verbose --file=music
drwxrwxrwx myself/user 0 May 31 21:49 1990 practice/
-rw-rw-rw- myself/user 42 May 21 13:29 1990 practice/blues
-rw-rw-rw- myself/user 62 May 23 10:55 1990 practice/folk
-rw-rw-rw- myself/user 40 May 21 13:30 1990 practice/jazz
-rw-rw-rw- myself/user 10240 May 31 21:49 1990 practice/records
% 
@end example

Note that when you use @samp{--verbose} with @samp{--list}, @code{tar}
doesn't print the names of files as they are being acted on, though
the @samp{--verbose} option will have this effect when used with all
other operations.

@node Specific File, Comparing Files, Additional File Info, Listing Archive Contents
@subsection List A Specific File in an Archive

To to see if a particular file is in an archive, use the name of the
file in question as a file-name argument while specifying the
@samp{--list} operation.  For example, to see whether the file
@file{folk} is in the archive file @file{music}, do the following:

@itemize @bullet
@item
Invoke @code{tar}, and specify the @samp{--list} operation
(@samp{--list} or @samp{-t}).

@item
Specify the archive file to be acted on (@samp{--file
@var{archive-name}} or @samp{-f @var{archive-name}}).

@item
Specify the files to look for, by typing their names as file-name
arguments.  You have to type the file name as it appears in the
archive (normally, as it is relative to the relative to the directory
from which the archive was created).  <<< xref absolute-paths -ringo
@end itemize

Type:

@example
% tar --list --file=music practice/folk
@end example

@noindent 
@code{tar} responds:

@example
practice/folk
@end example

@noindent
If the file were not stored in the archive (for example, the file
@file{practice/rock}), the example above would look like:

@example
% tar --list --file=music practice/rock
tar: practice/rock not found in archive
@end example

@noindent
If you had used @samp{--verbose} mode, the example above would look
like: 

@example
% tar --list --file=music practice/folk
-rw-rw-rw- myself/user 62 May 23 10:55 1990 practice/folk
@end example

@node Listing Directories, , ,
@subsection Listing the Contents of a Stored Directory

To get information about the contents of an archived directory, use
the directory name as a file-name argument in conjunction with
@samp{--list}.  To find out file attributes, include the
@samp{--verbose} option.

For example, to find out about files in the directory @file{practice},
in the archive file @file{music}, type:

@example
% tar --list --file=music practice
@end example

@noindent @code{tar} responds:

@example
drwxrwxrwx myself/user 0 May 31 21:49 1990 practice/
-rw-rw-rw- myself/user 42 May 21 13:29 1990 practice/blues
-rw-rw-rw- myself/user 62 May 23 10:55 1990 practice/folk
-rw-rw-rw- myself/user 40 May 21 13:30 1990 practice/jazz
-rw-rw-rw- myself/user 10240 May 31 21:49 1990 practice/records
@end example

When you use a directory name as a file-name argument, @code{tar} acts
on all the files (including sub-directories) in that directory.

@node Comparing Files, , ,
@section Comparing Files in an Archive with Files in the File System

To compare the attributes of archive members with the attributes of
their counterparts in the file system, use the @samp{--compare},
@samp{--diff}, or @samp{-d}) operation.  While you could use
@samp{--list --verbose} to manually compare some file attributes, it is
simpler to have @code{tar} itself compare file attributes and report
back on file differences.
@c <<<"manually"?  suggestions?  -ringo

The @samp{--compare} operation, as its name implies, compares archive
members with files of the same name in the file system, and reports
back differences in file size, mode, owner and modification date.
@samp{tar --compare} acts only on archive members---it ignores files in
the file system that are not stored in the archive.  If you give
@samp{tar --compare} a file-name argument that does not correspond to
the name of an archive member, @code{tar} responds with an error
message.

To compare archive members in the archive file @file{records} with
files in the @file{~/practice} directory, first change into the
@file{practice} directory.  Then:

@itemize @bullet
@item
Invoke @code{tar} and specify the @samp{--compare} operation.
(@samp{--compare}, @samp{--diff}, or @samp{-d}).

@item
Specify the archive where the files to be compared are stored
(@samp{--file=@var{archive-name}} or @samp{-f @var{archive-name}})

@item
Specify the archive members to be compared.  (In this example you are
comparing all the archive members in the archive.  Since this is the
default, you don't need to use any file-name arguments).
@end itemize

@example
% tar --compare --file=records
%
@end example

@noindent
While it looks like nothing has happened, @code{tar} has, in fact,
done the comparison---and found nothing to report.  

Use the @samp{--verbose} option to list the names of archive members as
they are being compared with their counterparts of the same name in
the file system:

@example
% tar --compare --verbose --file=records
blues
folk
jazz
%
@end example

@noindent
If @code{tar} had had anything to report, it would have done so as it
was comparing each file.  

If you remove the file @file{jazz} from the file system (@samp{rm
jazz}), and modify the file @file{blues} (for instance, by adding text
to it with an editor such as Emacs), the above example would look
like:

@example
% tar --compare --verbose --file=records
blues
blues: mod time differs
blues: size differs
folk
jazz
jazz: does not exist
% 
@end example

Note again that while @samp{tar --compare} reports the names of archive
members that do not have counterparts in the file system, @samp{tar
--compare} ignores files in the file system that do not have
counterparts in the archive.  To demonstrate this, create a file in
the @file{practice} directory called @file{rock} (using any text
editor).  The new file appears when you list the directory's contents:

@example
% ls
blues    folk     records  rock
@end example

@noindent
If you type the @samp{--compare} example again, @code{tar} prints the
following:

@example
% tar --compare --verbose --file=records
blues
blues: mod time differs
blues: size differs
folk
jazz
jazz: does not exist
% 
@end example

@noindent
@code{tar} ignores the file @file{rock} because @code{tar} is
comparing archive members to files in the file system, not vice versa.

If you specify @file{rock} explicitly (using a file-name argument),
@code{tar} prints an error message:

@example
% tar --compare --verbose --file=records rock
tar: rock not found in archive
% 
@end example

@menu
* Comparing Directories::       Using Compare on Directories
@end menu

@node Comparing Directories,  ,  ,  
@subsubsection Using Compare on Directories

In addition to using @samp{--compare} to compare text files, you can
use @samp{--compare} to compare directories.  To illustrate this,
re-create the examples above using your home directory as the working
directory, and using the archive file @file{~/music} instead of the
archive file @file{~/practice/records}.

First, change into your home directory (@samp{cd ~}).  Then, try the
above example using @file{music} as the specified archive file, and
@file{practice} as a file-name argument.

@example
% tar --compare --verbose --file=music practice
@end example

@noindent
If you have been following along with the tutorial, @code{tar} will
respond:

@example
practice
practice/blues
practice/blues: mod time differs
practice/blues: size differs
practice/folk
practice/jazz
practice/jazz: does not exist
practice/records
@end example

@node Adding to Archives, Concatenate, Listing Archive Contents, Tutorial
@section Adding Files to Existing Archives

@c >>> we want people to use the script for backups, so I an not going to
@c >>> use backups as an explanation in the tutorial.  (people can still
@c >>> do it if they really want to)  -ringo

While you can use @code{tar} to create a new archive every time you
want to store a file, it is more sometimes efficient to add files to
an existing archive.  

To add new files to an existing archive, use the @samp{--add-file},
@samp{--append} or @samp{-r} operation.  To add newer versions of
archive members to an archive, use the @samp{--update} or @samp{-u}
operation.

@menu
* Append::                      Appending Files to an Archive
* Update::                      Updating Files in an Archive
@end menu

@node Append, Update, Adding to Archives, Adding to Archives
@subsection Appending Files to an Archive

The simplest method of adding a file to an existing archive is the
@samp{--add-file}, @samp{-r} or @samp{--append} operation, which writes
files into the archive without regard to whether or not they are
already archive members.  When you use @samp{--add-file} you must use
file-name arguments; there is no default.  If you specify a file that
is already stored in the archive, @code{tar} adds another copy of the
file to the archive.

If you have been following the previous examples, you should have a
text file called @file{~/practice/rock} which has not been stored in
either the archive file @file{~/practice/records}, or the archive file
@file{~/music}.  To add @file{rock} to @file{records}, first make
@file{practice} the working directory (@samp{cd practice}).  Then:

@itemize @bullet
@item
Invoke @code{tar} and specify the @samp{--add-file} operation
(@samp{--add-file}, @samp{-r} or @samp{--append})

@item
Specify the archive to which the file will be added
(@samp{--file=@var{archive-name}} or @samp{-f @var{archive-name}})

@item
Specify the files to be added to the archive, using file-name
arguments
@end itemize

@noindent
For example:

@example
% tar --add-file --file=records rock
@end example

@noindent
If you list the archive members in @file{records}, you will see that
@file{rock} has been added to the archive:

@example
% tar --list --file=records
blues
folk
jazz
rock
@end example

@c <<<  this should be some kind of node.  

You can use @samp{--add-file} to keep archive members current with
active files.  Because @samp{--add-file} stores a file whether or not
there is already an archive member with the same file name, you can
use @samp{--add-file} to add newer versions of archive members to an
archive.  When you extract the file, 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.  

If you recall from the examples using @samp{--compare} above,
@file{blues} was changed after the archive @file{records} was created.
It is simple, however, to use @samp{--add-file} to add the new version
of @file{blues} to @file{records}:

@example
% tar --add-file --verbose --file=records blues
blues
@end example

@noindent
If you now list the contents of the archive, you will obtain the following:

@example
% tar --list -f records
blues
folk
jazz
rock
blues
@end example

@noindent
The newest version of @file{blues} is at the end of the archive.  When
the files in @file{records} are extracted, the newer version of
@file{blues} (which has the same name as the older) will overwrite the
version stored first.  When @samp{tar --extract} is finished, only the
newer version of @file{blues} is in the file system.  <<<xref
keep-old-files>>>

@node Update,  , Append, Adding to Archives
@subsection Updating Files in an Archive

To keep archive members up to date with their counterparts of the same
name in the file system, use the @samp{--update} or @samp{-u}
operation.  @samp{tar --update} adds a specified file to an archive if
no file of that name is already stored in the archive.  If there is
already an archive member with the same name, @code{tar} checks the
modification date of the archive member, and adds the file only if its
modification date is later.  If a file is stored in the archive but no
longer exists under the same name in the active file system,
@code{tar} reports an error.

You could use the @samp{--add-file} option to keep an archive current,
but do so you would either have to use the @samp{--compare} and
@samp{--list} options to determine what files needed to be re-archived
(which could waste a lot of time), or you would have to be willing to
add identical copies of already archived files to the archive (which
could waste a lot of space).

You must use file-name arguments with the @samp{--update}
operation---if you don't specify any files, @code{tar} won't act on
any files. 

To see the @samp{--update} option at work, create a new file,
@file{~/practice/classical}, and modify the file
@file{~/practice/blues} (you can use a text editor, such as Emacs, to
do both these things).  Then, with @file{practice} as your working
directory, invoke @samp{tar --update} using the names of all the files
in the practice directory as file-name arguments, and specifying the
@samp{--verbose} option:

@example
% tar --update --verbose --file=records blues folk rock classical
blues
classical
%
@end example

@noindent
Because you specified verbose mode, @code{tar} printed out the names
of the files it acted on.  If you now list the archive members of the
archive, (@samp{tar --list --file=records}), you will see that the file
@file{classical} and another version of the file @file{blues} have
been added to @file{records}.

Note: When you update an archive, @code{tar} does not overwrite old
archive members when it stores newer versions of a file.  This is
because archive members appear in an archive in the order in which
they are stored, and some archive devices do not allow writing in the
middle of an archive.

@node Concatenate, Extracting Files Example, Adding to Archives, Tutorial
@comment  node-name,  next,  previous,  up
@section Concatenating Archives 

To concatenate archive files, use @samp{tar --concatenate} or @samp{tar
-A}.  This operation adds other archives to the end of an archive.
While it may seem intuitive to concatenate archives using @code{cat},
the utility for adding files together, archive files which have been
"catted" together cannot be read properly by @code{tar}.  Archive
files incorporate an end of file marker---if archives are concatenated
using @code{cat}, this marker will appear before the end of the new
archive.  This will interfere with operations on that archive.
@c <<<xref ignore-zeros>>>

In earlier examples, you stored the @file{~/practice} directory in an
archive file, @file{~/music}.  If you have been following the
examples, you have since changed the contents of the @file{~/practice}
directory.  There is a current version of the files in the
@file{practice} directory, however, stored in the archive file
@file{~/practice/records}.

To store current versions of the files in @file{practice} in the
archive file @file{music}, you can use @samp{tar --concatenate} to add
the archive file @file{~/practice/records} to @file{music}.  First,
make sure you are in your home directory (@samp{cd ~}).  Then:

@itemize @bullet
@item
Invoke @code{tar}, and specify the @samp{--concatenate} operation
(@samp{-A} or @samp{--concatenate})

@item
Specify the archive file to be added to
(@samp{--file=@var{archive-name}} or @samp{-f @var{archive-name}})

@item
Specify the archives to be added, using file-name arguments.  In this
case, the file-name arguments are, unusually, the names of archive
files.  (Remember to include the path in the archive name, if the
archive file is not in your working directory.)
@end itemize

@example
% cd ~
% tar --concatenate --file=music practice/records
@end example

If you now list the contents of the @file{music}, you see it now
contains the archive members of @file{practice/records}:

@example
%tar --list --file=music
blues
folk
jazz
rock
blues
practice/blues
practice/folk
practice/jazz
practice/rock
practice/blues
practice/classical
@end example

@node Deleting Files,  ,  , Tutorial
@comment  node-name,  next,  previous,  up
@section Deleting Files From an Archive

In some instances, you may want to remove some files from an archive
stored on disk

@quotation
@emph{Caution:} you should never delete files from an archive stored
on tape---because of the linear nature of tape storage, doing this is
likely to scramble the archive.
@end quotation

To remove archive members from an archive, use the @samp{--delete}
operation.  You must specify the names of files to be removed as
file-name arguments.  All versions of the named file are removed from
the archive.  

Execution of the @samp{--delete} operation can be very slow.

To delete all versions of the file @file{blues} from the archive
@file{records} in the @file{practice} directory, make sure you are in
that directory, and then:

@itemize @bullet
@item
List the contents of the archive file @file{records} (see above for
the steps involved) to insure that the file(s) you wish to delete are
stored in the archive.  (This step is optional)

@item
Invoke @code{tar} and specify the @samp{--delete} operation
(@samp{--delete}).

@item
Specify the name of the archive file that the file(s) will be deleted
from (@samp{--file=@var{archive-name}} or @samp{-f @var{archive-name}})

@item
Specify the files to be deleted, using file-name arguments.

@item
List the contents of the archive file again---note that the files have
been removed.  (this step is also optional)
@end itemize

@example
% tar --list --file=records
blues
folk
jazz
% tar --delete --file=records blues
% tar --list --file=records
folk
jazz
% 
@end example

@node Wizardry, Archive Structure, Tutorial, Top
@chapter Wizardry

<<<This section needs to be written  -ringo

@strong{To come:} using Unix file linking capability to recreate directory
structures---linking files into one subdirectory and then tarring that
directory.   

@strong{to come:} nice hairy example using absolute-paths, newer, etc.


Piping one @code{tar} to another is an easy way to copy a directory's
contents from one disk to another, while preserving the dates, modes, owners
and link-structure of all the files therein.

@example
cd sourcedirectory; tar cf - . | (cd targetdir; tar xf -)
@end example

@noindent
or

<<<  the following using standard input/output correct??
@example
cd sourcedirectory; tar --create --file=- . | (cd targetdir; tar --extract --file=-)
@end example

@noindent

Archive files can be used for transporting 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, 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 @code{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 @code{tar} program.
@findex uuencode
<<< mention uuencode on a paragraph of its own

<<<<<end construction>>>>>

@node Archive Structure, Reading and Writing, Wizardry, Top
@chapter The Structure of an Archive

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.


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. 

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}.

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.

@menu
* Old Style File Information::  Old Style File Information
* Archive Label::               
* Format Variations::           
@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

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.

@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

@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

@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. 

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

@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

@node Format Variations,  , Archive Label, Archive Structure
@section Format Variations
@cindex Format Parameters
@cindex Format Options
@cindex Options to specify archive format.

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, @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.


@menu
* Multi-Volume Archives::       
* Sparse Files::                
* Blocking Factor::             
* Compressed Archives::         
@end menu

@node Multi-Volume Archives, Sparse Files, Format Variations, Format Variations
@subsection Archives Longer than One Tape or Disk
@cindex Multi-volume archives

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.

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.

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}.

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.

@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.


<<< There should be a sample program here, including an exit before
<<< end.

@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.

@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

@node Sparse Files, Blocking Factor, Multi-Volume Archives, Format Variations
@subsection Archiving Sparse Files
@cindex Sparse Files

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.

@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.  

@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

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

@code{tar} ignores the @samp{--sparse} option when reading an archive.

@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 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.

@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

@node Compressed Archives,  , Blocking Factor, Format Variations
@subsection Creating and Reading Compressed Archives
@cindex Compressed archives
@cindex Storing archives in compressed format

@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.

@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.

@quotation
@strong{Please Note:} The @code{compress} program may be covered by a patent,
and therefore we recommend you stop using it.  We hope to have a
different compress program in the future.  We may change the name of
this option at that time.
@end quotation

@table @samp
@item --compress
@itemx --uncompress
@itemx -z
@itemx -Z
When this option is specified, @code{tar} will compress (when writing
an archive), or uncompress (when reading an archive).  Used in
conjunction with the @samp{--create}, @samp{--extract}, @samp{--list} and
@samp{--compare} operations.

@item --compress-block
@itemx -z -z
Acts like @samp{--compress}, but pads the archive out to the next block
boundary as it is written when used in conjunction with the
@samp{--create} operation.
@end table

@c >>> MIB -- why not use -Z instead of -z -z ?  -ringo

@node Reading and Writing, Insuring Accuracy, Archive Structure, Top
@chapter Reading and Writing Archives

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.

@menu
* Archive Name::                The name of an archive 
* Creating in Detail::          Creating in detail
* Modifying::                   Modifying archives
* Listing Contents::            Listing the contents of an archive
* Extracting From Archives::    Extracting files from an archive 
@end menu

@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?

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.  

@c >>> MIB--does standard input and output redirection work with all
@c >>> operations?  
@c >>> need example for standard input and output (screen and keyboard?)

@cindex Standard input and output
@cindex tar to standard input and output

To specify an archive file on a device attached to a remote machine,
use the following:

@example
--file=@var{hostname}:/@var{dev}/@var{file name}
@end example

@noindent
@code{tar} will complete the remote connection, if possible, and
prompt you for a username and password.  If you use
@samp{--file=@@@var{hostname}:/@var{dev}/@var{file-name}}, @code{tar}
will complete the remote connection, if possible, using your username
as the username on the remote machine.  

@c >>>MIB --- is this clear?

@table @samp
@item --file=@var{archive-name}
@itemx -f @var{archive-name}
Names the archive to create or operate on.  Use in conjunction with
any operation.
@end table

@node Creating in Detail, Modifying, Archive Name, Reading and Writing
@section Creating in Detail
@c operations should probably have examples, not tables.
@cindex Writing new archives
@cindex Archive creation

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.

The following example creates an archive named @file{stooges},
containing the files @file{larry}, @file{moe} and @file{curley}:

@example
tar --create --file=stooges larry moe curley
@end example

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}:

@example
tar --create --file=hail/hail/fredonia marx
@end example

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:

@example
tar --create --file=home
@end example

@xref{File Name Lists}, for other ways to specify files to archive.

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.

@node Modifying, Listing Contents, Creating in Detail, Reading and Writing
@section Modifying Archives
@cindex Modifying archives

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.  

To find out what files are already stored in an archive, use @samp{tar
--list --file=@var{archive-name}}.  @xref{Listing Contents}.

@menu
* Adding Files::                
* Appending Archives::          
* Deleting Archive Files::      Deleting Files From an Archive
* Matching Format Parameters::  
@end menu

@node Adding Files, Appending Archives, Modifying, Modifying
@subsection Adding Files to an Archive
@cindex Adding files to an archive
@cindex Updating an archive

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

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.

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):

@example
tar --add-archive --file=stooges hail/hail/fredonia
@end example

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}).

@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

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.

The following example removes the file @file{curley} from the archive
@file{stooges}:

@example
tar --delete --file=stooges curley
@end example

You can only use @samp{tar --delete} on an archive if the archive
device allows you to write to any point on the media.

@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

@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=-).

@node Matching Format Parameters,  , Deleting Archive Files, Modifying
@subsection Matching the Format Parameters

Some format parameters must be taken into consideration when modifying
an archive:

Compressed archives cannot be modified.  

You have to specify the block size of the archive when modifying an
archive with a non-default block size.

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

To read archive members from the archive and write them into the file
system, use @samp{tar --extract}.  The archive itself is left
unchanged.

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.

The following example shows the extraction of the archive
@file{stooges} into an empty directory:

@example
tar --extract --file=stooges
@end example

@noindent
Generating a listing of the directory (@samp{ls}) produces:

@example
larry
moe
shemp
marx
@end example

@noindent
The subdirectory @file{marx} contains the files @file{julius},
@file{alexander} and @file{karl}.

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:

@example
tar --extract --file=stooges marx
@end example

@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

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}).

@menu
* Archive Reading Options::     
* File Writing Options::        
* Scarce Disk Space::           Recovering From Scarce Disk Space
@end menu

@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

@c <<< each option wants its own node.  summary after menu

Normally, @code{tar} will request data in full block increments from
an archive storage device.  If the device cannot return a full block,
@code{tar} will report an error.  However, some devices do not always
return full blocks, or do not require the last block of an archive to
be padded out to the next block boundary.  To keep reading until you
obtain a full block, or to accept an incomplete block if it contains
an end-of-archive marker, specify the @samp{--read-full-blocks} option
in conjunction with the @samp{--extract} or @samp{--list} operations.
@xref{Listing Contents}.

The @samp{--read-full-blocks} option is turned on by default when
@code{tar} reads an archive from standard input, or from a remote
machine.  This is because on BSD Unix systems, attempting to read a
pipe returns however much happens to be in the pipe, even if it is
less than was requested.  If this option were not enabled, @code{tar}
would fail as soon as it read an incomplete block from the pipe.

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}.

@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

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).

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.

@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

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.

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.

This option is probably never needed on modern computer systems.

@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

@c we don't need/want --preserve to exist any more

@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

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.

@table @samp
@item --keep-old files
@itemx -k 
Prevents @code{tar} from overwriting files in the file system during
extraction. 
@end table

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.

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}.

@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

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 <<<mib --- should be aliased to ignore-umask.  

@table @samp
@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
@samp{--extract}. 
@end table

@c <<< following paragraph needs to be rewritten:
@c <<< why doesnt' this cat files together, why is this useful.  is it
@c <<< really useful with more than one file?
To write the files extracted to the standard output, instead of
creating the files on the file system, use @samp{--to-stdout} in
conjunction with @samp{tar --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.

@table @samp
@item --to-stdout
@itemx -O
Writes files to the standard output.  Used in conjunction with
@samp{--extract}. 
@end table

@c <<< why would you want to do such a thing, how are files separated on
@c <<< the standard output? is this useful with more that one file?  are
@c <<< pipes the real reason?

@node Scarce Disk Space,  , File Writing Options, Extracting From Archives
@subsection Recovering From Scarce Disk Space
@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

If a previous attempt to extract files failed due to lack of disk
space, you can use @samp{--starting-file=@var{file-name}} to start
extracting only after file @var{file-name} when extracting files from
the archive.  This assumes, of course, that there is now free space,
or that you are now extracting into a different file system.

@table @samp
@item --starting-file=@var{file-name}
@itemx -K @var{file-name}
Starts an operation in the middle of an archive.  Use in conjunction
with @samp{--extract} or @samp{--list}.
@end table

If you notice you are running out of disk space during an extraction
operation, you can also suspend @code{tar}, remove unnecessary files
from the file system, and then restart the same @code{tar} operation.
In this case, @samp{--starting-file} is not necessary. 

@c <<< xref incremental,  xref --interactive,  xref --exclude

@node Insuring Accuracy, Selecting Archive Members, Reading and Writing, Top
@chapter Insuring the Accuracy of an Archive

You can insure the accuracy of an archive by comparing files in the
system with archive members.  @code{tar} can compare an archive to the
file system as the archive is being written, to verify a write
operation, or can compare a previously written archive, to insure that
it is up to date.

@menu
* Write Verification::          
* Comparing::                   
@end menu

@node Write Verification, Comparing, Insuring Accuracy, Insuring Accuracy
@section Verifying Data as It is Stored
@cindex Verifying a write operation
@cindex Double-checking a write operation

To check for discrepancies in an archive immediately after it is
written, use the @samp{--verify} option in conjunction with the
@samp{tar --create} operation.  When this option is specified,
@code{tar} checks archive members against their counterparts in the file
system, and reports discrepancies on the standard error.  In
multi-volume archives, each volume is verified after it is written,
before the next volume is written.

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.

@table @samp
@item --verify
@itemx -W
Checks for discrepancies in the archive immediately after it is
written.  Use in conjunction with @samp{tar --create}.
@end table

@node Comparing,  , Write Verification, Insuring Accuracy
@section Comparing an Archive with the File System
@cindex Verifying the currency of an archive

@samp{tar --compare} compares archive members in an existing archive
with their counterparts in the file system, and reports differences in
file size, mode, owner, modification date and contents.  If a file is
represented in the archive but does not exist in the file system,
@code{tar} reports a difference.

If you use @var{file-name} arguments in conjunction with @samp{tar
--compare}, @code{tar} compares the archived versions of the files
specified with their counterparts in the file system.  If you specify
a file that is not in the archive, @code{tar} will report an error.  If
you don't specify any files, @code{tar} compares all the files in the
archive.

Because @code{tar} only checks files in the archive against files in
the file system, and not vice versa, it ignores files in the file
system that do not exist in the archive.

The following example compares the archive members @file{larry},
@file{moe} and @file{curly} in the archive @file{stooges} with files
of the same name in the file system.

@example
tar --compare --file=stooges larry moe curly
@end example

@noindent 
If a file, for example @file{curly}, did not exist in the archive,
@code{tar} would report an error, as follows:

@example
curly: does not exist
@end example

@node Selecting Archive Members, User Interaction, Insuring Accuracy, Top
@chapter Selecting Archive Members
@cindex Specifying files to act on
@cindex Specifying archive members

@dfn{File-name arguments} specify which files in the file system
@code{tar} operates on, when creating or adding to an archive, or
which archive members @code{tar} operates on, when reading or
deleting from an archive.  (@pxref{Reading and Writing}.)

To specify file names, you can include them as the last arguments on
the command line, as follows:
@example
tar @var{operation} [@var{option1} @var{option2} ..] [@var{file-name-1} @var{file-name-2} ...]
@end example

If you specify a directory name as a file name argument, all the files
in that directory are operated on by @code{tar}. 

If you do not specify files when @code{tar} is invoked, @code{tar}
operates on all the non-directory files in the working directory (if
the operation is @samp{--create}), all the archive members in the
archive (if a read operation is specified), or does nothing (if any
other operation is specified).

@menu
* File Name Lists::             Reading File Names from a File
* File Name Interpretation::    this needs a better title
* File Exclusion::              so does this
@end menu

@node File Name Lists, File Name Interpretation, Selecting Archive Members, Selecting Archive Members
@section Reading a List of File Names from a File
@cindex Lists of file names
@cindex File-name arguments, alternatives

To read file names from a file on the file system, instead of from the
command line, use the @samp{--files-from=@var{file}} option.  If you
specify @samp{-} as @var{file}, the file names are read from standard
input.  Note that using both @samp{--files-from=-} and @samp{--file=-}
in the same command will not work unless the operation is
@samp{--create}.  @xref{Archive Name}, for an explanation of the
@samp{--file} option.

@table @samp
@item --files-from=@var{file}
@itemx -T @var{file}
Reads file-name arguments from a file on the file system, instead of
from the command line.  Use in conjunction with any operation.
@end table

@node File Name Interpretation, File Exclusion, File Name Lists, Selecting Archive Members
@section File Name Interpretation
@cindex File Names, interpreting

@c <<<<add some text  -ringo

@menu
* Absolute File Names::         
* Changing Working Directory::  
* Archiving with Symbolic Links::  Archiving Using Symbolic Links
@end menu

@node Absolute File Names, Changing Working Directory, File Name Interpretation, File Name Interpretation
@subsection Storing and Extracting Files Relative to Root

@c <<< is this what this does, or does it just preserve the slash?  
@c <<< is it still called --absolute-paths?

@c To archive or extract files relative to the root directory, specify
@c the @samp{--absolute-paths} option.

@c Normally, @code{tar} acts on files relative to the working
@c directory---ignoring superior directory names when archiving, and
@c ignoring leading slashes when extracting.

@c When you specify @samp{--absolute-paths}, @code{tar} stores file names
@c including all superior directory names, and preserves leading slashes.
@c If you only invoked @code{tar} from the root directory you would never
@c need the @samp{--absolute-paths} option, but using this option may be
@c more convenient than switching to root.

@c >>> should be an example in the tutorial/wizardry section using this
@c >>> to transfer files between systems.

@c >>>  is write access an issue?

@table @samp
@item --absolute-paths
Preserves full file names (inclusing superior dirctory names) when
archiving files.  Preserves leading slash when extracting files.
@end table

@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

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,

@example 
tar --create iggy ziggy --directory=baz melvin
@end example

@noindent
will place the files @file{iggy} and @file{ziggy} from the current
directory into the archive, followed by the file @file{melvin} from
the directory @file{baz}.  This option is especially useful when you
have several widely separated files that you want to store in the same
directory in the archive.

Note that the file @file{melvin} is recorded in the archive under the
precise name @file{melvin}, @emph{not} @file{baz/melvin}.  Thus, the
archive will contain three files that all appear to have come from the
same directory; if the archive is extracted with plain @samp{tar
--extract}, all three files will be written in the current directory.

Contrast this with the command

@example
tar -c iggy ziggy bar/melvin
@end example

@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}.

@table @samp
@item --directory=@file{directory}
@itemx -C @file{directory}
Changes the working directory.  
@end table

@c <<<need to test how extract deals with this, and add an example  -ringo

@node Archiving with Symbolic Links,  , Changing Working Directory, File Name Interpretation
@subsection Archiving Using Symbolic Links
@cindex File names, using symbolic links
@cindex Symbolic link as file name

@samp{--dereference} is used with @samp{tar --create}, and causes
@code{tar} to archive files which are referenced by a symbolic link,
using the name of the link as the file name.

<<<this needs to be checked by MIB and then re-written, with an example
The name under which the file is stored in the file system is not
recorded in the archive.  To record both the symbolic link name and
the file name in the system, archive the file under both names.  If
all links were recorded automatically by @code{tar}, an extracted file
might be linked to a file name that no longer exists in the file
system.

@c <<< is the following still true? - ringo
If a linked-to file is encountered again by @code{tar} while creating
the same archive, an entire second copy of it will be stored.  This
could be considered a bug.

@table @samp
@item --dereference
@itemx -h
Stores files referenced by a symbolic link, using the name of the link
as the file name.  Use in conjunction with any write operation.
@end table

@node File Exclusion,  , File Name Interpretation, Selecting Archive Members
@section Selecting Files by Characteristic
@cindex File names, excluding files by
@cindex Excluding files by name and pattern
@cindex Excluding files by file system
@cindex File system boundaries, not crossing
@cindex Excluding file by age
@cindex Modification time, excluding files by
@cindex Age, excluding files by

To avoid crossing file system boundaries when archiving parts of a
directory tree, use @samp{--one-file-system}.  This option only affects
files that are archived because they are in a directory that is being
archived; files explicitly named on the command line are archived
regardless of where they reside.

This option is useful for making full or incremental archival backups
of a file system.

If this option is used in conjunction with @samp{--verbose}, files that
are excluded are mentioned by name on the standard error.

@table @samp
@item --one-file-system
@itemx -l
Prevents @code{tar} from crossing file system boundaries when
archiving.  Use in conjunction with any write operation.
@end table

To avoid operating on files whose names match a particular pattern,
use the @samp{--exclude=@var{pattern}} or
@samp{--exclude-from=@var{file}} options.  

When you specify the @samp{--exclude=@var{pattern}} option, @code{tar}
ignores files which match the @var{pattern}, which can be a single
file name or a more complex expression.  Thus, if you invoke
@code{tar} with @samp{tar --create --exclude=*.o}, no files whose names
end in @file{.o} are included in the archive.  
@c <<< what other things can you use besides "*"?

@samp{--exclude-from=@var{file}} acts like @samp{--exclude}, but
specifies a file @var{file} containing a list of patterns.  @code{tar}
ignores files with names that fit any of these patterns.

You can use either option more than once in a single command.

@table @samp
@item --exclude=@var{pattern}
Causes @code{tar} to ignore files that match the @var{pattern}.

@item --exclude-from=@var{file}
Causes @code{tar} to ignore files that match the patterns listed in
@var{file}.
@end table
@c --exclude-from used to be "--exclude", --exclude didn't used to exist.

To operate only on files with modification or status-change times
after a particular date, use @samp{--after-date=@var{date}}.  You can
use this option with @samp{tar --create} or @samp{tar --add-file} to
insure only new files are archived, or with @samp{tar --extract} to
insure only recent files are resurrected. @refill
@c --after-date @var{date} or  --newer @var{date}

@samp{--newer-mtime=@var{date}} acts like @samp{--after-date=@var{date}},
but tests just the modification times of the files, ignoring
status-change times.

@c <<<need example of --newer-mtime with quoted argument
Remember that the entire date argument should be quoted if it contains
any spaces.


@strong{Please Note:} @samp{--after-date} and @samp{--newer-mtime}
should not be used for incremental backups.  Some files (such as those
in renamed directories) are not selected up properly by these options.
@c  xref to incremental backup chapter when node name is decided.

@table @samp
@item --after-date=@var{date}
@itemx --newer=@var{date}
@itemx -N @var{date}
Acts on files only if their modification or inode-changed times are
later than @var{date}.  Use in conjunction with any operation.
@item --newer-mtime=@var{date}
Acts like @samp{--after-date}, but only looks at modification times.
@end table

@c <<< following is the getdate date format --- needs to be re-written,
@c <<< made a sub-node:

Time/Date Formats Accepted by getdate
(omitting obscure constructions)

The input consists of one or more of: time zone day date year
in any order.

Those in turn consist of (`|' and `/' mean `or', `[]' means `optional'):

time: H am/pm | H:M [am/pm] | H:M:S [am/pm]
zone: timezone-name | timezone-name dst
day: day-name | day-name, | N day-name
date: M/D | M/D/Y | month-name D | month-name D, Y | D month-name | D month-name Y
year: Y

am can also be a.m., pm can also be p.m.
case and spaces around punctuation are not significant.
month and day names can be abbreviated.  >>>

@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}

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.

@menu
* Additional Information::      
* Interactive Operation::       
@end menu

@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.

@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

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.

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

@table @samp
@item --record-number
@itemx -R 
Prints the record number whenever a message is generated by
@code{tar}.  Use in conjunction with any operation.
@end table

@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:

@example
tar --version
@end example

@noindent
might return:

@example
GNU tar version 1.09
@end example
@c used to be an option.  has been fixed.

@node Interactive Operation,  , Additional Information, User Interaction
@section Asking for Confirmation During Operations
@cindex Interactive operation

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

@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

@node Backups and Restoration, Media, User Interaction, Top
@chapter Performing Backups and Restoring Files

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}.

@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

@node Backup Levels, Backup Scripts, Backups and Restoration, Backups and Restoration
@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 a incremental dump.  A @dfn{level
one} dump archives all the files that have changed since the last full
dump.

A typical dump strategy would be to perform a full dump once a week,
and a level one dump once a day.  This means some versions of files
will in fact be archived more than once, but this dump strategy makes
it possible to restore a file system to within one day of accuracy by
only extracting two archives---the last weekly (full) dump and the
last daily (level one) dump.  The only information lost would be in
files changed or created since the last daily backup.  (Doing dumps
more than once a day is usually not worth the trouble).

@node Backup Scripts, incremental and listed-incremental, Backup Levels, Backups and Restoration
@section Using Scripts to Perform Backups and Restoration

GNU @code{tar} comes with scripts you can use to do full and level-one
dumps.  Using scripts (shell programs) to perform backups and
restoration is a convenient and reliable alternative to typing out
file name lists and @code{tar} commands by hand.

Before you use these scripts, you need to edit the file
@file{backup-specs}, which specifies parameters used by the backup
scripts and by the restore script.  @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

@node Backup Parameters, Scripted Backups, Backup Scripts, Backup Scripts
@subsection Setting Parameters for Backups and Restoration

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.

@c <<< This about backup scripts needs to be written:
@c <<<BS is a shell script ....  thus ... @file{backup-specs} is in shell
@c  script syntax.  @xref{Script Syntax}, for an explanation of this
@c syntax.

@c whats a parameter ....  looked at by the backup scripts ... which will
@c be expecting to find ... now syntax ... value is linked to lame ...
@c @file{backup-specs} specifies the following parameters:


@table @code
@item ADMINISTRATOR
The user name of the backup administrator.

@item BACKUP_HOUR
The hour at which the backups are done.  This can be a number from 0
to 23, or the string @samp{now}.

@item TAPE_FILE
The device @code{tar} writes the archive to.  This device should be
attached to the host on which the dump scripts are run.
@c <<< examples for all  ...

@item TAPE_STATUS
The command to use to obtain the status of the archive device,
including error count.  On some tape drives there may not be such a
command; in that case, simply use `TAPE_STATUS=false'.

@item BLOCKING
The blocking factor @code{tar} will use when writing the dump archive.
@xref{Blocking Factor}.

@item BACKUP_DIRS
A list of file systems to be dumped.  You can include any directory
name in the list---subdirectories on that file system will be
included, regardless of how they may look to other networked machines.
Subdirectories on other file systems will be ignored.

The host name specifies which host to run @code{tar} on, and should
normally be the host that actually contains the file system.  However,
the host machine must have GNU @code{tar} installed, and must be able
to access the directory containing the backup scripts and their
support files using the same file name that is used on the machine
where the scripts are run (ie. what @code{pwd} will print when in that
directory on that machine).  If the host that contains the file system
does not have this capability, you can specify another host as long as
it can access the file system through NFS.

@item BACKUP_FILES
A list of individual files to be dumped.  These should be accessible
from the machine on which the backup script is run.  
@c <<<same file name, be specific.  through nfs ...
@end table

@menu
* backup-specs example::        An Example Text of @file{Backup-specs}
* Script Syntax::               Syntax for @file{Backup-specs}
@end menu

@node backup-specs example, Script Syntax, Backup Parameters, Backup Parameters
@subsubsection An Example Text of @file{Backup-specs}

The following is the text of @file{backup-specs} as it appears at FSF:

@example
# site-specific parameters for file system backup.

ADMINISTRATOR=friedman
BACKUP_HOUR=1
TAPE_FILE=/dev/nrsmt0
TAPE_STATUS="mts -t $TAPE_FILE"
BLOCKING=124
BACKUP_DIRS="
        albert:/fs/fsf
        apple-gunkies:/gd
        albert:/fs/gd2
        albert:/fs/gp
        geech:/usr/jla
        churchy:/usr/roland
        albert:/
        albert:/usr
        apple-gunkies:/
        apple-gunkies:/usr
        gnu:/hack
        gnu:/u
        apple-gunkies:/com/mailer/gnu
        apple-gunkies:/com/archive/gnu"

BACKUP_FILES="/com/mailer/aliases /com/mailer/league*[a-z]"

@end example

@node Script Syntax,  , backup-specs example, Backup Parameters
@subsubsection Syntax for @file{Backup-specs}

@file{backup-specs} is in shell script syntax.  The following
conventions should be considered when editing the script:
@c <<<   "conventions?"

A quoted string is considered to be contiguous, even if it is on more
than one line.  Therefore, you cannot include commented-out lines
within a multi-line quoted string.  BACKUP_FILES and BACKUP_DIRS are
the two most likely parameters to be multi-line.

A quoted string typically cannot contain wildcards.  In
@file{backup-specs}, however, the parameters BACKUP_DIRS and
BACKUP_FILES can contain wildcards.

@node Scripted Backups, Scripted Restoration, Backup Parameters, Backup Scripts
@subsection Using the Backup Scripts

The syntax for running a backup script is:

@example
@file{script-name} [@var{time-to-be-run}]
@end example

where @var{time-to-be-run} can be a specific system time, or can be
@kbd{now}.  If you do not specify a time, the script runs at the time
specified in @file{backup-specs} (@pxref{Script Syntax}).

You should start a script with a tape or disk mounted.  Once you start
a script, it prompts you for new tapes or disks as it needs them.
Media volumes don't have to correspond to archive files---a
multi-volume archive can be started in the middle of a tape that
already contains the end of another multi-volume archive.  The
@code{restore} script prompts for media by its archive volume, so to
avoid an error message you should keep track of which tape (or disk)
contains which volume of the archive.  @xref{Scripted Restoration}.

@c <<<have file names changed?  -ringo
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.  @xref{incremental and listed-incremental}, for a more
detailed explanation of this file.

The second file is a log file containing the names of the file systems
and files dumped, what time the backup was made, and any error
messages that were generated, as well as how much space was left in
the media volume after the last volume of the archive was written.
You should check this log file after every backup.  The file name is
@file{log-@var{mmm-ddd-yyyy}-level-1} or
@file{log-@var{mmm-ddd-yyyy}-full}.

The script also prints the name of each system being dumped to the
standard output.
@c <<<the section on restore scripts is commented out.
@c <<< a section on non-scripted testore mya be a good idea
@ignore
@node Scripted Restoration,  , Scripted Backups, Backup Scripts
@subsection Using the Restore Script
@c  subject to change as things develop

To restore files that were archived using a scripted backup, use the
@code{restore} script.  The syntax for the script is:


where ##### are the file systems to restore from, and
##### is a regular expression which specifies which files to
restore.  If you specify --all, the script restores all the files
in the file system.

You should start the restore script with the media containing the
first volume of the archive mounted.  The script will prompt for other
volumes as they are needed.  If the archive is on tape, you don't need
to rewind the tape to to its beginning---if the tape head is
positioned past the beginning of the archive, the script will rewind
the tape as needed.  @xref{Media}, for a discussion of tape
positioning.

If you specify @samp{--all} as the @var{files} argument, the
@code{restore} script extracts all the files in the archived file
system into the active file system.  

@quotation
@strong{Warning:}The script will delete files from the active file
system if they were not in the file system when the archive was made.
@end quotation

@xref{incremental and listed-incremental}, for an explanation of how
the script makes that determination.
@c this may be an option, not a given
@end ignore

@node incremental and listed-incremental, Problems, Backup Scripts, Backups and Restoration
@section The @code{--incremental} and @code{--listed-incremental} Options

@samp{--incremental} is used in conjunction with @samp{--create},
@samp{--extract} or @samp{--list} when backing up and restoring file
systems.  An archive cannot be extracted or listed with the
@samp{--incremental} option specified unless it was created with the
option specified.  This option should only be used by a script, not by
the user, and is usually disregarded in favor of
@samp{--listed-incremental}, which is described below.

@samp{--incremental} in conjunction with @samp{--create} causes
@code{tar} to write, at the beginning of the archive, an entry for
each of the directories that will be archived.  The entry for a
directory includes a list of all the files in the directory at the
time the archive was created and a flag for each file indicating
whether or not the file is going to be put in the archive.
 
Note that this option causes @code{tar} to create a non-standard
archive that may not be readable by non-GNU versions of the @code{tar}
program.  

@samp{--incremental} in conjunction with @samp{--extract} causes
@code{tar} to read the lists of directory contents previously stored
in the archive, @emph{delete} files in the file system that did not
exist in their directories when the archive was created, and then
extract the files in the archive.

This behavior is convenient when restoring a damaged file system from
a succession of incremental backups: it restores the entire state of
the file system to that which obtained when the backup was made.  If
@samp{--incremental} isn't specified, the file system will probably
fill up with files that shouldn't exist any more.

@samp{--incremental} in conjunction with @samp{--list}, causes 
@code{tar} to print, for each directory in the archive, the list of
files in that directory at the time the archive was created.  This
information is put out in a format that is not easy for humans to
read, but which is unambiguous for a program: each file name is
preceded by either a @samp{Y} if the file is present in the archive,
an @samp{N} if the file is not included in the archive, or a @samp{D}
if the file is a directory (and is included in the archive).  Each
file name is terminated by a null character.  The last file is followed
by an additional null and a newline to indicate the end of the data.

@samp{--listed-incremental}=@var{file} acts like @samp{--incremental},
but when used in conjunction with @samp{--create} will also cause
@code{tar} to use the file @var{file}, which contains information
about the state of the file system at the time of the last backup, to
decide which files to include in the archive being created.  That file
will then be updated by @code{tar}.  If the file @var{file} does not
exist when this option is specified, @code{tar} will create it, and
include all appropriate files in the archive.

The file @var{file}, which is archive independent, contains the date
it was last modified and a list of devices, inode numbers and
directory names.  @code{tar} will archive files with newer mod dates
or inode change times, and directories with an unchanged inode number
and device but a changed directory name.  The file is updated after
the files to be archived are determined, but before the new archive is
actually created.

@c <<< this section needs to be written
@node Problems,  , incremental and listed-incremental, Backups and Restoration
@section Some Common Problems and their Solutions

errors from system:
permission denied
no such file or directory
not owner

errors from tar:
directory checksum error
header format error

errors from media/system:
i/o error
device busy

@node Media, Quick Reference, Backups and Restoration, Top
@chapter Tapes and Other Archive Media

Archives are usually written on dismountable media---tape cartridges,
mag tapes, or floppy disks.

The amount of data a tape or disk holds depends not only on its size,
but also on how it is formatted.  A 2400 foot long reel of mag tape
holds 40 megabytes of data when formated at 1600 bits per inch.  The
physically smaller EXABYTE tape cartridge holds 2.3 gigabytes.  

Magnetic media are re-usable---once the archive on a tape is no longer
needed, the archive can be erased and the tape or disk used over.
Media quality does deteriorate with use, however.  Most tapes or disks
should be disgarded when they begin to produce data errors.  EXABYTE
tape cartridges should be disgarded when they generate an @dfn{error
count} (number of non-usable bits) of more than 10k.

Magnetic media are written and erased using magnetic fields, and
should be protected from such fields to avoid damage to stored data.
Sticking a floppy disk to a filing cabinet using a magnet is probably
not a good idea.


@menu
* Write Protection::            Write Protection
* Tape Positioning::            Tape Positions and Tape Marks
@end menu

@node Write Protection, Tape Positioning, Media, Media
@section Write Protection

All tapes and disks can be @dfn{write protected}, to protect data on
them from being changed.  Once an archive is written, you should write
protect the media to prevent the archive from being accidently
overwritten or deleted.  (This will protect the archive from being
changed with a tape or floppy drive---it will not protect it from
magnet fields or other physical hazards).

The write protection device itself is usually an integral part of the
physical media, and can be a two position (write enabled/write
disabled) switch, a notch which can be popped out or covered, a ring
which can be removed from the center of a tape reel, or some other
changeable feature.

@node Tape Positioning,  , Write Protection, Media
@section Tape Positions and Tape Marks

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.

If you think of data as a series of "0000"'s, and tape marks as "x"'s,
a tape might look like the following:

@example
0000x000000x00000x00x00000xx-------------------------
@end example

Tape devices read and write tapes using a read/write @dfn{tape
head}---a physical part of the device which can only access one point
on the tape at a time.  When you use @code{tar} to read or write
archive data from a tape device, the device will begin reading or
writing from wherever on the tape the tape head happens to be,
regardless of which archive or what part of the archive the tape head
is on.  Before writing an archive, you should make sure that no data
on the tape will be overwritten (unless it is no longer needed).
Before reading an archive, you should make sure the tape head is at
the beginning of the archive you want to read.  (The @code{restore}
script will find the archive automatically.  @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:

@example
0000x000000x00000x00x00000x000x0000xx----------------
@end example

@menu
* mt::                          The @code{mt} Utility
@end menu

@node mt,  , Tape Positioning, Tape Positioning
@subsection The @code{mt} Utility

<<< is it true that this only works on non-block devices?  should
<<< explain the difference, xref to block-size (fixed or variable).

You can use the @code{mt} utility to advance or rewind a tape past a
specified number of archive files on the tape.  This will allow you to
move to the beginning of an archive before extracting or reading it,
or to the end of all the archives before writing a new one.
@c why isn't there an "advance 'til you find two tape marks together"? 

The syntax of the @code{mt} command is:

@example
mt [-f @var{tapename}] @var{operation} [@var{number}]
@end example

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:

@table @code
@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
<<< is there a better way to frob the spacing on the list?  -ringo

If you don't specify a @var{tapename}, @code{mt} uses the environment
variable TAPE; if TAPE does not exist, @code{mt} uses the device
@file{/dev/rmt12}.

@code{mt} returns a 0 exit status when the operation(s) were
successful, 1 if the command was unrecognized, and 2 if an operation
failed.

@c <<< new node on how to find an archive?  -ringo
If you use @code{tar --extract} with the
@samp{--label=@var{archive-name}} option specified, @code{tar} will
read an archive label (the tape head has to be positioned on it) and
print an error if the archive label doesn't match the
@var{archive-name} specified.  @var{archive-name} can be any regular
expression.  If the labels match, @code{tar} extracts the archive.
@xref{Archive Label}.  @xref{Matching Format Parameters}.
<<< fix cross references

@code{tar --list --label} will cause @code{tar} to print the label.

@c <<< MIB -- program to list all the labels on a tape?

@node Quick Reference, Data Format Details, Media, Top
@appendix A Quick Reference Guide to @code{tar} Operations and Options
@c  put in proper form for appendix.  (unnumbered?)

@menu
* Operations::                  A Table of Operations
* Options::                     Table of Options
@end menu

@node Operations, Options, Quick Reference, Quick Reference
@appendixsec A Table of Operations
@c add xrefs, note synonyms

The operation argument to @code{tar} specifies which action you want to
take.

@table @samp
@item -A
Adds copies of an archive or archives to the end of another archive.

@item -c
Creates a new archive.  

@item -d
Compares files in the archive with their counterparts in the file
system, and reports differences in file size, mode, owner,
modification date and contents.

@item -r
Adds files to the end of the archive.  

@item -t
Prints a list of the contents of the archive.

@item -x
Reads files from the archive and writes them into the active file
system.

@item -u
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.

@item --add-archive
Adds copies of an archive or archives to the end of another archive.

@item --add-file
Adds files to the end of the archive.  

@item --append
Adds files to the end of the archive.  

@item --catenate
Adds copies of an archive or archives to the end of another archive.

@item --compare
Compares files in the archive with their counterparts in the file
system, and reports differences in file size, mode, owner,
modification date and contents.

@item --concatenate
Adds copies of an archive or archives to the end of another archive.

@item --create
Creates a new archive.  

@item --delete
Deletes files from the archive.  All versions of the files are deleted.

@item --diff
Compares files in the archive with their counterparts in the file
system, and reports differences in file size, mode, owner,
modification date and contents.

@item --extract
Reads files from the archive and writes them into the active file
system.

@item --get
Reads files from the archive and writes them into the active file
system.

@item --help
Prints a list of @code{tar} operations and options.

@item --list
Prints a list of the contents of the archive.

@item --update
Adds files to the end of the archive, but only if they are newer than
their counterparts already in the archive, or if they do not already
exist in the archive.

@item --version
Prints the version number of the @code{tar} program to the standard
error.
@end table

@node Options,  , Operations, Quick Reference
@appendixsec Table of Options

Options change the way @code{tar} performs an operation.

@table @samp
@item --absolute-paths   
WILL BE INPUT WHEN QUESTION IS RESOLVED

@item --after-date=@var{date}
Limit the operation to files changed after the given date.
@xref{File Exclusion}.

@item --block-size=@var{number}
Specify the blocking factor of an archive.  @xref{Blocking Factor}.

@item --compress
Specify a compressed archive.  @xref{Compressed Archives}.

@item --compress-block. 
Create a whole block sized compressed archive.  @xref{Compressed Archives}.

@item --confirmation
Solicit confirmation for each file.  @xref{Interactive Operation}
<<< --selective should be a synonym. 

@item --dereference
Treat a symbolic link as an alternate name for the file the link
points to.  @xref{Symbolic Links}.

@item --directory=@file{directory}
Change the working directory.  @xref{Changing Working Directory}.

@item --exclude=@var{pattern}
Exclude files which match the regular expression @var{pattern}.
@xref{File Exclusion}.

@item --exclude-from=@file{file}
Exclude files which match any of the regular expressions listed in
the file @file{file}.  @xref{File Exclusion}.

@item --file=@var{archive-name}
Name the archive.  @xref{Archive Name}).

@item --files-from=@file{file}
Read file-name arguments from a file on the file system.
@xref{File Name Lists}. 

@item --ignore-umask
Set modes of extracted files to those recorded in the archive.
@xref{File Writing Options}.

@item --ignore-zeros
Ignore end-of-archive entries.  @xref{Archive Reading Options}.
<<< this should be changed to --ignore-end 

@item --listed-incremental=@var{file-name}   (-g)
Take a file name argument always.  If the file doesn't exist, run a level
zero dump, creating the file.  If the file exists, uses that file to see
what has changed.

@item --incremental (-G)
@c <<<look it up>>>

@item --tape-length=@var{n}  (-L)
@c <<<alternate way of doing multi archive, will go to that length and
@c prompts for new tape, automatically turns on multi-volume. >>>
@c <<< this needs to be written into main body as well -ringo

@item --info-script=@var{program-file}
Create a multi-volume archive via a script.  @xref{Multi-Volume Archives}.

@item --interactive
Ask for confirmation before performing any operation on a file or
archive member.

@item --keep-old-files
Prevent overwriting during extraction.  @xref{File Writing Options}.

@item --label=@var{archive-label}
Include an archive-label in the archive being created.  @xref{Archive
Label}.

@item --modification-time
Set the modification time of extracted files to the time they were
extracted.  @xref{File Writing Options}.

@item --multi-volume
Specify a multi-volume archive.  @xref{Multi-Volume Archives}.

@item --newer=@var{date}
Limit the operation to files changed after the given date.
@xref{File Exclusion}.

@item --newer-mtime=@var{date}
Limit the operation to files modified after the given date.  @xref{File
Exclusion}.

@item --old
Create an old format archive.  @xref{Old Style File Information}.
@c <<< did we agree this should go away as a synonym?

@item --old-archive
Create an old format archive.  @xref{Old Style File Information}.

@item --one-file-system
Prevent @code{tar} from crossing file system boundaries when
archiving.  @xref{File Exclusion}.

@item --portable
Create an old format archive.  @xref{Old Style File Information}.
@c <<< was portability, may still need to be changed

@item --preserve-order
Help process large lists of file-names on machines with small amounts of
memory.  @xref{Archive Reading Options}.

@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 --same-order
Help process large lists of file-names on machines with small amounts of
memory.  @xref{Archive Reading Options}.

@item --same-permission
Set the modes of extracted files to those recorded in the archive.
@xref{File Writing Options}.

@item --sparse
Archive sparse files sparsely.  @xref{Sparse Files}.

@item --starting-file=@var{file-name}
Begin reading in the middle of an archive.  @xref{Scarce Disk Space}.

@item --to-stdout
Write files to the standard output.  @xref{File Writing Options}.

@item --uncompress
Specifdo  a compressed archive.  @xref{Compressed Archives}.

@item -V @var{archive-label}
Include an archive-label in the archive being created.  @xref{Archive
Label}.
@c was --volume

@item --verbose
Print the names of files or archive members as they are being
operated on.  @xref{Additional Information}.

@item --verify
Check for discrepancies in the archive immediately after it is
written.  @xref{Write Verification}.

@item -B 
Read an archive with a smaller than specified block size or which
contains incomplete blocks.  @xref{Archive Reading Options}).

@item -K @var{file-name}
Begin reading in the middle of an archive.  @xref{Scarce Disk Space}.

@item -M
Specify a multi-volume archive.  @xref{Multi-Volume Archives}.

@item -N @var{date}
Limit operation to files changed after the given date.  @xref{File Exclusion}.

@item -O
Write files to the standard output.  @xref{File Writing Options}.

@c <<<<- P is absolute paths, add when resolved.  -ringo>>>

@item -R 
Print the record number where a message is generated.
@xref{Additional Information}.

@item -S
Archive sparse files sparsely.  @xref{Sparse Files}.

@item -T @var{file}
Read file-name arguments from a file on the file system.
@xref{File Name Lists}. 

@item -W
Check for discrepancies in the archive immediately after it is
written.  @xref{Write Verification}.

@item -Z
Specify a compressed archive.  @xref{Compressed Archives}.

@item -b @var{number}
Specify the blocking factor of an archive.  @xref{Blocking Factor}.

@item -f @var{archive-name}
Name the archive.  @xref{Archive Name}).

@item -h
Treat a symbolic link as an alternate name for the file the link
points to.  @xref{Symbolic Links}.

@item -i
Ignore end-of-archive entries.  @xref{Archive Reading Options}.

@item -k 
Prevent overwriting during extraction.  @xref{File Writing Options}.

@item -l
Prevent @code{tar} from crossing file system boundaries when
archiving.  @xref{File Exclusion}.

@item -m
Set the modification time of extracted files to the time they were
extracted.  @xref{File Writing Options}.

@item -o
Create an old format archive.  @xref{Old Style File Information}.

@item -p
Set the modes of extracted files to those recorded in the archive.
@xref{File Writing Options}.

@item -s
Help process large lists of file-names on machines with small amounts of
memory.  @xref{Archive Reading Options}.

@item -v
Print the names of files or archive members they are being operated
on.  @xref{Additional Information}.

@item -w
@c <<<see --interactive.  WILL BE INPUT WHEN QUESTIONS ARE RESOLVED.>>>

@item -z
Specify a compressed archive.  @xref{Compressed Archives}.

@item -z -z
Create a whole block sized compressed archive.  @xref{Compressed Archives}.
@c I would rather this were -Z.  it is the only double letter short
@c form.

@item -C @file{directory}
Change the working directory.  @xref{Changing Working Directory}.

@item -F @var{program-file}
Create a multi-volume archive via a script.  @xref{Multi-Volume Archives}.

@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

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.

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.

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.  

@menu
* Header Data::                 The Distribution of Data in the Header
* Header Fields::               The Meaning of Header Fields
* Sparse File Handling::        Fields to Handle Sparse Files
@end menu

@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.

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.

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.

The @code{longnames} field supports a feature that is not yet
implemented.  This field should be empty.

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.

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.

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}.

@menu
* File Types::                  File Types
* GNU File Types::              Additional File Types Supported by GNU
@end menu

@node File Types, GNU File Types, Header Fields, Header Fields
@appendixsubsec File Types

The following flags are used to describe file types:

@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.

@item LF_FIFO
Indicates a FIFO special file.  Note that archiving a FIFO file
archives the existence of the file and not its contents.

@item LF_CONTIG
Indicates a contiguous file.  Contiguous files are the same as normal
files except that, in operating systems that support it, all the
files' disk space is allocated contiguously.  Operating systems which
do not allow contiguous allocation should silently treat this type as
a normal file.

@item 'A' @dots{}
@itemx 'Z'
These are reserved for custom implementations.  Some of these are used
in the GNU modified format, which is described below.  @xref{GNU File
Types}.  
@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.

@node GNU File Types,  , File Types, Header Fields
@appendixsubsec Additional File Types Supported by GNU

GNU @code{tar} uses additional file types to describe new types of
files in an archive.  These are listed below.

@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.

@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.
@end table

@node Sparse File Handling,  , Header Fields, Data Format Details
@appendixsec Fields to Handle Sparse Files

The following header information was added to deal with sparse files
(@pxref{Sparse Files}):

@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.

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.

The @code{isextended} flag is only set for sparse files, and then only
if extended header records are needed when archiving the file.

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.

@c so is @code{extended_header} the right way to write this?

@node Concept Index,  , Data Format Details, Top
@unnumbered Concept Index

@printindex cp

@summarycontents
@contents
@bye