GNU tar 1.15.92
***************

GNU tar: an archiver tool
*************************


This manual is for GNU tar (version
1.15.92, 26 June 2006), which creates and extracts files
from archives.

Copyright (C) 1992, 1994, 1995, 1996, 1997, 1999, 2000, 2001,
2003, 2004, 2005, 2006 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1 or
any later version published by the Free Software Foundation; with no
Invariant Sections, with the Front-Cover Texts being "A GNU Manual,"
and with the Back-Cover Texts as in (a) below.  A copy of the license
is included in the section entitled "GNU Free Documentation License".

(a) The FSF's Back-Cover Text is: "You are free to copy and modify
this GNU Manual.  Buying copies from GNU Press supports the FSF in
developing GNU and promoting software freedom."


The first part of this master menu lists the major nodes in this Info
document.  The rest of the menu lists all the lower level nodes.


* Introduction::
* Tutorial::
* tar invocation::
* operations::
* Backups::
* Choosing::
* Date input formats::
* Formats::
* Media::

Appendices

* Changes::
* Configuring Help Summary::
* Tar Internals::
* Genfile::
* Free Software Needs Free Documentation::
* Copying This Manual::
* Index of Command Line Options::
* Index::

 -- The Detailed Node Listing --

Introduction

* Book Contents::               What this Book Contains
* Definitions::                 Some Definitions
* What tar Does::               What tar Does
* Naming tar Archives::         How tar Archives are Named
* Authors::                     GNU tar Authors
* Reports::                     Reporting bugs or suggestions

Tutorial Introduction to tar

* assumptions::
* stylistic conventions::
* basic tar options::           Basic tar Operations and Options
* frequent operations::
* Two Frequent Options::
* create::                      How to Create Archives
* list::                        How to List Archives
* extract::                     How to Extract Members from an Archive
* going further::

Two Frequently Used Options

* file tutorial::
* verbose tutorial::
* help tutorial::

How to Create Archives

* prepare for examples::
* Creating the archive::
* create verbose::
* short create::
* create dir::

How to List Archives

* list dir::

How to Extract Members from an Archive

* extracting archives::
* extracting files::
* extract dir::
* extracting untrusted archives::
* failing commands::

Invoking GNU tar

* Synopsis::
* using tar options::
* Styles::
* All Options::
* help::
* defaults::
* verbose::
* interactive::

The Three Option Styles

* Long Options::                Long Option Style
* Short Options::               Short Option Style
* Old Options::                 Old Option Style
* Mixing::                      Mixing Option Styles

All tar Options

* Operation Summary::
* Option Summary::
* Short Option Summary::

GNU tar Operations

* Basic tar::
* Advanced tar::
* create options::
* extract options::
* backup::
* Applications::
* looking ahead::

Advanced GNU tar Operations

* Operations::
* append::
* update::
* concatenate::
* delete::
* compare::

How to Add Files to Existing Archives: `--append'

* appending files::             Appending Files to an Archive
* multiple::

Updating an Archive

* how to update::

Options Used by `--create'

* override::                  Overriding File Metadata.
* Ignore Failed Read::

Options Used by `--extract'

* Reading::                     Options to Help Read Archives
* Writing::                     Changing How tar Writes Files
* Scarce::                      Coping with Scarce Resources

Options to Help Read Archives

* read full records::
* Ignore Zeros::

Changing How tar Writes Files

* Dealing with Old Files::
* Overwrite Old Files::
* Keep Old Files::
* Keep Newer Files::
* Unlink First::
* Recursive Unlink::
* Data Modification Times::
* Setting Access Permissions::
* Directory Modification Times and Permissions::
* Writing to Standard Output::
* Writing to an External Program::
* remove files::

Coping with Scarce Resources

* Starting File::
* Same Order::

Performing Backups and Restoring Files

* Full Dumps::                  Using tar to Perform Full Dumps
* Incremental Dumps::           Using tar to Perform Incremental Dumps
* Backup Levels::               Levels of Backups
* Backup Parameters::           Setting Parameters for Backups and Restoration
* Scripted Backups::            Using the Backup Scripts
* Scripted Restoration::        Using the Restore Script

Setting Parameters for Backups and Restoration

* General-Purpose Variables::
* Magnetic Tape Control::
* User Hooks::
* backup-specs example::        An Example Text of `Backup-specs'

Choosing Files and Names for tar

* file::                        Choosing the Archive's Name
* Selecting Archive Members::
* files::                       Reading Names from a File
* exclude::                     Excluding Some Files
* wildcards::                   Wildcards Patterns and Matching
* quoting styles::              Ways of Quoting Special Characters in Names
* transform::                   Modifying File and Member Names
* after::                       Operating Only on New Files
* recurse::                     Descending into Directories
* one::                         Crossing File System Boundaries

Reading Names from a File

* nul::

Excluding Some Files

* problems with exclude::

Wildcards Patterns and Matching

* controlling pattern-matching::

Crossing File System Boundaries

* directory::                   Changing Directory
* absolute::                    Absolute File Names

Date input formats

* General date syntax::            Common rules.
* Calendar date items::            19 Dec 1994.
* Time of day items::              9:20pm.
* Time zone items::                EST, PDT, GMT.
* Day of week items::              Monday and others.
* Relative items in date strings:: next tuesday, 2 years ago.
* Pure numbers in date strings::   19931219, 1440.
* Seconds since the Epoch::        @1078100502.
* Specifying time zone rules::     TZ="America/New_York", TZ="UTC0".
* Authors of get_date::            Bellovin, Eggert, Salz, Berets, et al.

Controlling the Archive Format

* Portability::                 Making tar Archives More Portable
* Compression::                 Using Less Space through Compression
* Attributes::                  Handling File Attributes
* cpio::                        Comparison of tar and cpio

Making tar Archives More Portable

* Portable Names::              Portable Names
* dereference::                 Symbolic Links
* old::                         Old V7 Archives
* ustar::                       Ustar Archives
* gnu::                         GNU and old GNU format archives.
* posix::                       POSIX archives
* Checksumming::                Checksumming Problems
* Large or Negative Values::    Large files, negative time stamps, etc.
* Other Tars::                  How to Extract GNU-Specific Data Using
                                Other tar Implementations

GNU tar and POSIX tar

* PAX keywords:: Controlling Extended Header Keywords.

How to Extract GNU-Specific Data Using Other tar Implementations

* Split Recovery::       Members Split Between Volumes
* Sparse Recovery::      Sparse Members

Using Less Space through Compression

* gzip::                        Creating and Reading Compressed Archives
* sparse::                      Archiving Sparse Files

Tapes and Other Archive Media

* Device::                      Device selection and switching
* Remote Tape Server::
* Common Problems and Solutions::
* Blocking::                    Blocking
* Many::                        Many archives on one tape
* Using Multiple Tapes::        Using Multiple Tapes
* label::                       Including a Label in the Archive
* verify::
* Write Protection::

Blocking

* Format Variations::           Format Variations
* Blocking Factor::             The Blocking Factor of an Archive

Many Archives on One Tape

* Tape Positioning::            Tape Positions and Tape Marks
* mt::                          The mt Utility

Using Multiple Tapes

* Multi-Volume Archives::       Archives Longer than One Tape or Disk
* Tape Files::                  Tape Files
* Tarcat::                      Concatenate Volumes into a Single Archive


Tar Internals

* Standard::           Basic Tar Format
* Extensions::         GNU Extensions to the Archive Format
* Sparse Formats::     Storing Sparse Files
* Snapshot Files::
* Dumpdir::

Storing Sparse Files

* Old GNU Format::
* PAX 0::                PAX Format, Versions 0.0 and 0.1
* PAX 1::                PAX Format, Version 1.0

Genfile

* Generate Mode::     File Generation Mode.
* Status Mode::       File Status Mode.
* Exec Mode::         Synchronous Execution mode.

Copying This Manual

* GNU Free Documentation License::  License for copying this manual


1 Introduction
**************

GNU tar creates
and manipulates archives which are actually collections of
many other files; the program provides users with an organized and
systematic method for controlling a large amount of data.
The name "tar" originally came from the phrase "Tape ARchive", but
archives need not (and these days, typically do not) reside on tapes.

* Book Contents::               What this Book Contains
* Definitions::                 Some Definitions
* What tar Does::               What tar Does
* Naming tar Archives::         How tar Archives are Named
* Authors::                     GNU tar Authors
* Reports::                     Reporting bugs or suggestions

1.1 What this Book Contains
===========================

The first part of this chapter introduces you to various terms that will
recur throughout the book.  It also tells you who has worked on GNU tar
and its documentation, and where you should send bug reports
or comments.

The second chapter is a tutorial (see Tutorial) which provides a
gentle introduction for people who are new to using tar.  It is
meant to be self contained, not requiring any reading from subsequent
chapters to make sense.  It moves from topic to topic in a logical,
progressive order, building on information already explained.

Although the tutorial is paced and structured to allow beginners to
learn how to use tar, it is not intended solely for beginners.
The tutorial explains how to use the three most frequently used
operations (`create', `list', and `extract') as well as
two frequently used options (`file' and `verbose').  The other
chapters do not refer to the tutorial frequently; however, if a section
discusses something which is a complex variant of a basic concept, there
may be a cross reference to that basic concept.  (The entire book,
including the tutorial, assumes that the reader understands some basic
concepts of using a Unix-type operating system; see Tutorial.)

The third chapter presents the remaining five operations, and
information about using tar options and option syntax.

  The other chapters are meant to be used as a
reference.  Each chapter presents everything that needs to be said
about a specific topic.

One of the chapters (see Date input formats) exists in its
entirety in other GNU manuals, and is mostly self-contained.
In addition, one section of this manual (see Standard) contains a
big quote which is taken directly from tar sources.

In general, we give both long and short (abbreviated) option names
at least once in each section where the relevant option is covered, so
that novice readers will become familiar with both styles.  (A few
options have no short versions, and the relevant sections will
indicate this.)

1.2 Some Definitions
====================

The tar program is used to create and manipulate tar
archives.  An archive is a single file which contains the contents
of many files, while still identifying the names of the files, their
owner(s), and so forth.  (In addition, archives record access
permissions, user and group, size in bytes, and data modification time.
Some archives also record the file names in each archived directory, as
well as other file and directory information.)  You can use tar
to create a new archive in a specified directory.

The files inside an archive are called members.  Within this
manual, we use the term file to refer only to files accessible in
the normal ways (by ls, cat, and so forth), and the term
member to refer only to the members of an archive.  Similarly, a
file name is the name of a file, as it resides in the file system,
and a member name is the name of an archive member within the
archive.

The term extraction refers to the process of copying an archive
member (or multiple members) into a file in the file system.  Extracting
all the members of an archive is often called extracting the
archive.  The term unpack can also be used to refer to the
extraction of many or all the members of an archive.  Extracting an
archive does not destroy the archive's structure, just as creating an
archive does not destroy the copies of the files that exist outside of
the archive.  You may also list the members in a given archive
(this is often thought of as "printing" them to the standard output,
or the command line), or append members to a pre-existing archive.
All of these operations can be performed using tar.

1.3 What tar Does
=================

The tar program provides the ability to create tar
archives, as well as various other kinds of manipulation.  For example,
you can use tar on previously created archives to extract files,
to store additional files, or to update or list files which were already
stored.

Initially, tar archives were used to store files conveniently on
magnetic tape.  The name tar comes from this use; it stands for
tape archiver.  Despite the utility's name, tar can
direct its output to available devices, files, or other programs (using
pipes).  tar may even access remote devices or files (as archives).

You can use tar archives in many ways.  We want to stress a few
of them: storage, backup, and transportation.


Storage
Often, tar archives are used to store related files for
convenient file transfer over a network.  For example, the
GNU Project distributes its software bundled into
tar archives, so that all the files relating to a particular
program (or set of related programs) can be transferred as a single
unit.

A magnetic tape can store several files in sequence.  However, the tape
has no names for these files; it only knows their relative position on
the tape.  One way to store several files on one tape and retain their
names is by creating a tar archive.  Even when the basic transfer
mechanism can keep track of names, as FTP can, the nuisance of handling
multiple files, directories, and multiple links makes tar
archives useful.

Archive files are also used for long-term storage.  You can think of
this as transportation from the present into the future.  (It is a
science-fiction idiom that you can move through time as well as in
space; the idea here is that tar can be used to move archives in
all dimensions, even time!)

Backup
Because the archive created by tar is capable of preserving
file information and directory structure, tar is commonly
used for performing full and incremental backups of disks.  A backup
puts a collection of files (possibly pertaining to many users and
projects) together on a disk or a tape.  This guards against
accidental destruction of the information in those files.
GNU tar has special features that allow it to be
used to make incremental and full dumps of all the files in a
file system.

Transportation
You can create an archive on one system, transfer it to another system,
and extract the contents there.  This allows you to transport a group of
files from one system to another.

1.4 How tar Archives are Named
==============================

Conventionally, tar archives are given names ending with
`.tar'.  This is not necessary for tar to operate properly,
but this manual follows that convention in order to accustom readers to
it and to make examples more clear.

Often, people refer to tar archives as "tar files," and
archive members as "files" or "entries".  For people familiar with
the operation of tar, this causes no difficulty.  However, in
this manual, we consistently refer to "archives" and "archive
members" to make learning to use tar easier for novice users.

1.5 GNU tar Authors
===================

GNU tar was originally written by John Gilmore,
and modified by many people.  The GNU enhancements were
written by Jay Fenlason, then Joy Kendall, and the whole package has
been further maintained by Thomas Bushnell, n/BSG, Franc,ois
Pinard, Paul Eggert, and finally Sergey Poznyakoff with the help of
numerous and kind users.

We wish to stress that tar is a collective work, and owes much to
all those people who reported problems, offered solutions and other
insights, or shared their thoughts and suggestions.  An impressive, yet
partial list of those contributors can be found in the `THANKS'
file from the GNU tar distribution.





Jay Fenlason put together a draft of a GNU tar
manual, borrowing notes from the original man page from John Gilmore.
This was withdrawn in version 1.11.  Thomas Bushnell, n/BSG and Amy
Gorin worked on a tutorial and manual for GNU tar.
Franc,ois Pinard put version 1.11.8 of the manual together by
taking information from all these sources and merging them.  Melissa
Weisshaus finally edited and redesigned the book to create version
1.12.  The book for versions from 1.14 up to 1.15.92 were edited
by the current maintainer, Sergey Poznyakoff.

For version 1.12, Daniel Hagerty contributed a great deal of technical
consulting.  In particular, he is the primary author of Backups.

In July, 2003 GNU tar was put on CVS at savannah.gnu.org
(see <http://savannah.gnu.org/projects/tar>), and
active development and maintenance work has started
again.  Currently GNU tar is being maintained by Paul Eggert, Sergey
Poznyakoff and Jeff Bailey.

Support for POSIX archives was added by Sergey Poznyakoff.

1.6 Reporting bugs or suggestions
=================================

If you find problems or have suggestions about this program or manual,
please report them to `bug-tar@gnu.org'.

When reporting a bug, please be sure to include as much detail as
possible, in order to reproduce it.  .

2 Tutorial Introduction to tar
******************************

This chapter guides you through some basic examples of three tar
operations: `--create', `--list', and `--extract'.  If
you already know how to use some other version of tar, then you
may not need to read this chapter.  This chapter omits most complicated
details about how tar works.

* assumptions::
* stylistic conventions::
* basic tar options::           Basic tar Operations and Options
* frequent operations::
* Two Frequent Options::
* create::                      How to Create Archives
* list::                        How to List Archives
* extract::                     How to Extract Members from an Archive
* going further::

2.1 Assumptions this Tutorial Makes
===================================

This chapter is paced to allow beginners to learn about tar
slowly.  At the same time, we will try to cover all the basic aspects of
these three operations.  In order to accomplish both of these tasks, we
have made certain assumptions about your knowledge before reading this
manual, and the hardware you will be using:

* Before you start to work through this tutorial, you should understand
what the terms "archive" and "archive member" mean
(see Definitions).  In addition, you should understand something
about how Unix-type operating systems work, and you should know how to
use some basic utilities.  For example, you should know how to create,
list, copy, rename, edit, and delete files and directories; how to
change between directories; and how to figure out where you are in the
file system.  You should have some basic understanding of directory
structure and how files are named according to which directory they are
in.  You should understand concepts such as standard output and standard
input, what various definitions of the term "argument" mean, and the
differences between relative and absolute path names.  

* This manual assumes that you are working from your own home directory
(unless we state otherwise).  In this tutorial, you will create a
directory to practice tar commands in.  When we show path names,
we will assume that those paths are relative to your home directory.
For example, my home directory path is `/home/fsf/melissa'.  All of
my examples are in a subdirectory of the directory named by that path
name; the subdirectory is called `practice'.

* In general, we show examples of archives which exist on (or can be
written to, or worked with from) a directory on a hard disk.  In most
cases, you could write those archives to, or work with them on any other
device, such as a tape drive.  However, some of the later examples in
the tutorial and next chapter will not work on tape drives.
Additionally, working with tapes is much more complicated than working
with hard disks.  For these reasons, the tutorial does not cover working
with tape drives.  See Media, for complete information on using
tar archives with tape drives.



2.2 Stylistic Conventions
=========================

In the examples, `$' represents a typical shell prompt.  It
precedes lines you should type; to make this more clear, those lines are
shown in this font, as opposed to lines which represent the
computer's response; those lines are shown in this font, or
sometimes `like this'.


2.3 Basic tar Operations and Options
====================================

tar can take a wide variety of arguments which specify and define
the actions it will have on the particular set of files or the archive.
The main types of arguments to tar fall into one of two classes:
operations, and options.

Some arguments fall into a class called operations; exactly one of
these is both allowed and required for any instance of using tar;
you may not specify more than one.  People sometimes speak of
operating modes.  You are in a particular operating mode when you
have specified the operation which specifies it; there are eight
operations in total, and thus there are eight operating modes.

The other arguments fall into the class known as options.  You are
not required to specify any options, and you are allowed to specify more
than one at a time (depending on the way you are using tar at
that time).  Some options are used so frequently, and are so useful for
helping you type commands more carefully that they are effectively
"required".  We will discuss them in this chapter.

You can write most of the tar operations and options in any
of three forms: long (mnemonic) form, short form, and old style.  Some
of the operations and options have no short or "old" forms; however,
the operations and options which we will cover in this tutorial have
corresponding abbreviations.  We will indicate those abbreviations appropriately to get
you used to seeing them.  (Note that the "old style" option forms
exist in GNU tar for compatibility with Unix
tar.  In this book we present a full discussion of this way
of writing options and operations (see Old Options), and we discuss
the other two styles of writing options (See Long Options, and
see Short Options).

In the examples and in the text of this tutorial, we usually use the
long forms of operations and options; but the "short" forms produce
the same result and can make typing long tar commands easier.
For example, instead of typing

tar --create --verbose --file=afiles.tar apple angst aspic

you can type
tar -c -v -f afiles.tar apple angst aspic

or even
tar -cvf afiles.tar apple angst aspic

For more information on option syntax, see Advanced tar.  In
discussions in the text, when we name an option by its long form, we
also give the corresponding short option in parentheses.

The term, "option", can be confusing at times, since "operations"
are often lumped in with the actual, optional "options" in certain
general class statements.  For example, we just talked about "short and
long forms of options and operations".  However, experienced tar
users often refer to these by shorthand terms such as, "short and long
options".  This term assumes that the "operations" are included, also.
Context will help you determine which definition of "options" to use.

Similarly, the term "command" can be confusing, as it is often used in
two different ways.  People sometimes refer to tar "commands".
A tar command is the entire command line of user input
which tells tar what to do -- including the operation, options,
and any arguments (file names, pipes, other commands, etc).  However,
you will also sometimes hear the term "the tar command".  When
the word "command" is used specifically like this, a person is usually
referring to the tar operation, not the whole line.
Again, use context to figure out which of the meanings the speaker
intends.

2.4 The Three Most Frequently Used Operations
=============================================

Here are the three most frequently used operations (both short and long
forms), as well as a brief description of their meanings.  The rest of
this chapter will cover how to use these operations in detail.  We will
present the rest of the operations in the next chapter.

`--create'
`-c'
Create a new tar archive.
`--list'
`-t'
List the contents of an archive.
`--extract'
`-x'
Extract one or more members from an archive.

2.5 Two Frequently Used Options
===============================

To understand how to run tar in the three operating modes listed
previously, you also need to understand how to use two of the options to
tar: `--file' (which takes an archive file as an argument)
and `--verbose'.  (You are usually not required to specify
either of these options when you run tar, but they can be very
useful in making things more clear and helping you avoid errors.)

* file tutorial::
* verbose tutorial::
* help tutorial::

The `--file' Option
-------------------

`--file=archive-name'
`-f archive-name'
Specify the name of an archive file.

You can specify an argument for the `--file=archive-name' (`-f archive-name') option whenever you
use tar; this option determines the name of the archive file
that tar will work on.

If you don't specify this argument, then tar will examine
the environment variable TAPE.  If it is set, its value will be
used as the archive name.  Otherwise, tar will use the
default archive, determined at the compile time. Usually it is
standard output or some physical tape drive attached to your machine
(you can verify what the default is by running tar
--show-defaults, see defaults).  If there is no tape drive
attached, or the default is not meaningful, then tar will
print an error message.  The error message might look roughly like one
of the following:

tar: can't open /dev/rmt8 : No such device or address
tar: can't open /dev/rsmt0 : I/O error

To avoid confusion, we recommend that you always specify an archive file
name by using `--file=archive-name' (`-f archive-name') when writing your tar commands.
For more information on using the `--file=archive-name' (`-f archive-name') option, see
file.

The `--verbose' Option
----------------------

`--verbose'
`-v'
Show the files being worked on as tar is running.

`--verbose' (`-v') shows details about the results of running
tar.  This can be especially useful when the results might not be
obvious.  For example, if you want to see the progress of tar as
it writes files into the archive, you can use the `--verbose'
option.  In the beginning, you may find it useful to use
`--verbose' at all times; when you are more accustomed to
tar, you will likely want to use it at certain times but not at
others.  We will use `--verbose' at times to help make something
clear, and we will give many examples both using and not using
`--verbose' to show the differences.

Each instance of `--verbose' on the command line increases the
verbosity level by one, so if you need more details on the output, 
specify it twice.

When reading archives (`--list', `--extract',
`--diff'), tar by default prints only the names of
the members being extracted.  Using `--verbose' will show a full,
ls style member listing.

In contrast, when writing archives (`--create', `--append',
`--update'), tar does not print file names by
default.  So, a single `--verbose' option shows the file names
being added to the archive, while two `--verbose' options
enable the full listing.
   
For example, to create an archive in verbose mode:

$ tar -cvf afiles.tar apple angst aspic
apple
angst
aspic

Creating the same archive with the verbosity level 2 could give:

$ tar -cvvf afiles.tar apple angst aspic
-rw-r--r-- gray/staff    62373 2006-06-09 12:06 apple
-rw-r--r-- gray/staff    11481 2006-06-09 12:06 angst
-rw-r--r-- gray/staff    23152 2006-06-09 12:06 aspic

This works equally well using short or long forms of options.  Using
long forms, you would simply write out the mnemonic form of the option
twice, like this:

$ tar --create --verbose --verbose ...

Note that you must double the hyphens properly each time.

Later in the tutorial, we will give examples using `--verbose
--verbose'.


The full output consists of six fields:

* File type and permissions in symbolic form.
These are displayed in the same format as the first column of
ls -l output (see Verbose listing in GNU file utilities).

* Owner name and group separated by a slash character.
If these data are not available (for example, when listing a `v7' format
archive), numeric ID values are printed instead.

* Size of the file, in bytes.

* File modification date in ISO 8601 format.

* File modification time.

* File name.
If the name contains any special characters (white space, newlines,
etc.) these are displayed in an unambiguous form using so called
quoting style.  For the detailed discussion of available styles
and on how to use them, see quoting styles.

Depending on the file type, the name can be followed by some
additional information, described in the following table:

`-> link-name'
The file or archive member is a symbolic link and
link-name is the name of file it links to.

`link to link-name'
The file or archive member is a hard link and link-name is
the name of file it links to.

`--Long Link--'
The archive member is an old GNU format long link.  You will normally
not encounter this.

`--Long Name--'
The archive member is an old GNU format long name.  You will normally
not encounter this.

`--Volume Header--'
The archive member is a GNU volume header (see Tape Files).

`--Continued at byte n--'
Encountered only at the beginning of a multy-volume archive
(see Using Multiple Tapes).  This archive member is a continuation
from the previous volume. The number n gives the offset where
the original file was split.  

`--Mangled file names--'
This archive member contains mangled file names declarations,
a special member type that was used by early versions of GNU tar.
You probably will never encounter this, unless you are reading a very
old archive.

`unknown file type c'
An archive member of unknown type. c is the type character from
the archive header.  If you encounter such a message, it means that
either your archive contains proprietary member types GNU tar is not
able to handle, or the archive is corrupted.


For example, here is an archive listing containing most of the special
suffixes explained above:

V--------- 0/0          1536 2006-06-09 13:07 MyVolume--Volume Header--
-rw-r--r-- gray/staff 456783 2006-06-09 12:06 aspic--Continued at
byte 32456--
-rw-r--r-- gray/staff  62373 2006-06-09 12:06 apple
lrwxrwxrwx gray/staff      0 2006-06-09 13:01 angst -> apple
-rw-r--r-- gray/staff  35793 2006-06-09 12:06 blues
hrw-r--r-- gray/staff      0 2006-06-09 12:06 music link to blues


Getting Help: Using the `--help' Option
---------------------------------------

`--help'

The `--help' option to tar prints out a very brief list of
all operations and option available for the current version of
tar available on your system.

2.6 How to Create Archives
==========================
(This message will disappear, once this node revised.)

One of the basic operations of tar is `--create' (`-c'), which
you use to create a tar archive.  We will explain
`--create' first because, in order to learn about the other
operations, you will find it useful to have an archive available to
practice on.

To make this easier, in this section you will first create a directory
containing three files.  Then, we will show you how to create an
archive (inside the new directory).  Both the directory, and
the archive are specifically for you to practice on.  The rest of this
chapter and the next chapter will show many examples using this
directory and the files you will create: some of those files may be
other directories and other archives.

The three files you will archive in this example are called
`blues', `folk', and `jazz'.  The archive is called
`collection.tar'.

This section will proceed slowly, detailing how to use `--create'
in verbose mode, and showing examples using both short and long
forms.  In the rest of the tutorial, and in the examples in the next
chapter, we will proceed at a slightly quicker pace.  This section
moves more slowly to allow beginning users to understand how
tar works.

* prepare for examples::
* Creating the archive::
* create verbose::
* short create::
* create dir::

2.6.1 Preparing a Practice Directory for Examples
-------------------------------------------------

To follow along with this and future examples, create a new directory
called `practice' containing files called `blues', `folk'
and `jazz'.  The files can contain any information you like:
ideally, they should contain information which relates to their names,
and be of different lengths.  Our examples assume that `practice'
is a subdirectory of your home directory.

Now cd to the directory named `practice'; `practice'
is now your working directory.  (Please note: Although
the full path name of this directory is
`/homedir/practice', in our examples we will refer to
this directory as `practice'; the homedir is presumed.

In general, you should check that the files to be archived exist where
you think they do (in the working directory) by running ls.
Because you just created the directory and the files and have changed to
that directory, you probably don't need to do that this time.

It is very important to make sure there isn't already a file in the
working directory with the archive name you intend to use (in this case,
`collection.tar'), or that you don't care about its contents.
Whenever you use `create', tar will erase the current
contents of the file named by `--file=archive-name' (`-f archive-name') if it exists.  tar
will not tell you if you are about to overwrite an archive unless you
specify an option which does this (see backup, for the
information on how to do so).  To add files to an existing archive,
you need to use a different option, such as `--append' (`-r'); see
append for information on how to do this.

2.6.2 Creating the Archive
--------------------------

To place the files `blues', `folk', and `jazz' into an
archive named `collection.tar', use the following command:

$ tar --create --file=collection.tar blues folk jazz

The order of the arguments is not very important, when using long
option forms.  You could also say:

$ tar blues --create folk --file=collection.tar jazz

However, you can see that this order is harder to understand; this is
why we will list the arguments in the order that makes the commands
easiest to understand (and we encourage you to do the same when you use
tar, to avoid errors).

Note that the sequence
`--file=collection.tar' is considered to be one argument.
If you substituted any other string of characters for
collection.tar,  then that string would become the name of the
archive file you create.

The order of the options becomes more important when you begin to use
short forms.  With short forms, if you type commands in the wrong order
(even if you type them correctly in all other ways), you may end up with
results you don't expect.  For this reason, it is a good idea to get
into the habit of typing options in the order that makes inherent sense.
See short create, for more information on this.

In this example, you type the command as shown above: `--create'
is the operation which creates the new archive
(`collection.tar'), and `--file' is the option which lets
you give it the name you chose.  The files, `blues', `folk',
and `jazz', are now members of the archive, `collection.tar'
(they are file name arguments to the `--create' operation.
See Choosing, for the detailed discussion on these.) Now that they are
in the archive, they are called archive members, not files.
(see members).

When you create an archive, you must specify which files you
want placed in the archive.  If you do not specify any archive
members, GNU tar will complain.

If you now list the contents of the working directory (ls), you will
find the archive file listed as well as the files you saw previously:

blues   folk   jazz   collection.tar

Creating the archive `collection.tar' did not destroy the copies of
the files in the directory.

Keep in mind that if you don't indicate an operation, tar will not
run and will prompt you for one.  If you don't name any files, tar
will complain.  You must have write access to the working directory,
or else you will not be able to create an archive in that directory.

Caution: Do not attempt to use `--create' (`-c') to add files to
an existing archive; it will delete the archive and write a new one.
Use `--append' (`-r') instead.  See append.

2.6.3 Running `--create' with `--verbose'
-----------------------------------------

If you include the `--verbose' (`-v') option on the command line,
tar will list the files it is acting on as it is working.  In
verbose mode, the create example above would appear as:

$ tar --create --verbose --file=collection.tar blues folk jazz
blues
folk
jazz

This example is just like the example we showed which did not use
`--verbose', except that tar generated the remaining lines

In the rest of the examples in this chapter, we will frequently use
verbose mode so we can show actions or tar responses that
you would otherwise not see, and which are important for you to
understand.

2.6.4 Short Forms with `create'
-------------------------------

As we said before, the `--create' (`-c') operation is one of the most
basic uses of tar, and you will use it countless times.
Eventually, you will probably want to use abbreviated (or "short")
forms of options.  A full discussion of the three different forms that
options can take appears in Styles; for now, here is what the
previous example (including the `--verbose' (`-v') option) looks like
using short option forms:

$ tar -cvf collection.tar blues folk jazz
blues
folk
jazz

As you can see, the system responds the same no matter whether you use
long or short option forms.

 One difference between using
short and long option forms is that, although the exact placement of
arguments following options is no more specific when using short forms,
it is easier to become confused and make a mistake when using short
forms.  For example, suppose you attempted the above example in the
following way:

$ tar -cfv collection.tar blues folk jazz

In this case, tar will make an archive file called `v',
containing the files `blues', `folk', and `jazz', because
the `v' is the closest "file name" to the `-f' option, and
is thus taken to be the chosen archive file name.  tar will try
to add a file called `collection.tar' to the `v' archive file;
if the file `collection.tar' did not already exist, tar will
report an error indicating that this file does not exist.  If the file
`collection.tar' does already exist (e.g., from a previous command
you may have run), then tar will add this file to the archive.
Because the `-v' option did not get registered, tar will not
run under `verbose' mode, and will not report its progress.

The end result is that you may be quite confused about what happened,
and possibly overwrite a file.  To illustrate this further, we will show
you how an example we showed previously would look using short forms.

This example,

$ tar blues --create folk --file=collection.tar jazz

is confusing as it is.  When shown using short forms, however, it
becomes much more so:

$ tar blues -c folk -f collection.tar jazz

It would be very easy to put the wrong string of characters
immediately following the `-f', but doing that could sacrifice
valuable data.

For this reason, we recommend that you pay very careful attention to
the order of options and placement of file and archive names,
especially when using short option forms.  Not having the option name
written out mnemonically can affect how well you remember which option
does what, and therefore where different names have to be placed.

2.6.5 Archiving Directories
---------------------------

You can archive a directory by specifying its directory name as a
file name argument to tar.  The files in the directory will be
archived relative to the working directory, and the directory will be
re-created along with its contents when the archive is extracted.

To archive a directory, first move to its superior directory.  If you
have followed the previous instructions in this tutorial, you should
type:

$ cd ..
$

This will put you into the directory which contains `practice',
i.e., your home directory.  Once in the superior directory, you can
specify the subdirectory, `practice', as a file name argument.  To
store `practice' in the new archive file `music.tar', type:

$ tar --create --verbose --file=music.tar practice

tar should output:

practice/
practice/blues
practice/folk
practice/jazz
practice/collection.tar

Note that the archive thus created is not in the subdirectory
`practice', but rather in the current working directory--the
directory from which tar was invoked.  Before trying to archive a
directory from its superior directory, you should make sure you have
write access to the superior directory itself, not only the directory
you are trying archive with tar.  For example, you will probably
not be able to store your home directory in an archive by invoking
tar from the root directory; See absolute.  (Note
also that `collection.tar', the original archive file, has itself
been archived.  tar will accept any file as a file to be
archived, regardless of its content.  When `music.tar' is
extracted, the archive file `collection.tar' will be re-written
into the file system).

If you give tar a command such as

$ tar --create --file=foo.tar .

tar will report `tar: ./foo.tar is the archive; not
dumped'.  This happens because tar creates the archive
`foo.tar' in the current directory before putting any files into
it.  Then, when tar attempts to add all the files in the
directory `.' to the archive, it notices that the file
`./foo.tar' is the same as the archive `foo.tar', and skips
it.  (It makes no sense to put an archive into itself.)  GNU tar
will continue in this case, and create the archive
normally, except for the exclusion of that one file.  (Please
note: Other implementations of tar may not be so clever;
they will enter an infinite loop when this happens, so you should not
depend on this behavior unless you are certain you are running
GNU tar.  In general, it is wise to always place the archive outside
of the directory being dumped.

2.7 How to List Archives
========================

Frequently, you will find yourself wanting to determine exactly what a
particular archive contains.  You can use the `--list'
(`-t') operation to get the member names as they currently
appear in the archive, as well as various attributes of the files at
the time they were archived.  For example, you can examine the archive
`collection.tar' that you created in the last section with the
command, 

$ tar --list --file=collection.tar

The output of tar would then be:

blues
folk
jazz

The archive `bfiles.tar' would list as follows:

./birds
baboon
./box

Be sure to use a `--file=archive-name' (`-f
archive-name') option just as with `--create'
(`-c') to specify the name of the archive.

If you use the `--verbose' (`-v') option with
`--list', then tar will print out a listing
reminiscent of `ls -l', showing owner, file size, and so
forth.  This output is described in detail in verbose member listing.

If you had used `--verbose' (`-v') mode, the example
above would look like:

$ tar --list --verbose --file=collection.tar folk
-rw-r--r-- myself user 62 1990-05-23 10:55 folk


It is important to notice that the output of tar --list
--verbose does not necessarily match that produced by tar
--create --verbose while creating the archive.  It is because
GNU tar, unless told explicitly not to do so, removes some directory
prefixes from file names before storing them in the archive
(See absolute, for more information).  In other
words, in verbose mode GNU tar shows file names when creating
an archive and member names when listing it.  Consider this
example:

$ tar cfv archive /etc/mail
tar: Removing leading `/' from member names
/etc/mail/
/etc/mail/sendmail.cf
/etc/mail/aliases
$ tar tf archive
etc/mail/
etc/mail/sendmail.cf
etc/mail/aliases

  This default behavior can sometimes be inconvenient.  You can force
GNU tar show member names when creating archive by supplying
`--show-stored-names' option.

`--show-stored-names'
Print member (as opposed to file) names when creating the archive.

You can specify one or more individual member names as arguments when
using `list'.  In this case, tar will only list the
names of members you identify.  For example, tar --list
--file=afiles.tar apple would only print `apple'.

Because tar preserves paths, file names must be specified as
they appear in the archive (i.e., relative to the directory from which
the archive was created).  Therefore, it is essential when specifying
member names to tar that you give the exact member names.
For example, tar --list --file=bfiles.tar birds would produce an
error message something like `tar: birds: Not found in archive',
because there is no member named `birds', only one named
`./birds'.  While the names `birds' and `./birds' name
the same file, member names by default are compared verbatim.

However, tar --list --file=bfiles.tar baboon would respond
with `baboon', because this exact member name is in the archive file
`bfiles.tar'.  If you are not sure of the exact file name,
use globbing patterns, for example:

$ tar --list --file=bfiles.tar --wildcards '*b*'

will list all members whose name contains `b'.  See wildcards,
for a detailed discussion of globbing patterns and related
tar command line options.

* list dir::

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
`--list' (`-t').  To find out file attributes, include the
`--verbose' (`-v') option.

For example, to find out about files in the directory `practice', in
the archive file `music.tar', type:

$ tar --list --verbose --file=music.tar practice

tar responds:

drwxrwxrwx myself user 0 1990-05-31 21:49 practice/
-rw-r--r-- myself user 42 1990-05-21 13:29 practice/blues
-rw-r--r-- myself user 62 1990-05-23 10:55 practice/folk
-rw-r--r-- myself user 40 1990-05-21 13:30 practice/jazz
-rw-r--r-- myself user 10240 1990-05-31 21:49 practice/collection.tar

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

2.8 How to Extract Members from an Archive
==========================================
(This message will disappear, once this node revised.)

Creating an archive is only half the job--there is no point in storing
files in an archive if you can't retrieve them.  The act of retrieving
members from an archive so they can be used and manipulated as
unarchived files again is called extraction.  To extract files
from an archive, use the `--extract' (`--get' or
`-x') operation.  As with `--create', specify the name
of the archive with `--file' (`-f') option. Extracting
an archive does not modify the archive in any way; you can extract it
multiple times if you want or need to.

Using `--extract', you can extract an entire archive, or specific
files.  The files can be directories containing other files, or not.  As
with `--create' (`-c') and `--list' (`-t'), you may use the short or the
long form of the operation without affecting the performance.

* extracting archives::
* extracting files::
* extract dir::
* extracting untrusted archives::
* failing commands::

2.8.1 Extracting an Entire Archive
----------------------------------

To extract an entire archive, specify the archive file name only, with
no individual file names as arguments.  For example,

$ tar -xvf collection.tar

produces this:

-rw-r--r-- me user     28 1996-10-18 16:31 jazz
-rw-r--r-- me user     21 1996-09-23 16:44 blues
-rw-r--r-- me user     20 1996-09-23 16:44 folk

2.8.2 Extracting Specific Files
-------------------------------

To extract specific archive members, give their exact member names as
arguments, as printed by `--list' (`-t').  If you had
mistakenly deleted one of the files you had placed in the archive
`collection.tar' earlier (say, `blues'), you can extract it
from the archive without changing the archive's structure.  Its
contents will be identical to the original file `blues' that you
deleted. 

First, make sure you are in the `practice' directory, and list the
files in the directory.  Now, delete the file, `blues', and list
the files in the directory again.

You can now extract the member `blues' from the archive file
`collection.tar' like this:

$ tar --extract --file=collection.tar blues

If you list the files in the directory again, you will see that the file
`blues' has been restored, with its original permissions, data
modification times, and owner.(1)  (These parameters will be identical to those which
the file had when you originally placed it in the archive; any changes
you may have made before deleting the file from the file system,
however, will not have been made to the archive member.)  The
archive file, `collection.tar', is the same as it was before you
extracted `blues'.  You can confirm this by running tar with
`--list' (`-t').

Remember that as with other operations, specifying the exact member
name is important.  tar --extract --file=bfiles.tar birds
will fail, because there is no member named `birds'.  To extract
the member named `./birds', you must specify tar
--extract --file=bfiles.tar ./birds.  If you don't remember the
exact member names, use `--list' (`-t') option
(see list).  You can also extract those members that match a
specific globbing pattern.  For example, to extract from
`bfiles.tar' all files that begin with `b', no matter their
directory prefix, you could type:

$ tar -x -f bfiles.tar --wildcards --no-anchored 'b*'

Here, `--wildcards' instructs tar to treat
command line arguments as globbing patterns and `--no-anchored'
informs it that the patterns apply to member names after any `/'
delimiter.  The use of globbing patterns is discussed in detail in
See wildcards. 

You can extract a file to standard output by combining the above options
with the `--to-stdout' (`-O') option (see Writing to Standard Output).

If you give the `--verbose' option, then `--extract'
will print the names of the archive members as it extracts them.

2.8.3 Extracting Files that are Directories
-------------------------------------------

Extracting directories which are members of an archive is similar to
extracting other files.  The main difference to be aware of is that if
the extracted directory has the same name as any directory already in
the working directory, then files in the extracted directory will be
placed into the directory of the same name.  Likewise, if there are
files in the pre-existing directory with the same names as the members
which you extract, the files from the extracted archive will replace
the files already in the working directory (and possible
subdirectories).  This will happen regardless of whether or not the
files in the working directory were more recent than those extracted
(there exist, however, special options that alter this behavior
see Writing).

However, if a file was stored with a directory name as part of its file
name, and that directory does not exist under the working directory when
the file is extracted, tar will create the directory.

We can demonstrate how to use `--extract' to extract a directory
file with an example.  Change to the `practice' directory if you
weren't there, and remove the files `folk' and `jazz'.  Then,
go back to the parent directory and extract the archive
`music.tar'.  You may either extract the entire archive, or you may
extract only the files you just deleted.  To extract the entire archive,
don't give any file names as arguments after the archive name
`music.tar'.  To extract only the files you deleted, use the
following command:

$ tar -xvf music.tar practice/folk practice/jazz
practice/folk
practice/jazz

If you were to specify two `--verbose' (`-v') options, tar
would have displayed more detail about the extracted files, as shown
in the example below:

$ tar -xvvf music.tar practice/folk practice/jazz
-rw-r--r-- me user     28 1996-10-18 16:31 practice/jazz
-rw-r--r-- me user     20 1996-09-23 16:44 practice/folk

Because you created the directory with `practice' as part of the
file names of each of the files by archiving the `practice'
directory as `practice', you must give `practice' as part
of the file names when you extract those files from the archive.

2.8.4 Extracting Archives from Untrusted Sources
------------------------------------------------

Extracting files from archives can overwrite files that already exist.
If you receive an archive from an untrusted source, you should make a
new directory and extract into that directory, so that you don't have
to worry about the extraction overwriting one of your existing files.
For example, if `untrusted.tar' came from somewhere else on the
Internet, and you don't necessarily trust its contents, you can
extract it as follows:

$ mkdir newdir
$ cd newdir
$ tar -xvf ../untrusted.tar

It is also a good practice to examine contents of the archive
before extracting it, using `--list' (`-t') option, possibly combined
with `--verbose' (`-v').

2.8.5 Commands That Will Fail
-----------------------------

Here are some sample commands you might try which will not work, and why
they won't work.

If you try to use this command,

$ tar -xvf music.tar folk jazz

you will get the following response:

tar: folk: Not found in archive
tar: jazz: Not found in archive
$

This is because these files were not originally in the parent
directory `..', where the archive is located; they were in the
`practice' directory, and their file names reflect this:

$ tar -tvf music.tar
practice/folk
practice/jazz
practice/rock



Likewise, if you try to use this command,

$ tar -tvf music.tar folk jazz

you would get a similar response.  Members with those names are not in the
archive.  You must use the correct member names, or wildcards, in order
to extract the files from the archive.

If you have forgotten the correct names of the files in the archive,
use tar --list --verbose to list them correctly.



2.9 Going Further Ahead in this Manual
======================================



3 Invoking GNU tar
******************
(This message will disappear, once this node revised.)

This chapter is about how one invokes the GNU tar
command, from the command synopsis (see Synopsis).  There are
numerous options, and many styles for writing them.  One mandatory
option specifies the operation tar should perform
(see Operation Summary), other options are meant to detail how
this operation should be performed (see Option Summary).
Non-option arguments are not always interpreted the same way,
depending on what the operation is.

You will find in this chapter everything about option styles and rules for
writing them (see Styles).  On the other hand, operations and options
are fully described elsewhere, in other chapters.  Here, you will find
only synthetic descriptions for operations and options, together with
pointers to other parts of the tar manual.

Some options are so special they are fully described right in this
chapter.  They have the effect of inhibiting the normal operation of
tar or else, they globally alter the amount of feedback the user
receives about what is going on.  These are the `--help' and
`--version' (see help), `--verbose' (see verbose)
and `--interactive' options (see interactive).

* Synopsis::
* using tar options::
* Styles::
* All Options::
* help::
* defaults::
* verbose::
* interactive::

3.1 General Synopsis of tar
===========================

The GNU tar program is invoked as either one of:

tar option... [name]...
tar letter... [argument]... [option]... [name]...

The second form is for when old options are being used.

You can use tar to store files in an archive, to extract them from
an archive, and to do other types of archive manipulation.  The primary
argument to tar, which is called the operation, specifies
which action to take.  The other arguments to tar are either
options, which change the way tar performs an operation,
or file names or archive members, which specify the files or members
tar is to act on.

You can actually type in arguments in any order, even if in this manual
the options always precede the other arguments, to make examples easier
to understand.  Further, the option stating the main operation mode
(the tar main command) is usually given first.

Each name in the synopsis above is interpreted as an archive member
name when the main command is one of `--compare'
(`--diff', `-d'), `--delete', `--extract'
(`--get', `-x'), `--list' (`-t') or
`--update' (`-u').  When naming archive members, you
must give the exact name of the member in the archive, as it is
printed by `--list'.  For `--append' (`-r') and
`--create' (`-c'), these name arguments specify
the names of either files or directory hierarchies to place in the archive.
These files or hierarchies should already exist in the file system,
prior to the execution of the tar command.

tar interprets relative file names as being relative to the
working directory.  tar will make all file names relative
(by removing leading slashes when archiving or restoring files),
unless you specify otherwise (using the `--absolute-names'
option).  See absolute, for more information about
`--absolute-names'.

If you give the name of a directory as either a file name or a member
name, then tar acts recursively on all the files and directories
beneath that directory.  For example, the name `/' identifies all
the files in the file system to tar.

The distinction between file names and archive member names is especially
important when shell globbing is used, and sometimes a source of confusion
for newcomers.  See wildcards, for more information about globbing.
The problem is that shells may only glob using existing files in the
file system.  Only tar itself may glob on archive members, so when
needed, you must ensure that wildcard characters reach tar without
being interpreted by the shell first.  Using a backslash before `*'
or `?', or putting the whole argument between quotes, is usually
sufficient for this.

Even if names are often specified on the command line, they
can also be read from a text file in the file system, using the
`--files-from=file-of-names' (`-T file-of-names') option.

If you don't use any file name arguments, `--append' (`-r'),
`--delete' and `--concatenate' (`--catenate',
`-A') will do nothing, while `--create' (`-c')
will usually yield a diagnostic and inhibit tar execution.
The other operations of tar (`--list',
`--extract', `--compare', and `--update')
will act on the entire contents of the archive.

Besides successful exits, GNU tar may fail for
many reasons.  Some reasons correspond to bad usage, that is, when the
tar command is improperly written.  Errors may be
encountered later, while encountering an error processing the archive
or the files.  Some errors are recoverable, in which case the failure
is delayed until tar has completed all its work.  Some
errors are such that it would not meaningful, or at least risky, to
continue processing: tar then aborts processing immediately.
All abnormal exits, whether immediate or delayed, should always be
clearly diagnosed on stderr, after a line stating the nature of
the error.

GNU tar returns only a few exit statuses.  I'm really
aiming simplicity in that area, for now.  If you are not using the
`--compare' `--diff', `-d') option, zero means
that everything went well, besides maybe innocuous warnings.  Nonzero
means that something went wrong. Right now, as of today, "nonzero"
is almost always 2, except for remote operations, where it may be
128.

3.2 Using tar Options
=====================

GNU tar has a total of eight operating modes which
allow you to perform a variety of tasks.  You are required to choose
one operating mode each time you employ the tar program by
specifying one, and only one operation as an argument to the
tar command (two lists of four operations each may be found
at frequent operations and Operations).  Depending on
circumstances, you may also wish to customize how the chosen operating
mode behaves.  For example, you may wish to change the way the output
looks, or the format of the files that you wish to archive may require
you to do something special in order to make the archive look right.

You can customize and control tar's performance by running
tar with one or more options (such as `--verbose'
(`-v'), which we used in the tutorial).  As we said in the
tutorial, options are arguments to tar which are (as
their name suggests) optional. Depending on the operating mode, you
may specify one or more options. Different options will have different
effects, but in general they all change details of the operation, such
as archive format, archive name, or level of user interaction.  Some
options make sense with all operating modes, while others are
meaningful only with particular modes. You will likely use some
options frequently, while you will only use others infrequently, or
not at all.  (A full list of options is available in see All Options.)


The TAR_OPTIONS environment variable specifies default options to
be placed in front of any explicit options.  For example, if
TAR_OPTIONS is `-v --unlink-first', tar behaves as
if the two options `-v' and `--unlink-first' had been
specified before any explicit options.  Option specifications are
separated by whitespace.  A backslash escapes the next character, so it
can be used to specify an option containing whitespace or a backslash.

Note that tar options are case sensitive.  For example, the
options `-T' and `-t' are different; the first requires an
argument for stating the name of a file providing a list of names,
while the second does not require an argument and is another way to
write `--list' (`-t').

In addition to the eight operations, there are many options to
tar, and three different styles for writing both: long (mnemonic)
form, short form, and old style.  These styles are discussed below.
Both the options and the operations can be written in any of these three
styles.



3.3 The Three Option Styles
===========================

There are three styles for writing operations and options to the command
line invoking tar.  The different styles were developed at
different times during the history of tar.  These styles will be
presented below, from the most recent to the oldest.

Some options must take an argument.  (For example, `--file'
(`-f')) takes the name of an archive file as an argument.  If
you do not supply an archive file name, tar will use a
default, but this can be confusing; thus, we recommend that you always
supply a specific archive file name.)  Where you place the
arguments generally depends on which style of options you choose.  We
will detail specific information relevant to each option style in the
sections on the different option styles, below.  The differences are
subtle, yet can often be very important; incorrect option placement
can cause you to overwrite a number of important files.  We urge you
to note these differences, and only use the option style(s) which
makes the most sense to you until you feel comfortable with the others.

Some options may take an argument.  Such options may have at
most long and short forms, they do not have old style equivalent.  The
rules for specifying an argument for such options are stricter than
those for specifying mandatory arguments.  Please, pay special
attention to them. 

* Long Options::                Long Option Style
* Short Options::               Short Option Style
* Old Options::                 Old Option Style
* Mixing::                      Mixing Option Styles

3.3.1 Long Option Style
-----------------------

Each option has at least one long (or mnemonic) name starting with two
dashes in a row, e.g., `--list'.  The long names are more clear than
their corresponding short or old names.  It sometimes happens that a
single long option has many different different names which are
synonymous, such as `--compare' and `--diff'.  In addition,
long option names can be given unique abbreviations.  For example,
`--cre' can be used in place of `--create' because there is no
other long option which begins with `cre'.  (One way to find
this out is by trying it and seeing what happens; if a particular
abbreviation could represent more than one option, tar will tell
you that that abbreviation is ambiguous and you'll know that that
abbreviation won't work.  You may also choose to run `tar --help'
to see a list of options.  Be aware that if you run tar with a
unique abbreviation for the long name of an option you didn't want to
use, you are stuck; tar will perform the command as ordered.)

Long options are meant to be obvious and easy to remember, and their
meanings are generally easier to discern than those of their
corresponding short options (see below).  For example:

$ tar --create --verbose --blocking-factor=20 --file=/dev/rmt0

gives a fairly good set of hints about what the command does, even
for those not fully acquainted with tar.

Long options which require arguments take those arguments
immediately following the option name.  There are two ways of
specifying a mandatory argument.  It can be separated from the
option name either by an equal sign, or by any amount of
white space characters.  For example, the `--file' option (which
tells the name of the tar archive) is given a file such as
`archive.tar' as argument by using any of the following notations:
`--file=archive.tar' or `--file archive.tar'.

In contrast, optional arguments must always be introduced using
an equal sign.  For example, the `--backup' option takes
an optional argument specifying backup type.  It must be used
as `--backup=backup-type'.

3.3.2 Short Option Style
------------------------

Most options also have a short option name.  Short options start with
a single dash, and are followed by a single character, e.g., `-t'
(which is equivalent to `--list').  The forms are absolutely
identical in function; they are interchangeable.

The short option names are faster to type than long option names.

Short options which require arguments take their arguments immediately
following the option, usually separated by white space.  It is also
possible to stick the argument right after the short option name, using
no intervening space.  For example, you might write `-f
archive.tar' or `-farchive.tar' instead of using
`--file=archive.tar'.  Both `--file=archive-name' and
`-f archive-name' denote the option which indicates a
specific archive, here named `archive.tar'.

Short options which take optional arguments take their arguments
immediately following the option letter, without any intervening
white space characters.

Short options' letters may be clumped together, but you are not
required to do this (as compared to old options; see below).  When
short options are clumped as a set, use one (single) dash for them
all, e.g., `tar -cvf'.  Only the last option in
such a set is allowed to have an argument(2).

When the options are separated, the argument for each option which requires
an argument directly follows that option, as is usual for Unix programs.
For example:

$ tar -c -v -b 20 -f /dev/rmt0

If you reorder short options' locations, be sure to move any arguments
that belong to them.  If you do not move the arguments properly, you may
end up overwriting files.

3.3.3 Old Option Style
----------------------
(This message will disappear, once this node revised.)

Like short options, old options are single letters.  However, old options
must be written together as a single clumped set, without spaces separating
them or dashes preceding them(3).  This set
of letters must be the first to appear on the command line, after the
tar program name and some white space; old options cannot appear
anywhere else.  The letter of an old option is exactly the same letter as
the corresponding short option.  For example, the old option `t' is
the same as the short option `-t', and consequently, the same as the
long option `--list'.  So for example, the command `tar
cv' specifies the option `-v' in addition to the operation `-c'.

When options that need arguments are given together with the command,
all the associated arguments follow, in the same order as the options.
Thus, the example given previously could also be written in the old
style as follows:

$ tar cvbf 20 /dev/rmt0

Here, `20' is the argument of `-b' and `/dev/rmt0' is
the argument of `-f'.

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 `tar cvbf 20 /dev/rmt0', for example,
`20' is the argument for `-b', `/dev/rmt0' is the
argument for `-f', and `-v' does not have a corresponding
argument.  Even using short options like in `tar -c -v -b 20 -f
/dev/rmt0' is clearer, putting all arguments next to the option they
pertain to.

If you want to reorder the letters in the old option argument, be
sure to reorder any corresponding argument appropriately.

This old way of writing tar options can surprise even experienced
users.  For example, the two commands:

tar cfz archive.tar.gz file
tar -cfz archive.tar.gz file

are quite different.  The first example uses `archive.tar.gz' as
the value for option `f' and recognizes the option `z'.  The
second example, however, uses `z' as the value for option
`f' -- probably not what was intended.

Old options are kept for compatibility with old versions of tar.

This second example could be corrected in many ways, among which the
following are equivalent:

tar -czf archive.tar.gz file
tar -cf archive.tar.gz -z file
tar cf archive.tar.gz -z file

As far as we know, all tar programs, GNU and
non-GNU, support old options.  GNU tar
supports them not only for historical reasons, but also because many
people are used to them.  For compatibility with Unix tar,
the first argument is always treated as containing command and option
letters even if it doesn't start with `-'.  Thus, `tar c' is
equivalent to `tar -c': both of them specify the
`--create' (`-c') command to create an archive.

3.3.4 Mixing Option Styles
--------------------------

All three styles may be intermixed in a single tar command,
so long as the rules for each style are fully
respected(4).  Old style options and either of the modern styles of
options may be mixed within a single tar command.  However,
old style options must be introduced as the first arguments only,
following the rule for old options (old options must appear directly
after the tar command and some white space).  Modern options
may be given only after all arguments to the old options have been
collected.  If this rule is not respected, a modern option might be
falsely interpreted as the value of the argument to one of the old
style options.

For example, all the following commands are wholly equivalent, and
illustrate the many combinations and orderings of option styles.

tar --create --file=archive.tar
tar --create -f archive.tar
tar --create -farchive.tar
tar --file=archive.tar --create
tar --file=archive.tar -c
tar -c --file=archive.tar
tar -c -f archive.tar
tar -c -farchive.tar
tar -cf archive.tar
tar -cfarchive.tar
tar -f archive.tar --create
tar -f archive.tar -c
tar -farchive.tar --create
tar -farchive.tar -c
tar c --file=archive.tar
tar c -f archive.tar
tar c -farchive.tar
tar cf archive.tar
tar f archive.tar --create
tar f archive.tar -c
tar fc archive.tar

On the other hand, the following commands are not equivalent to
the previous set:

tar -f -c archive.tar
tar -fc archive.tar
tar -fcarchive.tar
tar -farchive.tarc
tar cfarchive.tar

These last examples mean something completely different from what the
user intended (judging based on the example in the previous set which
uses long options, whose intent is therefore very clear).  The first
four specify that the tar archive would be a file named
`-c', `c', `carchive.tar' or `archive.tarc',
respectively.  The first two examples also specify a single non-option,
name argument having the value `archive.tar'.  The last
example contains only old style option letters (repeating option
`c' twice), not all of which are meaningful (eg., `.',
`h', or `i'), with no argument value.  

3.4 All tar Options
===================

The coming manual sections contain an alphabetical listing of all
tar operations and options, with brief descriptions and cross
references to more in-depth explanations in the body of the manual.
They also contain an alphabetically arranged table of the short option
forms with their corresponding long option.  You can use this table as
a reference for deciphering tar commands in scripts.

* Operation Summary::
* Option Summary::
* Short Option Summary::

3.4.1 Operations
----------------

`--append'
`-r'

Appends files to the end of the archive.  See append.

`--catenate'
`-A'

Same as `--concatenate'.  See concatenate.

`--compare'
`-d'

Compares archive members with their counterparts in the file
system, and reports differences in file size, mode, owner,
modification date and contents.  See compare.

`--concatenate'
`-A'

Appends other tar archives to the end of the archive.
See concatenate.

`--create'
`-c'

Creates a new tar archive.  See create.

`--delete'

Deletes members from the archive.  Don't try this on a archive on a
tape!  See delete.

`--diff'
`-d'

Same `--compare'.  See compare.

`--extract'
`-x'

Extracts members from the archive into the file system.  See extract.

`--get'
`-x'

Same as `--extract'.  See extract.

`--list'
`-t'

Lists the members in an archive.  See list.

`--update'
`-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. See update.


3.4.2 tar Options
-----------------

`--absolute-names'
`-P'

Normally when creating an archive, tar strips an initial
`/' from member names.  This option disables that behavior.
See absolute.

`--after-date'

(See `--newer', see after)

`--anchored'
A pattern must match an initial subsequence of the name's components.
See controlling pattern-matching.

`--atime-preserve'
`--atime-preserve=replace'
`--atime-preserve=system'

Attempt to preserve the access time of files when reading them.  This
option currently is effective only on files that you own, unless you
have superuser privileges.

`--atime-preserve=replace' remembers the access time of a file
before reading it, and then restores the access time afterwards.  This
may cause problems if other programs are reading the file at the same
time, as the times of their accesses will be lost.  On most platforms
restoring the access time also requires tar to restore the
data modification time too, so this option may also cause problems if
other programs are writing the file at the same time.  (Tar attempts
to detect this situation, but cannot do so reliably due to race
conditions.)  Worse, on most platforms restoring the access time also
updates the status change time, which means that this option is
incompatible with incremental backups.

`--atime-preserve=system' avoids changing time stamps on files,
without interfering with time stamp updates
caused by other programs, so it works better with incremental backups.
However, it requires a special O_NOATIME option from the
underlying operating and file system implementation, and it also requires
that searching directories does not update their access times.  As of
this writing (November 2005) this works only with Linux, and only with
Linux kernels 2.6.8 and later.  Worse, there is currently no reliable
way to know whether this feature actually works.  Sometimes
tar knows that it does not work, and if you use
`--atime-preserve=system' then tar complains and
exits right away.  But other times tar might think that the
option works when it actually does not.

Currently `--atime-preserve' with no operand defaults to
`--atime-preserve=replace', but this may change in the future
as support for `--atime-preserve=system' improves.

If your operating system does not support
`--atime-preserve=system', you might be able to preserve access
times reliably by by using the mount command.  For example,
you can mount the file system read-only, or access the file system via
a read-only loopback mount, or use the `noatime' mount option
available on some systems.  However, mounting typically requires
superuser privileges and can be a pain to manage.

`--backup=backup-type'

Rather than deleting files from the file system, tar will
back them up using simple or numbered backups, depending upon
backup-type.  See backup.

`--block-number'
`-R'

With this option present, tar prints error messages for read errors
with the block number in the archive file.  See block-number.

`--blocking-factor=blocking'
`-b blocking'

Sets the blocking factor tar uses to blocking x 512 bytes per
record.  See Blocking Factor.

`--bzip2'
`-j'

This option tells tar to read or write archives through
bzip2.  See gzip.

`--checkpoint[=number]'

This option directs tar to print periodic checkpoint
messages as it reads through the archive.  It is intended for when you
want a visual indication that tar is still running, but
don't want to see `--verbose' output.  For a detailed
description, see Progress information.

`--check-links'
`-l'
If this option was given, tar will check the number of links
dumped for each processed file.  If this number does not match the
total number of hard links for the file, a warning message will be
output (5). 

`--compress'
`--uncompress'
`-Z'

tar will use the compress program when reading or
writing the archive.  This allows you to directly act on archives
while saving space.  See gzip.

`--confirmation'

(See `--interactive'.)  See interactive.

`--delay-directory-restore'

Delay setting modification times and permissions of extracted
directories until the end of extraction. See Directory Modification Times and Permissions.

`--dereference'
`-h'

When creating a tar archive, tar will archive the
file that a symbolic link points to, rather than archiving the
symlink.  See dereference.

`--directory=dir'
`-C dir'

When this option is specified, tar will change its current directory
to dir before performing any operations.  When this option is used
during archive creation, it is order sensitive.  See directory.

`--exclude=pattern'

When performing operations, tar will skip files that match
pattern.  See exclude.

`--exclude-from=file'
`-X file'

Similar to `--exclude', except tar will use the list of
patterns in the file file.  See exclude.

`--exclude-caches'

Automatically excludes all directories
containing a cache directory tag.  See exclude.

`--file=archive'
`-f archive'

tar will use the file archive as the tar archive it
performs operations on, rather than tar's compilation dependent
default.  See file tutorial.

`--files-from=file'
`-T file'

tar will use the contents of file as a list of archive members
or files to operate on, in addition to those specified on the
command-line.  See files.

`--force-local'

Forces tar to interpret the filename given to `--file'
as a local file, even if it looks like a remote tape drive name.
See local and remote archives.

`--format=format'
`-H format'

Selects output archive format.  Format may be one of the
following:

`v7'
Creates an archive that is compatible with Unix V7 tar.

`oldgnu'
Creates an archive that is compatible with GNU tar version
1.12 or earlier.

`gnu'
Creates archive in GNU tar 1.13 format.  Basically it is the same as
`oldgnu' with the only difference in the way it handles long
numeric fields.

`ustar'
Creates a POSIX.1-1988 compatible archive.

`posix'
Creates a POSIX.1-2001 archive.


See Formats, for a detailed discussion of these formats.

`--group=group'

Files added to the tar archive will have a group id of group,
rather than the group from the source file.  group is first decoded
as a group symbolic name, but if this interpretation fails, it has to be
a decimal numeric group ID.  See override.

Also see the comments for the `--owner=user' option.

`--gzip'
`--gunzip'
`--ungzip'
`-z'

This option tells tar to read or write archives through
gzip, allowing tar to directly operate on several
kinds of compressed archives transparently.  See gzip.

`--help'
`-?'

tar will print out a short message summarizing the operations and
options to tar and exit. See help.

`--ignore-case'
Ignore case when matching member or file names with
patterns. See controlling pattern-matching. 

`--ignore-command-error'
Ignore exit codes of subprocesses. See Writing to an External Program.

`--ignore-failed-read'

Do not exit unsuccessfully merely because an unreadable file was encountered.
See Reading.

`--ignore-zeros'
`-i'

With this option, tar will ignore zeroed blocks in the
archive, which normally signals EOF.  See Reading.

`--incremental'
`-G'

Used to inform tar that it is working with an old
GNU-format incremental backup archive.  It is intended
primarily for backwards compatibility only.  See Incremental Dumps,
for a detailed discussion of incremental archives.

`--index-file=file'

Send verbose output to file instead of to standard output.

`--info-script=script-file'
`--new-volume-script=script-file'
`-F script-file'

When tar is performing multi-tape backups, script-file is run
at the end of each tape.  If script-file exits with nonzero status,
tar fails immediately.  See info-script, for a detailed
discussion of script-file.

`--interactive'
`--confirmation'
`-w'

Specifies that tar should ask the user for confirmation before
performing potentially destructive options, such as overwriting files.
See interactive.

`--keep-newer-files'

Do not replace existing files that are newer than their archive copies
when extracting files from an archive.

`--keep-old-files'
`-k'

Do not overwrite existing files when extracting files from an archive.
See Keep Old Files.

`--label=name'
`-V name'

When creating an archive, instructs tar to write name
as a name record in the archive.  When extracting or listing archives,
tar will only operate on archives that have a label matching
the pattern specified in name.  See Tape Files.

`--listed-incremental=snapshot-file'
`-g snapshot-file'

During a `--create' operation, specifies that the archive that
tar creates is a new GNU-format incremental
backup, using snapshot-file to determine which files to backup.
With other operations, informs tar that the archive is in
incremental format.  See Incremental Dumps.

`--mode=permissions'

When adding files to an archive, tar will use
permissions for the archive members, rather than the permissions
from the files.  permissions can be specified either as an octal
number or as symbolic permissions, like with
chmod. See override.

`--mtime=date'

When adding files to an archive, tar will use date as
the modification time of members when creating archives, instead of
their actual modification times.  The value of date can be
either a textual date representation (see Date input formats) or a
name of the existing file, starting with `/' or `.'.  In the
latter case, the modification time of that file is used. See override.

`--multi-volume'
`-M'

Informs tar that it should create or otherwise operate on a
multi-volume tar archive.  See Using Multiple Tapes.

`--new-volume-script'

(see -info-script)

`--seek'
`-n'

Assume that the archive media supports seeks to arbitrary
locations.  Usually tar determines automatically whether
the archive can be seeked or not.  This option is intended for use
in cases when such recognition fails.

`--newer=date'
`--after-date=date'
`-N'

When creating an archive, tar will only add files that have changed
since date.  If date begins with `/' or `.', it
is taken to be the name of a file whose data modification time specifies
the date.  See after.

`--newer-mtime=date'

Like `--newer', but add only files whose
contents have changed (as opposed to just `--newer', which will
also back up files for which any status information has
changed).  See after.

`--no-anchored'
An exclude pattern can match any subsequence of the name's components.
See controlling pattern-matching.

`--no-delay-directory-restore'

Setting modification times and permissions of extracted
directories when all files from this directory has been
extracted. This is the default. See Directory Modification Times and Permissions.

`--no-ignore-case'
Use case-sensitive matching.
See controlling pattern-matching.

`--no-ignore-command-error'
Print warnings about subprocesses terminated with a non-zero exit
code. See Writing to an External Program.

`--no-overwrite-dir'

Preserve metadata of existing directories when extracting files
from an archive.  See Overwrite Old Files.

`--no-quote-chars=string'
Remove characters listed in string from the list of quoted
characters set by the previous `--quote-chars' option
(see quoting styles).

`--no-recursion'

With this option, tar will not recurse into directories.
See recurse.

`--no-same-owner'
`-o'

When extracting an archive, do not attempt to preserve the owner
specified in the tar archive.  This the default behavior
for ordinary users.

`--no-same-permissions'

When extracting an archive, subtract the user's umask from files from
the permissions specified in the archive.  This is the default behavior
for ordinary users.

`--no-unquote'
Treat all input file or member names literally, do not interpret
escape sequences.  See input name quoting.

`--no-wildcards'
Do not use wildcards.
See controlling pattern-matching.

`--no-wildcards-match-slash'
Wildcards do not match `/'.
See controlling pattern-matching.

`--null'

When tar is using the `--files-from' option, this option
instructs tar to expect filenames terminated with `NUL', so
tar can correctly work with file names that contain newlines.
See nul.

`--numeric-owner'

This option will notify tar that it should use numeric user
and group IDs when creating a tar file, rather than names.
See Attributes.

`-o'
The function of this option depends on the action tar is
performing.  When extracting files, `-o' is a synonym for
`--no-same-owner', i.e.  it prevents tar from
restoring ownership of files being extracted.

When creating an archive, it is a synonym for
`--old-archive'.  This behavior is for compatibility
with previous versions of GNU tar, and will be
removed in the future releases.

See Changes, for more information.

`--occurrence[=number]'

This option can be used in conjunction with one of the subcommands
`--delete', `--diff', `--extract' or
`--list' when a list of files is given either on the command
line or via `-T' option.

This option instructs tar to process only the numberth
occurrence of each named file.  Number defaults to 1, so

tar -x -f archive.tar --occurrence filename

will extract the first occurrence of the member `filename' from `archive.tar'
and will terminate without scanning to the end of the archive.

`--old-archive'
Synonym for `--format=v7'.

`--one-file-system'
Used when creating an archive.  Prevents tar from recursing into
directories that are on different file systems from the current
directory (6).

`--overwrite'

Overwrite existing files and directory metadata when extracting files
from an archive.  See Overwrite Old Files.

`--overwrite-dir'

Overwrite the metadata of existing directories when extracting files
from an archive.  See Overwrite Old Files.

`--owner=user'

Specifies that tar should use user as the owner of members
when creating archives, instead of the user associated with the source
file.  user is first decoded as a user symbolic name, but if
this interpretation fails, it has to be a decimal numeric user ID.
See override.

This option does not affect extraction from archives.

`--transform=sed-expr'

Transform file or member names using sed replacement expression
sed-expr.  For example,

$ tar cf archive.tar --transform 's,^\./,usr/,' .

will add to `archive' files from the current working directory,
replacing initial `./' prefix with `usr/'. For the detailed
discussion, See transform.

To see transformed member names in verbose listings, use
`--show-transformed-names' option
(see show-transformed-names).  

`--quote-chars=string'
Always quote characters from string, even if the selected
quoting style would not quote them (see quoting styles).

`--quoting-style=style'
Set quoting style to use when printing member and file names
(see quoting styles). Valid style values are:
literal, shell, shell-always, c,
escape, locale, and clocale. Default quoting
style is escape, unless overridden while configuring the
package.

`--pax-option=keyword-list'
This option is meaningful only with POSIX.1-2001 archives
(see posix).  It modifies the way tar handles the
extended header keywords.  Keyword-list is a comma-separated
list of keyword options.  See PAX keywords, for a detailed
discussion.

`--portability'
`--old-archive'
Synonym for `--format=v7'.

`--posix'
Same as `--format=posix'.

`--preserve'

Synonymous with specifying both `--preserve-permissions' and
`--same-order'.  See Setting Access Permissions.

`--preserve-order'

(See `--same-order'; see Reading.)

`--preserve-permissions'
`--same-permissions'
`-p'

When tar is extracting an archive, it normally subtracts the
users' umask from the permissions specified in the archive and uses
that number as the permissions to create the destination file.
Specifying this option instructs tar that it should use the
permissions directly from the archive.  See Setting Access Permissions.

`--read-full-records'
`-B'

Specifies that tar should reblock its input, for reading
from pipes on systems with buggy implementations.  See Reading.

`--record-size=size'

Instructs tar to use size bytes per record when accessing the
archive.  See Blocking Factor.

`--recursion'

With this option, tar recurses into directories.
See recurse.

`--recursive-unlink'

Remove existing
directory hierarchies before extracting directories of the same name
from the archive.  See Recursive Unlink.

`--remove-files'

Directs tar to remove the source file from the file system after
appending it to an archive.  See remove files.

`--restrict'

Disable use of some potentially harmful tar options.
Currently this option disables shell invocaton from multi-volume menu
(see Using Multiple Tapes).

`--rmt-command=cmd'

Notifies tar that it should use cmd instead of
the default `/usr/libexec/rmt' (see Remote Tape Server).

`--rsh-command=cmd'

Notifies tar that is should use cmd to communicate with remote
devices.  See Device.

`--same-order'
`--preserve-order'
`-s'

This option is an optimization for tar when running on machines with
small amounts of memory.  It informs tar that the list of file
arguments has already been sorted to match the order of files in the
archive.  See Reading.

`--same-owner'

When extracting an archive, tar will attempt to preserve the owner
specified in the tar archive with this option present.
This is the default behavior for the superuser; this option has an
effect only for ordinary users.  See Attributes.

`--same-permissions'

(See `--preserve-permissions'; see Setting Access Permissions.)

`--show-defaults'

Displays the default options used by tar and exits
successfully.  This option is intended for use in shell scripts.
Here is an example of what you can see using this option:

$ tar --show-defaults
--format=gnu -f- -b20 --quoting-style=escape \
--rmt-command=/usr/libexec/rmt --rsh-command=/usr/bin/rsh

`--show-omitted-dirs'

Instructs tar to mention directories its skipping over when
operating on a tar archive.  See show-omitted-dirs.

`--show-transformed-names'
`--show-stored-names'

Display file or member names after applying any transformations
(see transform).  In particular, when used in conjunction with one of
archive creation operations it instructs tar to list the member names
stored in the archive, as opposed to the actual file
names.  See listing member and file names.

`--sparse'
`-S'

Invokes a GNU extension when adding files to an archive that handles
sparse files efficiently.  See sparse.

`--sparse-version=version'

Specified the format version to use when archiving sparse
files.  Implies `--sparse'.  See sparse. For the description
of the supported sparse formats, See Sparse Formats.

`--starting-file=name'
`-K name'

This option affects extraction only; tar will skip extracting
files in the archive until it finds one that matches name.
See Scarce.

`--strip-components=number'
Strip given number of leading components from file names before
extraction.(7) For example, if archive `archive.tar' contained
`/some/file/name', then running

tar --extract --file archive.tar --strip-components=2

would extract this file to file `name'.

`--suffix=suffix'

Alters the suffix tar uses when backing up files from the default
`~'.  See backup.

`--tape-length=num'
`-L num'

Specifies the length of tapes that tar is writing as being
num x 1024 bytes long.  See Using Multiple Tapes.

`--test-label'

Reads the volume label.  If an argument is specified, test whether it
matches the volume label.  See --test-label option.

`--to-command=command'

During extraction tar will pipe extracted files to the
standard input of command.  See Writing to an External Program.

`--to-stdout'
`-O'

During extraction, tar will extract files to stdout rather
than to the file system.  See Writing to Standard Output.

`--totals[=signo]'

Displays the total number of bytes transferred when processing an
archive.  If an argument is given, these data are displayed on
request, when signal signo is delivered to tar.
See totals.

`--touch'
`-m'

Sets the data modification time of extracted files to the extraction time,
rather than the data modification time stored in the archive.
See Data Modification Times.

`--uncompress'

(See `--compress'. see gzip)

`--ungzip'

(See `--gzip'. see gzip)

`--unlink-first'
`-U'

Directs tar to remove the corresponding file from the file
system before extracting it from the archive.  See Unlink First.

`--unquote'
Enable unquoting input file or member names (default).  See input name quoting.

`--use-compress-program=prog'

Instructs tar to access the archive through prog, which is
presumed to be a compression program of some sort.  See gzip.

`--utc'

Display file modification dates in UTC.  This option implies
`--verbose'.

`--verbose'
`-v'

Specifies that tar should be more verbose about the operations its
performing.  This option can be specified multiple times for some
operations to increase the amount of information displayed.
See verbose.

`--verify'
`-W'

Verifies that the archive was correctly written when creating an
archive.  See verify.

`--version'

Print information about the program's name, version, origin and legal
status, all on standard output, and then exit successfully.
See help.

`--volno-file=file'

Used in conjunction with `--multi-volume'.  tar will
keep track of which volume of a multi-volume archive its working in
file.  See volno-file.

`--wildcards'
Use wildcards when matching member names with patterns.
See controlling pattern-matching.

`--wildcards-match-slash'
Wildcards match `/'.
See controlling pattern-matching.

3.4.3 Short Options Cross Reference
-----------------------------------

Here is an alphabetized list of all of the short option forms, matching
them with the equivalent long option.

 Short Option Reference
 -A --concatenate.
 -B --read-full-records.
 -C --directory.
 -F --info-script.
 -G --incremental.
 -K --starting-file.
 -L --tape-length.
 -M --multi-volume.
 -N --newer.
 -O --to-stdout.
 -P --absolute-names.
 -R --block-number.
 -S --sparse.
 -T --files-from.
 -U --unlink-first.
 -V --label.
 -W --verify.
 -X --exclude-from.
 -Z --compress.
 -b --blocking-factor.
 -c --create.
 -d --compare.
 -f --file.
 -g --listed-incremental.
 -h --dereference.
 -i --ignore-zeros.
 -j --bzip2.
 -k --keep-old-files.
 -l --check-links.
 -m --touch.
 -o When creating, --no-same-owner, when extracting --
--portability. 

The later usage is deprecated.  It is retained for compatibility with
the earlier versions of GNU tar.  In the future releases
`-o' will be equivalent to `--no-same-owner' only.
 -p --preserve-permissions.
 -r --append.
 -s --same-order.
 -t --list.
 -u --update.
 -v --verbose.
 -w --interactive.
 -x --extract.
 -z --gzip.

3.5 GNU tar documentation
=========================

Being careful, the first thing is really checking that you are using
GNU tar, indeed.  The `--version' option
causes tar to print information about its name, version,
origin and legal status, all on standard output, and then exit
successfully.  For example, `tar --version' might print:

tar (GNU tar) 1.15.92
Copyright (C) 2006 Free Software Foundation, Inc.
This is free software.  You may redistribute copies of it under the terms
of the GNU General Public License <http://www.gnu.org/licenses/gpl.html>.
There is NO WARRANTY, to the extent permitted by law.

Written by John Gilmore and Jay Fenlason.

The first occurrence of `tar' in the result above is the program
name in the package (for example, rmt is another program),
while the second occurrence of `tar' is the name of the package
itself, containing possibly many programs.  The package is currently
named `tar', after the name of the main program it
contains(8).

Another thing you might want to do is checking the spelling or meaning
of some particular tar option, without resorting to this
manual, for once you have carefully read it.  GNU tar
has a short help feature, triggerable through the
`--help' option.  By using this option, tar will
print a usage message listing all available options on standard
output, then exit successfully, without doing anything else and
ignoring all other options.  Even if this is only a brief summary, it
may be several screens long.  So, if you are not using some kind of
scrollable window, you might prefer to use something like:

$ tar --help | less

presuming, here, that you like using less for a pager.  Other
popular pagers are more and pg.  If you know about some
keyword which interests you and do not want to read all the
`--help' output, another common idiom is doing:

tar --help | grep keyword

for getting only the pertinent lines.  Notice, however, that some
tar options have long description lines and the above
command will list only the first of them.

The exact look of the option summary displayed by tar --help is
configurable. See Configuring Help Summary, for a detailed description.

If you only wish to check the spelling of an option, running tar
--usage may be a better choice.  This will display a terse list of
tar option without accompanying explanations.

The short help output is quite succinct, and you might have to get
back to the full documentation for precise points.  If you are reading
this paragraph, you already have the tar manual in some
form.  This manual is available in a variety of forms from
<http://www.gnu.org/software/tar/manual>.  It may be printed out of the GNU tar
distribution, provided you have TeX already installed somewhere,
and a laser printer around.  Just configure the distribution, execute
the command `make dvi', then print `doc/tar.dvi' the
usual way (contact your local guru to know how).  If GNU tar
has been conveniently installed at your place, this
manual is also available in interactive, hypertextual form as an Info
file.  Just call `info tar' or, if you do not have the
info program handy, use the Info reader provided within
GNU Emacs, calling `tar' from the main Info menu.

There is currently no man page for GNU tar.
If you observe such a man page on the system you are running,
either it does not belong to GNU tar, or it has not
been produced by GNU.  Some package maintainers convert
tar --help output to a man page, using help2man.  In
any case, please bear in mind that the authoritative source of
information about GNU tar is this Texinfo documentation.

3.6 Obtaining GNU tar default values
====================================

GNU tar has some predefined defaults that are used when you do not
explicitely specify another values.  To obtain a list of such
defaults, use `--show-defaults' option.  This will output the
values in the form of tar command line options:

tar --show-defaults
--format=gnu -f- -b20 --quoting-style=escape 
--rmt-command=/etc/rmt --rsh-command=/usr/bin/rsh

Notice, that this option outputs only one line.  The example output above
has been split to fit page boundaries.

The above output shows that this version of GNU tar defaults to
using `gnu' archive format (see Formats), it uses standard
output as the archive, if no `--file' option has been given
(see file tutorial), the default blocking factor is 20
(see Blocking Factor).  It also shows the default locations where
tar will look for rmt and rsh binaries.

3.7 Checking tar progress
=========================

Typically, tar performs most operations without reporting any
information to the user except error messages.  When using tar
with many options, particularly ones with complicated or
difficult-to-predict behavior, it is possible to make serious mistakes.
tar provides several options that make observing tar
easier.  These options cause tar to print information as it
progresses in its job, and you might want to use them just for being
more careful about what is going on, or merely for entertaining
yourself.  If you have encountered a problem when operating on an
archive, however, you may need more information than just an error
message in order to solve the problem.  The following options can be
helpful diagnostic tools.

Normally, the `--list' (`-t') command to list an archive
prints just the file names (one per line) and the other commands are
silent. When used with most operations, the `--verbose'
(`-v') option causes tar to print the name of each
file or archive member as it is processed.  This and the other options
which make tar print status information can be useful in
monitoring tar.

With `--create' or `--extract', `--verbose' used
once just prints the names of the files or members as they are processed.
Using it twice causes tar to print a longer listing
(See verbose member listing, for the description) for each member.
Since `--list' already prints  the names of the members,
`--verbose' used once with `--list' causes tar
to print an `ls -l' type listing of the files in the archive.
The following examples both extract members with long list output:

$ tar --extract --file=archive.tar --verbose --verbose
$ tar xvvf archive.tar

Verbose output appears on the standard output except when an archive is
being written to the standard output, as with `tar --create
--file=- --verbose' (`tar cfv -', or even `tar cv'--if the
installer let standard output be the default archive).  In that case
tar writes verbose output to the standard error stream.

If `--index-file=file' is specified, tar sends
verbose output to file rather than to standard output or standard
error.


The `--totals' option causes tar to print on the
standard error the total amount of bytes transferred when processing
an archive.  When creating or appending to an archive, this option
prints the number of bytes written to the archive and the average
speed at which they have been written, e.g.:

$ tar -c -f archive.tar --totals /home
Total bytes written: 7924664320 (7.4GiB, 85MiB/s)

When reading an archive, this option displays the number of bytes
read:

$ tar -x -f archive.tar --totals
Total bytes read: 7924664320 (7.4GiB, 95MiB/s)

Finally, when deleting from an archive, the `--totals' option
displays both numbers plus number of bytes removed from the archive:

$ tar --delete -f foo.tar --totals --wildcards '*~'
Total bytes read: 9543680 (9.2MiB, 201MiB/s)
Total bytes written: 3829760 (3.7MiB, 81MiB/s)
Total bytes deleted: 1474048

You can also obtain this information on request.  When
`--totals' is used with an argument, this argument is
interpreted as a symbolic name of a signal, upon delivery of which the
statistics is to be printed:

`--totals=signo'
Print statistics upon delivery of signal signo.  Valid arguments
are: SIGHUP, SIGQUIT, SIGINT, SIGUSR1 and
SIGUSR2.  Shortened names without `SIG' prefix are also
accepted. 

Both forms of `--totals' option can be used simultaneously.
Thus, tar -x --totals --totals=USR1 instructs tar to
extract all members from its default archive and print statistics
after finishing the extraction, as well as when receiving signal
SIGUSR1. 


The `--checkpoint' option prints an occasional message
as tar reads or writes the archive.  It is designed for
those who don't need the more detailed (and voluminous) output of
`--block-number' (`-R'), but do want visual confirmation
that tar is actually making forward progress.  By default it
prints a message each 10 records read or written.  This can be changed
by giving it a numeric argument after an equal sign:

$ tar -c --checkpoint=1000 /var
tar: Write checkpoint 1000
tar: Write checkpoint 2000
tar: Write checkpoint 3000

This example shows the default checkpoint message used by
tar.  If you place a dot immediately after the equal
sign, it will print a `.' at each checkpoint.  For example:

$ tar -c --checkpoint=.1000 /var
...


The `--show-omitted-dirs' option, when reading an archive--with
`--list' or `--extract', for example--causes a message
to be printed for each directory in the archive which is skipped.
This happens regardless of the reason for skipping: the directory might
not have been named on the command line (implicitly or explicitly),
it might be excluded by the use of the
`--exclude=pattern' option, or some other reason.


If `--block-number' (`-R') is used, tar prints, along with
every message it would normally produce, the block number within the
archive where the message was triggered.  Also, supplementary messages
are triggered when reading blocks full of NULs, or when hitting end of
file on the archive.  As of now, if the archive if properly terminated
with a NUL block, the reading of the file may stop before end of file
is met, so the position of end of file will not usually show when
`--block-number' (`-R') is used.  Note that GNU tar
drains the archive before exiting when reading the
archive from a pipe.

This option is especially useful when reading damaged archives, since
it helps pinpoint the damaged sections.  It can also be used with
`--list' (`-t') 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).  See backup.

3.8 Asking for Confirmation During Operations
=============================================

Typically, 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 (see Choosing), or by performing
an operation interactively, using the `--interactive' (`-w') option.
tar also accepts `--confirmation' for this option.

When the `--interactive' (`-w') option is specified, before
reading, writing, or deleting files, tar first prints a message
for each such file, telling what operation it intends to take, then asks
for confirmation on the terminal.  The actions which require
confirmation include adding a file to the archive, extracting a file
from the archive, deleting a file from the archive, and deleting a file
from disk.  To confirm the action, you must type a line of input
beginning with `y'.  If your input line begins with anything other
than `y', tar skips that file.

If tar is reading the archive from the standard input,
tar opens the file `/dev/tty' to support the interactive
communications.

Verbose output is normally sent to standard output, separate from
other error messages.  However, if the archive is produced directly
on standard output, then verbose output is mixed with errors on
stderr.  Producing the archive on standard output may be used
as a way to avoid using disk space, when the archive is soon to be
consumed by another process reading it, say.  Some people felt the need
of producing an archive on stdout, still willing to segregate between
verbose output and error output.  A possible approach would be using a
named pipe to receive the archive, and having the consumer process to
read from that named pipe.  This has the advantage of letting standard
output free to receive verbose output, all separate from errors.

4 GNU tar Operations
********************

* Basic tar::
* Advanced tar::
* create options::
* extract options::
* backup::
* Applications::
* looking ahead::

4.1 Basic GNU tar Operations
============================

The basic tar operations, `--create' (`-c'),
`--list' (`-t') and `--extract' (`--get',
`-x'), are currently presented and described in the tutorial
chapter of this manual.  This section provides some complementary notes
for these operations.

`--create'
`-c'

Creating an empty archive would have some kind of elegance.  One can
initialize an empty archive and later use `--append'
(`-r') for adding all members.  Some applications would not
welcome making an exception in the way of adding the first archive
member.  On the other hand, many people reported that it is
dangerously too easy for tar to destroy a magnetic tape with
an empty archive(9).  The two most common errors are:

1. Mistakingly using create instead of extract, when the
intent was to extract the full contents of an archive.  This error
is likely: keys c and x are right next to each other on
the QWERTY keyboard.  Instead of being unpacked, the archive then
gets wholly destroyed.  When users speak about exploding an
archive, they usually mean something else :-).

2. Forgetting the argument to file, when the intent was to create
an archive with a single file in it.  This error is likely because a
tired user can easily add the f key to the cluster of option
letters, by the mere force of habit, without realizing the full
consequence of doing so.  The usual consequence is that the single
file, which was meant to be saved, is rather destroyed.

So, recognizing the likelihood and the catastrophical nature of these
errors, GNU tar now takes some distance from elegance, and
cowardly refuses to create an archive when `--create' option is
given, there are no arguments besides options, and
`--files-from' (`-T') option is not used.  To get
around the cautiousness of GNU tar and nevertheless create an
archive with nothing in it, one may still use, as the value for the
`--files-from' option, a file with no names in it, as shown in
the following commands:

tar --create --file=empty-archive.tar --files-from=/dev/null
tar cfT empty-archive.tar /dev/null

`--extract'
`--get'
`-x'

A socket is stored, within a GNU tar archive, as a pipe.

``--list' (`-t')'

GNU tar now shows dates as `1996-08-30',
while it used to show them as `Aug 30 1996'. Preferably,
people should get used to ISO 8601 dates.  Local American dates should
be made available again with full date localization support, once
ready.  In the meantime, programs not being localizable for dates
should prefer international dates, that's really the way to go.

Look up <http://www.cl.cam.ac.uk/~mgk25/iso-time.html> if you
are curious, it contains a detailed explanation of the ISO 8601 standard.


4.2 Advanced GNU tar Operations
===============================

Now that you have learned the basics of using GNU tar, you may want
to learn about further ways in which tar can help you.

This chapter presents five, more advanced operations which you probably
won't use on a daily basis, but which serve more specialized functions.
We also explain the different styles of options and why you might want
to use one or another, or a combination of them in your tar
commands.  Additionally, this chapter includes options which allow you to
define the output from tar more carefully, and provide help and
error correction in special circumstances.



* Operations::
* append::
* update::
* concatenate::
* delete::
* compare::

4.2.1 The Five Advanced tar Operations
--------------------------------------
(This message will disappear, once this node revised.)

In the last chapter, you learned about the first three operations to
tar.  This chapter presents the remaining five operations to
tar: `--append', `--update', `--concatenate',
`--delete', and `--compare'.

You are not likely to use these operations as frequently as those
covered in the last chapter; however, since they perform specialized
functions, they are quite useful when you do need to use them.  We
will give examples using the same directory and files that you created
in the last chapter.  As you may recall, the directory is called
`practice', the files are `jazz', `blues', `folk',
`rock', and the two archive files you created are
`collection.tar' and `music.tar'.

We will also use the archive files `afiles.tar' and
`bfiles.tar'.  The archive `afiles.tar' contains the members `apple',
`angst', and `aspic'; `bfiles.tar' contains the members
`./birds', `baboon', and `./box'.

Unless we state otherwise, all practicing you do and examples you follow
in this chapter will take place in the `practice' directory that
you created in the previous chapter; see prepare for examples.
(Below in this section, we will remind you of the state of the examples
where the last chapter left them.)

The five operations that we will cover in this chapter are:

`--append'
`-r'
Add new entries to an archive that already exists.
`--update'
`-r'
Add more recent copies of archive members to the end of an archive, if
they exist.
`--concatenate'
`--catenate'
`-A'
Add one or more pre-existing archives to the end of another archive.
`--delete'
Delete items from an archive (does not work on tapes).
`--compare'
`--diff'
`-d'
Compare archive members to their counterparts in the file system.

4.2.2 How to Add Files to Existing Archives: `--append'
-------------------------------------------------------
(This message will disappear, once this node revised.)

If you want to add files to an existing archive, you don't need to
create a new archive; you can use `--append' (`-r').
The archive must already exist in order to use `--append'.  (A
related operation is the `--update' operation; you can use this
to add newer versions of archive members to an existing archive.  To learn how to
do this with `--update', see update.)

If you use `--append' to add a file that has the same name as an
archive member to an archive containing that archive member, then the
old member is not deleted.  What does happen, however, is somewhat
complex.  tar allows you to have infinite number of files
with the same name.  Some operations treat these same-named members no
differently than any other set of archive members: for example, if you
view an archive with `--list' (`-t'), you will see all
of those members listed, with their data modification times, owners, etc.

Other operations don't deal with these members as perfectly as you might
prefer; if you were to use `--extract' to extract the archive,
only the most recently added copy of a member with the same name as four
other members would end up in the working directory.  This is because
`--extract' extracts an archive in the order the members appeared
in the archive; the most recently archived members will be extracted
last.  Additionally, an extracted member will replace a file of
the same name which existed in the directory already, and tar
will not prompt you about this(10).  Thus, only the most recently archived
member will end up being extracted, as it will replace the one
extracted before it, and so on.

There exists a special option that allows you to get around this
behavior and extract (or list) only a particular copy of the file.
This is `--occurrence' option.  If you run tar with
this option, it will extract only the first copy of the file.  You
may also give this option an argument specifying the number of
copy to be extracted.  Thus, for example if the archive
`archive.tar' contained three copies of file `myfile', then
the command

tar --extract --file archive.tar --occurrence=2 myfile

would extract only the second copy.  See --occurrence, for the description of `--occurrence'
option.



If you want to replace an archive member, use `--delete' to
delete the member you want to remove from the archive, , and then use
`--append' to add the member you want to be in the archive.  Note
that you can not change the order of the archive; the most recently
added member will still appear last.  In this sense, you cannot truly
"replace" one member with another.  (Replacing one member with another
will not work on certain types of media, such as tapes; see delete
and Media, for more information.)

* appending files::             Appending Files to an Archive
* multiple::

4.2.2.1 Appending Files to an Archive
.....................................
(This message will disappear, once this node revised.)

The simplest way to add a file to an already existing archive is the
`--append' (`-r') operation, which writes specified
files into the archive whether or not they are already among the
archived files.

When you use `--append', you must specify file name
arguments, as there is no default.  If you specify a file that already
exists in the archive, another copy of the file will be added to the
end of the archive.  As with other operations, the member names of the
newly added files will be exactly the same as their names given on the
command line.  The `--verbose' (`-v') option will print
out the names of the files as they are written into the archive.

`--append' cannot be performed on some tape drives, unfortunately,
due to deficiencies in the formats those tape drives use.  The archive
must be a valid tar archive, or else the results of using this
operation will be unpredictable.  See Media.

To demonstrate using `--append' to add a file to an archive,
create a file called `rock' in the `practice' directory.
Make sure you are in the `practice' directory.  Then, run the
following tar command to add `rock' to
`collection.tar':

$ tar --append --file=collection.tar rock

If you now use the `--list' (`-t') operation, you will see that
`rock' has been added to the archive:

$ tar --list --file=collection.tar
-rw-r--r-- me user     28 1996-10-18 16:31 jazz
-rw-r--r-- me user     21 1996-09-23 16:44 blues
-rw-r--r-- me user     20 1996-09-23 16:44 folk
-rw-r--r-- me user     20 1996-09-23 16:44 rock

4.2.2.2 Multiple Members with the Same Name
...........................................

You can use `--append' (`-r') to add copies of files
which have been updated since the archive was created.  (However, we
do not recommend doing this since there is another tar
option called `--update'; See update, for more information.
We describe this use of `--append' here for the sake of
completeness.)  When you extract the archive, the older version will
be effectively lost.  This works because files are extracted from an
archive in the order in which they were archived.  Thus, when the
archive is extracted, a file archived later in time will replace a
file of the same name which was archived earlier, even though the
older version of the file will remain in the archive unless you delete
all versions of the file. 

Supposing you change the file `blues' and then append the changed
version to `collection.tar'.  As you saw above, the original
`blues' is in the archive `collection.tar'.  If you change the
file and append the new version of the file to the archive, there will
be two copies in the archive.  When you extract the archive, the older
version of the file will be extracted first, and then replaced by the
newer version when it is extracted.

You can append the new, changed copy of the file `blues' to the
archive in this way:

$ tar --append --verbose --file=collection.tar blues
blues

Because you specified the `--verbose' option, tar has
printed the name of the file being appended as it was acted on.  Now
list the contents of the archive:

$ tar --list --verbose --file=collection.tar
-rw-r--r-- me user     28 1996-10-18 16:31 jazz
-rw-r--r-- me user     21 1996-09-23 16:44 blues
-rw-r--r-- me user     20 1996-09-23 16:44 folk
-rw-r--r-- me user     20 1996-09-23 16:44 rock
-rw-r--r-- me user     58 1996-10-24 18:30 blues

The newest version of `blues' is now at the end of the archive
(note the different creation dates and file sizes).  If you extract
the archive, the older version of the file `blues' will be
replaced by the newer version.  You can confirm this by extracting
the archive and running `ls' on the directory.

If you wish to extract the first occurrence of the file `blues'
from the archive, use `--occurrence' option, as shown in
the following example:

$ tar --extract -vv --occurrence --file=collection.tar blues
-rw-r--r-- me user     21 1996-09-23 16:44 blues

See Writing, for more information on `--extract' and
See -occurrence, for the description of
`--occurrence' option.

4.2.3 Updating an Archive
-------------------------
(This message will disappear, once this node revised.)

In the previous section, you learned how to use `--append' to
add a file to an existing archive.  A related operation is
`--update' (`-u').  The `--update' operation
updates a tar archive by comparing the date of the specified
archive members against the date of the file with the same name.  If
the file has been modified more recently than the archive member, then
the newer version of the file is added to the archive (as with
`--append').

Unfortunately, you cannot use `--update' with magnetic tape drives.
The operation will fail.



Both `--update' and `--append' work by adding to the end
of the archive.  When you extract a file from the archive, only the
version stored last will wind up in the file system, unless you use
the `--backup' option.  See multiple, for a detailed discussion.

* how to update::

4.2.3.1 How to Update an Archive Using `--update'
.................................................

You must use file name arguments with the `--update'
(`-u') operation.  If you don't specify any files,
tar won't act on any files and won't tell you that it didn't
do anything (which may end up confusing you).


To see the `--update' option at work, create a new file,
`classical', in your practice directory, and some extra text to the
file `blues', using any text editor.  Then invoke tar with
the `update' operation and the `--verbose' (`-v')
option specified, using the names of all the files in the practice
directory as file name arguments:

$ tar --update -v -f collection.tar blues folk rock classical
blues
classical
$

Because we have specified verbose mode, tar prints out the names
of the files it is working on, which in this case are the names of the
files that needed to be updated.  If you run `tar --list' and look
at the archive, you will see `blues' and `classical' at its
end.  There will be a total of two versions of the member `blues';
the one at the end will be newer and larger, since you added text before
updating it.

(The reason tar does not overwrite the older file when updating
it is because writing to the middle of a section of tape is a difficult
process.  Tapes are not designed to go backward.  See Media, for more
information about tapes.

`--update' (`-u') is not suitable for performing backups for two
reasons: it does not change directory content entries, and it
lengthens the archive every time it is used.  The GNU tar
options intended specifically for backups are more
efficient.  If you need to run backups, please consult Backups.

4.2.4 Combining Archives with `--concatenate'
---------------------------------------------

Sometimes it may be convenient to add a second archive onto the end of
an archive rather than adding individual files to the archive.  To add
one or more archives to the end of another archive, you should use the
`--concatenate' (`--catenate', `-A') operation.

To use `--concatenate', give the first archive with
`--file' option and name the rest of archives to be
concatenated on the command line.  The members, and their member
names, will be copied verbatim from those archives to the first one.
(11)
The new, concatenated archive will be called by the same name as the
one given with the `--file' option.  As usual, if you omit
`--file', tar will use the value of the environment
variable TAPE, or, if this has not been set, the default archive name.



To demonstrate how `--concatenate' works, create two small archives
called `bluesrock.tar' and `folkjazz.tar', using the relevant
files from `practice':

$ tar -cvf bluesrock.tar blues rock
blues
rock
$ tar -cvf folkjazz.tar folk jazz
folk
jazz

If you like, You can run `tar --list' to make sure the archives
contain what they are supposed to:

$ tar -tvf bluesrock.tar
-rw-r--r-- melissa user    105 1997-01-21 19:42 blues
-rw-r--r-- melissa user     33 1997-01-20 15:34 rock
$ tar -tvf jazzfolk.tar
-rw-r--r-- melissa user     20 1996-09-23 16:44 folk
-rw-r--r-- melissa user     65 1997-01-30 14:15 jazz

We can concatenate these two archives with tar:

$ cd ..
$ tar --concatenate --file=bluesrock.tar jazzfolk.tar

If you now list the contents of the `bluesrock.tar', you will see
that now it also contains the archive members of `jazzfolk.tar':

$ tar --list --file=bluesrock.tar
blues
rock
folk
jazz

When you use `--concatenate', the source and target archives must
already exist and must have been created using compatible format
parameters.  Notice, that tar does not check whether the
archives it concatenates have compatible formats, it does not
even check if the files are really tar archives.

Like `--append' (`-r'), this operation cannot be performed on some
tape drives, due to deficiencies in the formats those tape drives use.

It may seem more intuitive to you to want or try to use cat to
concatenate two archives instead of using the `--concatenate'
operation; after all, cat is the utility for combining files.

However, tar archives incorporate an end-of-file marker which
must be removed if the concatenated archives are to be read properly as
one archive.  `--concatenate' removes the end-of-archive marker
from the target archive before each new archive is appended.  If you use
cat to combine the archives, the result will not be a valid
tar format archive.  If you need to retrieve files from an
archive that was added to using the cat utility, use the
`--ignore-zeros' (`-i') option.  See Ignore Zeros, for further
information on dealing with archives improperly combined using the
cat shell utility.

4.2.5 Removing Archive Members Using `--delete'
-----------------------------------------------
(This message will disappear, once this node revised.)

You can remove members from an archive by using the `--delete'
option.  Specify the name of the archive with `--file'
(`-f') and then specify the names of the members to be deleted;
if you list no member names, nothing will be deleted.  The
`--verbose' option will cause tar to print the names
of the members as they are deleted. As with `--extract', you
must give the exact member names when using `tar --delete'.
`--delete' will remove all versions of the named file from the
archive.  The `--delete' operation can run very slowly.

Unlike other operations, `--delete' has no short form.

This operation will rewrite the archive.  You can only use
`--delete' on an archive if the archive device allows you to
write to any point on the media, such as a disk; because of this, it
does not work on magnetic tapes.  Do not try to delete an archive member
from a magnetic tape; the action will not succeed, and you will be
likely to scramble the archive and damage your tape.  There is no safe
way (except by completely re-writing the archive) to delete files from
most kinds of magnetic tape.  See Media.

To delete all versions of the file `blues' from the archive
`collection.tar' in the `practice' directory, make sure you
are in that directory, and then,

$ tar --list --file=collection.tar
blues
folk
jazz
rock
$ tar --delete --file=collection.tar blues
$ tar --list --file=collection.tar
folk
jazz
rock
$



The `--delete' option has been reported to work properly when
tar acts as a filter from stdin to stdout.

4.2.6 Comparing Archive Members with the File System
----------------------------------------------------
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The `--compare' (`-d'), or `--diff' operation compares
specified archive members against files with the same names, and then
reports differences in file size, mode, owner, modification date and
contents.  You should only specify archive member names, not file
names.  If you do not name any members, then tar will compare the
entire archive.  If a file is represented in the archive but does not
exist in the file system, tar reports a difference.

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

tar ignores files in the file system that do not have
corresponding members in the archive.

The following example compares the archive members `rock',
`blues' and `funk' in the archive `bluesrock.tar' with
files of the same name in the file system.  (Note that there is no file,
`funk'; tar will report an error message.)

$ tar --compare --file=bluesrock.tar rock blues funk
rock
blues
tar: funk not found in archive

The spirit behind the `--compare' (`--diff',
`-d') option is to check whether the archive represents the
current state of files on disk, more than validating the integrity of
the archive media.  For this later goal, See verify. 

4.3 Options Used by `--create'
==============================

The previous chapter described the basics of how to use
`--create' (`-c') to create an archive from a set of files.
See create.  This section described advanced options to be used with
`--create'.

* override::                  Overriding File Metadata.
* Ignore Failed Read::

4.3.1 Overriding File Metadata
------------------------------

As described above, a tar archive keeps, for each member it contains,
its metadata, such as modification time, mode and ownership of
the file.  GNU tar allows to replace these data with other values
when adding files to the archive.  The options described in this
section affect creation of archives of any type.  For POSIX archives,
see also PAX keywords, for additional ways of controlling
metadata, stored in the archive.

`--mode=permissions'

When adding files to an archive, tar will use
permissions for the archive members, rather than the permissions
from the files.  permissions can be specified either as an octal
number or as symbolic permissions, like with
chmod (See File permissions in GNU file utilities.  This reference
also has useful information for those not being overly familiar with
the UNIX permission system).  Using latter syntax allows for
more flexibility.  For example, the value `a+rw' adds read and write
permissions for everybody, while retaining executable bits on directories
or on any other file already marked as executable:

$ tar -c -f archive.tar --mode='a+rw' .

`--mtime=date'

When adding files to an archive, tar will use date as
the modification time of members when creating archives, instead of
their actual modification times.  The argument date can be
either a textual date representation in almost arbitrary format
(see Date input formats) or a name of the existing file, starting
with `/' or `.'.  In the latter case, the modification time
of that file will be used.

The following example will set the modification date to 00:00:00 UTC,
January 1, 1970:

$ tar -c -f archive.tar --mtime='1970-01-01' .

When used with `--verbose' (see verbose tutorial) GNU tar
will try to convert the specified date back to its textual
representation and compare it with the one given with
`--mtime' options.  If the two dates differ, tar will
print a warning saying what date it will use.  This is to help user
ensure he is using the right date.

For example:

$ tar -c -f archive.tar -v --mtime=yesterday .
tar: Option --mtime: Treating date `yesterday' as 2006-06-20
13:06:29.152478
...

`--owner=user'

Specifies that tar should use user as the owner of members
when creating archives, instead of the user associated with the source
file.  The argument user can be either an existing user symbolic
name, or a decimal numeric user ID.

There is no value indicating a missing number, and `0' usually means
root.  Some people like to force `0' as the value to offer in
their distributions for the owner of files, because the root user is
anonymous anyway, so that might as well be the owner of anonymous
archives.  For example:

$ tar -c -f archive.tar --owner=0 .
# Or:
$ tar -c -f archive.tar --owner=root .

`--group=group'

Files added to the tar archive will have a group id of group,
rather than the group from the source file.  The argument group
can be either an existing group symbolic name, or a decimal numeric group ID.

4.3.2 Ignore Fail Read
----------------------

`--ignore-failed-read'
Do not exit with nonzero on unreadable files or directories.

4.4 Options Used by `--extract'
===============================
(This message will disappear, once this node revised.)

The previous chapter showed how to use `--extract' to extract
an archive into the file system.  Various options cause tar to
extract more information than just file contents, such as the owner,
the permissions, the modification date, and so forth.  This section
presents options to be used with `--extract' when certain special
considerations arise.  You may review the information presented in
extract for more basic information about the
`--extract' operation.

* Reading::                     Options to Help Read Archives
* Writing::                     Changing How tar Writes Files
* Scarce::                      Coping with Scarce Resources

4.4.1 Options to Help Read Archives
-----------------------------------
(This message will disappear, once this node revised.)

Normally, tar will request data in full record increments from
an archive storage device.  If the device cannot return a full record,
tar will report an error.  However, some devices do not always
return full records, or do not require the last record of an archive to
be padded out to the next record boundary.  To keep reading until you
obtain a full record, or to accept an incomplete record if it contains
an end-of-archive marker, specify the `--read-full-records' (`-B') option
in conjunction with the `--extract' or `--list' operations.
See Blocking.

The `--read-full-records' (`-B') option is turned on by default when
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, tar
would fail as soon as it read an incomplete record from the pipe.

If you're not sure of the blocking factor of an archive, you can
read the archive by specifying `--read-full-records' (`-B') and
`--blocking-factor=512-size' (`-b
512-size'), using a blocking factor larger than what the archive
uses.  This lets you avoid having to determine the blocking factor
of an archive.  See Blocking Factor.

* read full records::
* Ignore Zeros::

Reading Full Records
....................



`--read-full-records'
`-B'
Use in conjunction with `--extract' (`--get',
`-x') to read an archive which contains incomplete records, or
one which has a blocking factor less than the one specified.

Ignoring Blocks of Zeros
........................

Normally, tar stops reading when it encounters a block of zeros
between file entries (which usually indicates the end of the archive).
`--ignore-zeros' (`-i') allows tar to
completely read an archive which contains a block of zeros before the
end (i.e., a damaged archive, or one that was created by concatenating
several archives together).

The `--ignore-zeros' (`-i') option is turned off by default because many
versions of tar write garbage after the end-of-archive entry,
since that part of the media is never supposed to be read.  GNU tar
does not write after the end of an archive, but seeks to
maintain compatiblity among archiving utilities.

`--ignore-zeros'
`-i'
To ignore blocks of zeros (i.e., end-of-archive entries) which may be
encountered while reading an archive.  Use in conjunction with
`--extract' or `--list'.

4.4.2 Changing How tar Writes Files
-----------------------------------
(This message will disappear, once this node revised.)



* Dealing with Old Files::
* Overwrite Old Files::
* Keep Old Files::
* Keep Newer Files::
* Unlink First::
* Recursive Unlink::
* Data Modification Times::
* Setting Access Permissions::
* Directory Modification Times and Permissions::
* Writing to Standard Output::
* Writing to an External Program::
* remove files::

Options Controlling the Overwriting of Existing Files
.....................................................

When extracting files, if tar discovers that the extracted
file already exists, it normally replaces the file by removing it before
extracting it, to prevent confusion in the presence of hard or symbolic
links.  (If the existing file is a symbolic link, it is removed, not
followed.)  However, if a directory cannot be removed because it is
nonempty, tar normally overwrites its metadata (ownership,
permission, etc.).  The `--overwrite-dir' option enables this
default behavior.  To be more cautious and preserve the metadata of
such a directory, use the `--no-overwrite-dir' option.

To be even more cautious and prevent existing files from being replaced, use
the `--keep-old-files' (`-k') option.  It causes tar to refuse
to replace or update a file that already exists, i.e., a file with the
same name as an archive member prevents extraction of that archive
member.  Instead, it reports an error.

To be more aggressive about altering existing files, use the
`--overwrite' option.  It causes tar to overwrite
existing files and to follow existing symbolic links when extracting.

Some people argue that GNU tar should not hesitate
to overwrite files with other files when extracting.  When extracting
a tar archive, they expect to see a faithful copy of the
state of the file system when the archive was created.  It is debatable
that this would always be a proper behavior.  For example, suppose one
has an archive in which `usr/local' is a link to
`usr/local2'.  Since then, maybe the site removed the link and
renamed the whole hierarchy from `/usr/local2' to
`/usr/local'.  Such things happen all the time.  I guess it would
not be welcome at all that GNU tar removes the
whole hierarchy just to make room for the link to be reinstated
(unless it also simultaneously restores the full
`/usr/local2', of course!)  GNU tar is indeed
able to remove a whole hierarchy to reestablish a symbolic link, for
example, but only if `--recursive-unlink' is specified
to allow this behavior.  In any case, single files are silently
removed.

Finally, the `--unlink-first' (`-U') option can improve performance in
some cases by causing tar to remove files unconditionally
before extracting them.

Overwrite Old Files
...................

`--overwrite'
Overwrite existing files and directory metadata when extracting files
from an archive.

This causes tar to write extracted files into the file system without
regard to the files already on the system; i.e., files with the same
names as archive members are overwritten when the archive is extracted.
It also causes tar to extract the ownership, permissions,
and time stamps onto any preexisting files or directories.
If the name of a corresponding file name is a symbolic link, the file
pointed to by the symbolic link will be overwritten instead of the
symbolic link itself (if this is possible).  Moreover, special devices,
empty directories and even symbolic links are automatically removed if
they are in the way of extraction.

Be careful when using the `--overwrite' option, particularly when
combined with the `--absolute-names' (`-P') option, as this combination
can change the contents, ownership or permissions of any file on your
system.  Also, many systems do not take kindly to overwriting files that
are currently being executed.

`--overwrite-dir'
Overwrite the metadata of directories when extracting files from an
archive, but remove other files before extracting.

Keep Old Files
..............

`--keep-old-files'
`-k'
Do not replace existing files from archive.  The
`--keep-old-files' (`-k') option prevents tar
from replacing existing files with files with the same name from the
archive. The `--keep-old-files' option is meaningless with
`--list' (`-t').  Prevents tar from replacing
files in the file system during extraction.

Keep Newer Files
................

`--keep-newer-files'
Do not replace existing files that are newer than their archive
copies.  This option is meaningless with `--list' (`-t').

Unlink First
............

`--unlink-first'
`-U'
Remove files before extracting over them.
This can make tar run a bit faster if you know in advance
that the extracted files all need to be removed.  Normally this option
slows tar down slightly, so it is disabled by default.

Recursive Unlink
................

`--recursive-unlink'
When this option is specified, try removing files and directory hierarchies
before extracting over them.  This is a dangerous option!

If you specify the `--recursive-unlink' option,
tar removes anything that keeps you from extracting a file
as far as current permissions will allow it.  This could include removal
of the contents of a full directory hierarchy.

Setting Data Modification Times
...............................

Normally, tar sets the data modification times of extracted
files to the corresponding times recorded for the files in the archive, but
limits the permissions of extracted files by the current umask
setting.

To set the data modification times of extracted files to the time when
the files were extracted, use the `--touch' (`-m') option in
conjunction with `--extract' (`--get', `-x').

`--touch'
`-m'
Sets the data 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 `--extract' (`--get', `-x').

Setting Access Permissions
..........................

To set the modes (access permissions) of extracted files to those
recorded for those files in the archive, use `--same-permissions'
in conjunction with the `--extract' (`--get',
`-x') operation.  

`--preserve-permissions'
`--same-permissions'
`-p'
Set modes of extracted archive members to those recorded in the
archive, instead of current umask settings.  Use in conjunction with
`--extract' (`--get', `-x').

Directory Modification Times and Permissions
............................................

After sucessfully extracting a file member, GNU tar normally
restores its permissions and modification times, as described in the
previous sections.  This cannot be done for directories, because
after extracting a directory tar will almost certainly
extract files into that directory and this will cause the directory
modification time to be updated.  Moreover, restoring that directory
permissions may not permit file creation within it.  Thus, restoring
directory permissions and modification times must be delayed at least
until all files have been extracted into that directory.  GNU tar
restores directories using the following approach.

The extracted directories are created with the mode specified in the
archive, as modified by the umask of the user, which gives sufficient
permissions to allow file creation.  The meta-information about the
directory is recorded in the temporary list of directories.  When
preparing to extract next archive member, GNU tar checks if the
directory prefix of this file contains the remembered directory.  If
it does not, the program assumes that all files have been extracted
into that directory, restores its modification time and permissions
and removes its entry from the internal list.  This approach allows
to correctly restore directory meta-information in the majority of
cases, while keeping memory requirements sufficiently small.  It is
based on the fact, that most tar archives use the predefined
order of members: first the directory, then all the files and
subdirectories in that directory.

However, this is not always true.  The most important exception are
incremental archives (see Incremental Dumps).  The member order in
an incremental archive is reversed: first all directory members are
stored, followed by other (non-directory) members.  So, when extracting
from incremental archives, GNU tar alters the above procedure.  It
remebers all restored directories, and restores their meta-data
only after the entire archive has been processed.  Notice, that you do
not need to specity any special options for that, as GNU tar
automatically detects archives in incremental format.

There may be cases, when such processing is required for normal archives
too.  Consider the following example:

$ tar --no-recursion -cvf archive \
    foo foo/file1 bar bar/file foo/file2
foo/
foo/file1
bar/
bar/file
foo/file2

During the normal operation, after encountering `bar'
GNU tar will assume that all files from the directory `foo'
were already extracted and will therefore restore its timestamp and
permission bits.  However, after extracting `foo/file2' the
directory timestamp will be offset again.

To correctly restore directory meta-information in such cases, use
`delay-directory-restore' command line option:

`--delay-directory-restore'
Delays restoring of the modification times and permissions of extracted
directories until the end of extraction.  This way, correct
meta-information is restored even if the archive has unusual member
ordering.

`--no-delay-directory-restore'
Cancel the effect of the previous `--delay-directory-restore'.
Use this option if you have used `--delay-directory-restore' in
TAR_OPTIONS variable (see TAR_OPTIONS) and wish to
temporarily disable it.

Writing to Standard Output
..........................

To write the extracted files to the standard output, instead of
creating the files on the file system, use `--to-stdout' (`-O') in
conjunction with `--extract' (`--get', `-x').  This option is useful if you are
extracting files to send them through a pipe, and do not need to
preserve them in the file system.  If you extract multiple members,
they appear on standard output concatenated, in the order they are
found in the archive.

`--to-stdout'
`-O'
Writes files to the standard output.  Use only in conjunction with
`--extract' (`--get', `-x').  When this option is
used, instead of creating the files specified, tar writes
the contents of the files extracted to its standard output.  This may
be useful if you are only extracting the files in order to send them
through a pipe.  This option is meaningless with `--list'
(`-t').

This can be useful, for example, if you have a tar archive containing
a big file and don't want to store the file on disk before processing
it.  You can use a command like this:

tar -xOzf foo.tgz bigfile | process

or even like this if you want to process the concatenation of the files:

tar -xOzf foo.tgz bigfile1 bigfile2 | process

Hovewer, `--to-command' may be more convenient for use with
multiple files. See the next section.

Writing to an External Program
..............................

You can instruct tar to send the contents of each extracted
file to the standard input of an external program:

`--to-command=command'
Extract files and pipe their contents to the standard input of
command. When this option is used, instead of creating the
files specified, tar invokes command and pipes the
contents of the files to its standard output. Command may
contain command line arguments. The program is executed via
sh -c. Notice, that command is executed once for each regular file
extracted. Non-regular files (directories, etc.) are ignored when this
option is used.

The command can obtain the information about the file it processes
from the following environment variables:

TAR_FILETYPE
Type of the file. It is a single letter with the following meaning:

 f Regular file
 d Directory
 l Symbolic link
 h Hard link
 b Block device
 c Character device

Currently only regular files are supported.

TAR_MODE
File mode, an octal number.

TAR_FILENAME
The name of the file.

TAR_REALNAME
Name of the file as stored in the archive.

TAR_UNAME
Name of the file owner.

TAR_GNAME
Name of the file owner group.

TAR_ATIME
Time of last access. It is a decimal number, representing seconds
since the epoch.  If the archive provides times with nanosecond
precision, the nanoseconds are appended to the timestamp after a
decimal point.

TAR_MTIME
Time of last modification.

TAR_CTIME
Time of last status change.

TAR_SIZE
Size of the file.

TAR_UID
UID of the file owner.

TAR_GID
GID of the file owner.

In addition to these variables, TAR_VERSION contains the
GNU tar version number.

If command exits with a non-0 status, tar will print
an error message similar to the following:

tar: 2345: Child returned status 1

Here, `2345' is the PID of the finished process.

If this behavior is not wanted, use `--ignore-command-error':

`--ignore-command-error'
Ignore exit codes of subprocesses.  Notice that if the program
exits on signal or otherwise terminates abnormally, the error message
will be printed even if this option is used.

`--no-ignore-command-error'
Cancel the effect of any previous `--ignore-command-error'
option. This option is useful if you have set
`--ignore-command-error' in TAR_OPTIONS
(see TAR_OPTIONS) and wish to temporarily cancel it.

Removing Files
..............



`--remove-files'
Remove files after adding them to the archive.

4.4.3 Coping with Scarce Resources
----------------------------------
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* Starting File::
* Same Order::

Starting File
.............

`--starting-file=name'
`-K name'
Starts an operation in the middle of an archive.  Use in conjunction
with `--extract' (`--get', `-x') or `--list' (`-t').

If a previous attempt to extract files failed due to lack of disk
space, you can use `--starting-file=name' (`-K
name') to start extracting only after member name of the
archive.  This assumes, of course, that there is now free space, or
that you are now extracting into a different file system.  (You could
also choose to suspend tar, remove unnecessary files from
the file system, and then restart the same tar operation.
In this case, `--starting-file' is not necessary.
See Incremental Dumps, See interactive, and exclude.)

Same Order
..........

`--same-order'
`--preserve-order'
`-s'
To process large lists of file names on machines with small amounts of
memory.  Use in conjunction with `--compare' (`--diff',
`-d'), `--list' (`-t') or `--extract'
(`--get', `-x').

The `--same-order' (`--preserve-order', `-s') option tells tar that the list of file
names to be listed or extracted is sorted in the same order as the
files in the archive.  This allows a large list of names to be used,
even on a small machine that would not otherwise be able to hold all
the names in memory at the same time.  Such a sorted list can easily be
created by running `tar -t' on the archive and editing its output.

This option is probably never needed on modern computer systems.

4.5 Backup options
==================


GNU tar offers options for making backups of files
before writing new versions.  These options control the details of
these backups.  They may apply to the archive itself before it is
created or rewritten, as well as individual extracted members.  Other
GNU programs (cp, install, ln,
and mv, for example) offer similar options.

Backup options may prove unexpectedly useful when extracting archives
containing many members having identical name, or when extracting archives
on systems having file name limitations, making different members appear
has having similar names through the side-effect of name truncation.
(This is true only if we have a good scheme for truncated backup names,
which I'm not sure at all: I suspect work is needed in this area.)
When any existing file is backed up before being overwritten by extraction,
then clashing files are automatically be renamed to be unique, and the
true name is kept for only the last file of a series of clashing files.
By using verbose mode, users may track exactly what happens.

At the detail level, some decisions are still experimental, and may
change in the future, we are waiting comments from our users.  So, please
do not learn to depend blindly on the details of the backup features.
For example, currently, directories themselves are never renamed through
using these options, so, extracting a file over a directory still has
good chances to fail.  Also, backup options apply to created archives,
not only to extracted members.  For created archives, backups will not
be attempted when the archive is a block or character device, or when it
refers to a remote file.

For the sake of simplicity and efficiency, backups are made by renaming old
files prior to creation or extraction, and not by copying.  The original
name is restored if the file creation fails.  If a failure occurs after a
partial extraction of a file, both the backup and the partially extracted
file are kept.

`--backup[=method]'
Back up files that are about to be overwritten or removed.
Without this option, the original versions are destroyed.

Use method to determine the type of backups made.
If method is not specified, use the value of the VERSION_CONTROL
environment variable.  And if VERSION_CONTROL is not set,
use the `existing' method.

This option corresponds to the Emacs variable `version-control';
the same values for method are accepted as in Emacs.  This option
also allows more descriptive names.  The valid methods are:

`t'
`numbered'
Always make numbered backups.

`nil'
`existing'
Make numbered backups of files that already have them, simple backups
of the others.

`never'
`simple'
Always make simple backups.


`--suffix=suffix'
Append suffix to each backup file made with `--backup'.  If this
option is not specified, the value of the SIMPLE_BACKUP_SUFFIX
environment variable is used.  And if SIMPLE_BACKUP_SUFFIX is not
set, the default is `~', just as in Emacs.


4.6 Notable tar Usages
======================
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You can easily use archive files to transport a group of files from
one system to another: put all relevant files into an archive on one
computer system, transfer the archive to another system, and extract
the contents there.  The basic transfer medium might be magnetic tape,
Internet FTP, or even electronic mail (though you must encode the
archive with 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 tar program.

For example, here is how you might copy a directory's contents from
one disk to another, while preserving the dates, modes, owners and
link-structure of all the files therein.  In this case, the transfer
medium is a pipe, which is one a Unix redirection mechanism:

$ (cd sourcedir; tar -cf - .) | (cd targetdir; tar -xf -)

You can avoid subshells by using `-C' option:

$ tar -C sourcedir -cf - . | tar -C targetdir -xf -

The command also works using short option forms:

$ (cd sourcedir; tar --create --file=- . ) \
       | (cd targetdir; tar --extract --file=-)
# Or:
$ tar --directory sourcedir --create --file=- . ) \
       | tar --directory targetdir --extract --file=-

This is one of the easiest methods to transfer a tar archive.

4.7 Looking Ahead: The Rest of this Manual
==========================================

You have now seen how to use all eight of the operations available to
tar, and a number of the possible options.  The next chapter
explains how to choose and change file and archive names, how to use
files to store names of other files which you can then call as
arguments to tar (this can help you save time if you expect to
archive the same list of files a number of times), and so forth.


If there are too many files to conveniently list on the command line,
you can list the names in a file, and tar will read that file.
See files.

There are various ways of causing tar to skip over some files,
and not archive them.  See Choosing.

5 Performing Backups and Restoring Files
****************************************
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GNU tar is distributed along with the scripts
which the Free Software Foundation uses for performing backups.  There
is no corresponding scripts available yet for doing restoration of
files.  Even if there is a good chance those scripts may be satisfying
to you, they are not the only scripts or methods available for doing
backups and restore.  You may well create your own, or use more
sophisticated packages dedicated to that purpose.

Some users are enthusiastic about Amanda (The Advanced Maryland
Automatic Network Disk Archiver), a backup system developed by James
da Silva `jds@cs.umd.edu' and available on many Unix systems.
This is free software, and it is available at these places:

http://www.cs.umd.edu/projects/amanda/amanda.html
ftp://ftp.cs.umd.edu/pub/amanda



This chapter documents both the provided shell scripts and tar
options which are more specific to usage as a backup tool.

To back up a file system means to create archives that contain
all the files in that file system.  Those archives can then be used to
restore any or all of those files (for instance if a disk crashes or a
file is accidentally deleted).  File system backups are also
called dumps.

* Full Dumps::                  Using tar to Perform Full Dumps
* Incremental Dumps::           Using tar to Perform Incremental Dumps
* Backup Levels::               Levels of Backups
* Backup Parameters::           Setting Parameters for Backups and Restoration
* Scripted Backups::            Using the Backup Scripts
* Scripted Restoration::        Using the Restore Script

5.1 Using tar to Perform Full Dumps
===================================
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Full dumps should only be made when no other people or programs
are modifying files in the file system.  If files are modified while
tar is making the backup, they may not be stored properly in
the archive, in which case you won't be able to restore them if you
have to.  (Files not being modified are written with no trouble, and do
not corrupt the entire archive.)

You will want to use the `--label=archive-label'
(`-V archive-label') option to give the archive a
volume label, so you can tell what this archive is even if the label
falls off the tape, or anything like that.

Unless the file system you are dumping is guaranteed to fit on
one volume, you will need to use the `--multi-volume' (`-M') option.
Make sure you have enough tapes on hand to complete the backup.

If you want to dump each file system separately you will need to use
the `--one-file-system' option to prevent
tar from crossing file system boundaries when storing
(sub)directories.

The `--incremental' (`-G') (see Incremental Dumps)
option is not needed, since this is a complete copy of everything in
the file system, and a full restore from this backup would only be
done onto a completely
empty disk.

Unless you are in a hurry, and trust the tar program (and your
tapes), it is a good idea to use the `--verify' (`-W')
option, to make sure your files really made it onto the dump properly.
This will also detect cases where the file was modified while (or just
after) it was being archived.  Not all media (notably cartridge tapes)
are capable of being verified, unfortunately.

5.2 Using tar to Perform Incremental Dumps
==========================================

Incremental backup is a special form of GNU tar archive that
stores additional metadata so that exact state of the file system
can be restored when extracting the archive.

GNU tar currently offers two options for handling incremental
backups: `--listed-incremental=snapshot-file' (`-g
snapshot-file') and `--incremental' (`-G').

The option `--listed-incremental' instructs tar to operate on
an incremental archive with additional metadata stored in a standalone
file, called a snapshot file.  The purpose of this file is to help
determine which files have been changed, added or deleted since the
last backup, so that the next incremental backup will contain only
modified files.  The name of the snapshot file is given as an argument
to the option:

`--listed-incremental=file'
`-g file'
  Handle incremental backups with snapshot data in file.

To create an incremental backup, you would use
`--listed-incremental' together with `--create'
(see create).  For example:

$ tar --create \
           --file=archive.1.tar \
           --listed-incremental=/var/log/usr.snar \
           /usr

This will create in `archive.1.tar' an incremental backup of
the `/usr' file system, storing additional metadata in the file
`/var/log/usr.snar'.  If this file does not exist, it will be
created.  The created archive will then be a level 0 backup;
please see the next section for more on backup levels.

Otherwise, if the file `/var/log/usr.snar' exists, it
determines which files are modified.  In this case only these files will be
stored in the archive.  Suppose, for example, that after running the
above command, you delete file `/usr/doc/old' and create
directory `/usr/local/db' with the following contents:

$ ls /usr/local/db
/usr/local/db/data
/usr/local/db/index

Some time later you create another incremental backup.  You will
then see:

$ tar --create \
           --file=archive.2.tar \
           --listed-incremental=/var/log/usr.snar \
           /usr
tar: usr/local/db: Directory is new
usr/local/db/
usr/local/db/data
usr/local/db/index

The created archive `archive.2.tar' will contain only these
three members.  This archive is called a level 1 backup.  Notice
that `/var/log/usr.snar' will be updated with the new data, so if
you plan to create more `level 1' backups, it is necessary to
create a working copy of the snapshot file before running
tar.  The above example will then be modified as follows:

$ cp /var/log/usr.snar /var/log/usr.snar-1
$ tar --create \
           --file=archive.2.tar \
           --listed-incremental=/var/log/usr.snar-1 \
           /usr

Incremental dumps depend crucially on time stamps, so the results are
unreliable if you modify a file's time stamps during dumping (e.g.,
with the `--atime-preserve=replace' option), or if you set the clock
backwards.

Metadata stored in snapshot files include device numbers, which,
obviously is supposed to be a non-volatile value.  However, it turns
out that NFS devices have undependable values when an automounter
gets in the picture.  This can lead to a great deal of spurious
redumping in incremental dumps, so it is somewhat useless to compare
two NFS devices numbers over time.  The solution implemented currently
is to considers all NFS devices as being equal when it comes to
comparing directories; this is fairly gross, but there does not seem
to be a better way to go.

Note that incremental archives use tar extensions and may
not be readable by non-GNU versions of the tar program.

To extract from the incremental dumps, use
`--listed-incremental' together with `--extract'
option (see extracting files).  In this case, tar does
not need to access snapshot file, since all the data necessary for
extraction are stored in the archive itself.  So, when extracting, you
can give whatever argument to `--listed-incremental', the usual
practice is to use `--listed-incremental=/dev/null'.
Alternatively, you can use `--incremental', which needs no
arguments.  In general, `--incremental' (`-G') can be
used as a shortcut for `--listed-incremental' when listing or
extracting incremental backups (for more information, regarding this
option, see incremental-op).

When extracting from the incremental backup GNU tar attempts to
restore the exact state the file system had when the archive was
created.  In particular, it will delete those files in the file
system that did not exist in their directories when the archive was
created.  If you have created several levels of incremental files,
then in order to restore the exact contents the file system  had when
the last level was created, you will need to restore from all backups
in turn.  Continuing our example, to restore the state of `/usr'
file system, one would do(12):

$ tar --extract \
           --listed-incremental=/dev/null \
           --file archive.1.tar
$ tar --extract \
           --listed-incremental=/dev/null \
           --file archive.2.tar

To list the contents of an incremental archive, use `--list'
(see list), as usual.  To obtain more information about the
archive, use `--listed-incremental' or `--incremental'
combined with two `--verbose' options(13):

tar --list --incremental --verbose --verbose archive.tar

This command will print, for each directory in the archive, the list
of files in that directory at the time the archive was created.  This
information is put out in a format which is both human-readable and
unambiguous for a program: each file name is printed as

x file

where x is a letter describing the status of the file: `Y'
if the file  is present in the archive, `N' if the file is not
included in the archive, or a `D' if the file is a directory (and
is included in the archive).  See Dumpdir, for the detailed
description of dumpdirs and status codes.  Each such
line is terminated by a newline character.  The last line is followed
by an additional newline to indicate the end of the data.

The option `--incremental' (`-G')
gives the same behavior as `--listed-incremental' when used
with `--list' and `--extract' options.  When used with
`--create' option, it creates an incremental archive without
creating snapshot file.  Thus, it is impossible to create several
levels of incremental backups with `--incremental' option.

5.3 Levels of Backups
=====================

An archive containing all the files in the file system is called a
full backup or full dump.  You could insure your data by
creating a full dump every day.  This strategy, however, would waste a
substantial amount of archive media and user time, as unchanged files
are daily re-archived.

It is more efficient to do a full dump only occasionally.  To back up
files between full dumps, you can use incremental dumps.  A level
one dump archives all the files that have changed since the last full
dump.

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

GNU tar comes with scripts you can use to do full
and level-one (actually, even level-two and so on) dumps.  Using
scripts (shell programs) to perform backups and restoration is a
convenient and reliable alternative to typing out file name lists
and tar commands by hand.

Before you use these scripts, you need to edit the file
`backup-specs', which specifies parameters used by the backup
scripts and by the restore script.  This file is usually located
in `/etc/backup' directory.  See Backup Parameters, for its
detailed description.  Once the backup parameters are set, you can
perform backups or restoration by running the appropriate script.

The name of the backup script is backup.  The name of the
restore script is restore.  The following sections describe
their use in detail.

Please Note: The backup and restoration scripts are
designed to be used together.  While it is possible to restore files by
hand from an archive which was created using a backup script, and to create
an archive by hand which could then be extracted using the restore script,
it is easier to use the scripts.  See Incremental Dumps, before
making such an attempt.

5.4 Setting Parameters for Backups and Restoration
==================================================

The file `backup-specs' specifies backup parameters for the
backup and restoration scripts provided with tar.  You must
edit `backup-specs' to fit your system configuration and schedule
before using these scripts.

Syntactically, `backup-specs' is a shell script, containing
mainly variable assignments.  However, any valid shell construct
is allowed in this file.  Particularly, you may wish to define
functions within that script (e.g., see RESTORE_BEGIN below).
For more information about shell script syntax, please refer to
the definition of the Shell Command Language <http://www.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html#ta g_02>.  See also
Bash Features in Bash Reference Manual.

The shell variables controlling behavior of backup and
restore are described in the following subsections.

* General-Purpose Variables::
* Magnetic Tape Control::
* User Hooks::
* backup-specs example::        An Example Text of `Backup-specs'

5.4.1 General-Purpose Variables
-------------------------------

 -- Backup variable: ADMINISTRATOR
The user name of the backup administrator.  Backup scripts
sends a backup report to this address.

 -- Backup variable: BACKUP_HOUR
The hour at which the backups are done.  This can be a number from 0
to 23, or the time specification in form hours:minutes,
or the string `now'.

This variable is used by backup.  Its value may be overridden
using `--time' option (see Scripted Backups).

 -- Backup variable: TAPE_FILE

The device tar writes the archive to.  If TAPE_FILE
is a remote archive (see remote-dev), backup script will suppose
that your mt is able to access remote devices.  If RSH
(see RSH) is set, `--rsh-command' option will be added to
invocations of mt.

 -- Backup variable: BLOCKING

The blocking factor tar will use when writing the dump archive.
See Blocking Factor.

 -- Backup variable: BACKUP_DIRS

A list of file systems to be dumped (for backup), or restored
(for restore).  You can include any directory
name in the list -- subdirectories on that file system will be
included, regardless of how they may look to other networked machines.
Subdirectories on other file systems will be ignored.

The host name specifies which host to run tar on, and should
normally be the host that actually contains the file system.  However,
the host machine must have GNU 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 (i.e.  what pwd will print
when in that directory on that machine).  If the host that contains
the file system does not have this capability, you can specify another
host as long as it can access the file system through NFS.

If the list of file systems is very long you may wish to put it
in a separate file.  This file is usually named
`/etc/backup/dirs', but this name may be overridden in
`backup-specs' using DIRLIST variable.

 -- Backup variable: DIRLIST

A path to the file containing the list of the file systems to backup
or restore.  By default it is `/etc/backup/dirs'.

 -- Backup variable: BACKUP_FILES

A list of individual files to be dumped (for backup), or restored
(for restore).  These should be accessible from the machine on
which the backup script is run.

If the list of file systems is very long you may wish to store it
in a separate file.  This file is usually named
`/etc/backup/files', but this name may be overridden in
`backup-specs' using FILELIST variable.

 -- Backup variable: FILELIST

A path to the file containing the list of the individual files to backup
or restore.  By default it is `/etc/backup/files'.

 -- Backup variable: MT

Full file name of mt binary.

 -- Backup variable: RSH

Full file name of rsh binary or its equivalent.  You may wish to
set it to ssh, to improve security.  In this case you will have
to use public key authentication.

 -- Backup variable: RSH_COMMAND

Full file name of rsh binary on remote mashines.  This will
be passed via `--rsh-command' option to the remote invocation
of GNU tar.

 -- Backup variable: VOLNO_FILE

Name of temporary file to hold volume numbers.  This needs to be accessible
by all the machines which have file systems to be dumped.

 -- Backup variable: XLIST

Name of exclude file list.  An exclude file list is a file
located on the remote machine and containing the list of files to
be excluded from the backup.  Exclude file lists are searched in
/etc/tar-backup directory.  A common use for exclude file lists
is to exclude files containing security-sensitive information
(e.g., `/etc/shadow' from backups).

This variable affects only backup.

 -- Backup variable: SLEEP_TIME

Time to sleep between dumps of any two successive file systems

This variable affects only backup.

 -- Backup variable: DUMP_REMIND_SCRIPT

Script to be run when it's time to insert a new tape in for the next
volume.  Administrators may want to tailor this script for their site.
If this variable isn't set, GNU tar will display its built-in
prompt, and will expect confirmation from the console.  For the
description of the default prompt, see change volume prompt.


 -- Backup variable: SLEEP_MESSAGE

Message to display on the terminal while waiting for dump time.  Usually
this will just be some literal text.

 -- Backup variable: TAR

Full file name of the GNU tar executable.  If this is not set, backup
scripts will search tar in the current shell path.

5.4.2 Magnetic Tape Control
---------------------------

Backup scripts access tape device using special hook functions.
These functions take a single argument - the name of the tape
device.  Their names are kept in the following variables:

 -- Backup variable: MT_BEGIN
The name of begin function.  This function is called before
accessing the drive.  By default it retensions the tape:

MT_BEGIN=mt_begin

mt_begin() {
    mt -f "$1" retension
}

 -- Backup variable: MT_REWIND
The name of rewind function.  The default definition is as
follows:

MT_REWIND=mt_rewind

mt_rewind() {
    mt -f "$1" rewind
}


 -- Backup variable: MT_OFFLINE
The name of the function switching the tape off line.  By default
it is defined as follows:

MT_OFFLINE=mt_offline

mt_offline() {
    mt -f "$1" offl
}

 -- Backup variable: MT_STATUS
The name of the function used to obtain the status of the archive device,
including error count.  Default definition:

MT_STATUS=mt_status

mt_status() {
    mt -f "$1" status
}

5.4.3 User Hooks
----------------

User hooks are shell functions executed before and after
each tar invocation.  Thus, there are backup
hooks, which are executed before and after dumping each file
system, and restore hooks, executed before and
after restoring a file system.  Each user hook is a shell function
taking four arguments:

 -- User Hook Function: hook level host fs fsname
Its arguments are:

level
Current backup or restore level.

host
Name or IP address of the host machine being dumped or restored.

fs
Full path name to the file system being dumped or restored.

fsname
File system name with directory separators replaced with colons.  This
is useful, e.g., for creating unique files.

Following variables keep the names of user hook functions

 -- Backup variable: DUMP_BEGIN
Dump begin function.  It is executed before dumping the file system.

 -- Backup variable: DUMP_END
Executed after dumping the file system.

 -- Backup variable: RESTORE_BEGIN
Executed before restoring the file system.

 -- Backup variable: RESTORE_END
Executed after restoring the file system.

5.4.4 An Example Text of `Backup-specs'
---------------------------------------

The following is an example of `backup-specs':

# site-specific parameters for file system backup.

ADMINISTRATOR=friedman
BACKUP_HOUR=1
TAPE_FILE=/dev/nrsmt0

# Use ssh instead of the less secure rsh
RSH=/usr/bin/ssh
RSH_COMMAND=/usr/bin/ssh

# Override MT_STATUS function:
my_status() {
      mts -t $TAPE_FILE
}
MT_STATUS=my_status

# Disable MT_OFFLINE function
MT_OFFLINE=:

BLOCKING=124
BACKUP_DIRS="
        albert:/fs/fsf
        apple-gunkies:/gd
        albert:/fs/gd2
        albert:/fs/gp
        geech:/usr/jla
        churchy:/usr/roland
        albert:/
        albert:/usr
        apple-gunkies:/
        apple-gunkies:/usr
        gnu:/hack
        gnu:/u
        apple-gunkies:/com/mailer/gnu
        apple-gunkies:/com/archive/gnu"

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


5.5 Using the Backup Scripts
============================

The syntax for running a backup script is:

backup --level=level --time=time

The `level' option requests the dump level.  Thus, to produce
a full dump, specify --level=0 (this is the default, so
`--level' may be omitted if its value is 0).
(14)

The `--time' option determines when should the backup be
run.  Time may take three forms:

hh:mm

The dump must be run at hh hours mm minutes.

hh

The dump must be run at hh hours

now

The dump must be run immediately.

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 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 (see Scripted Restoration).

The backup scripts write two files on the file system.  The first is a
record file in `/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.  See Snapshot Files, 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
`log-mm-dd-yyyy-level-n', where mm-dd-yyyy
represents current date, and n represents current dump level number.

The script also prints the name of each system being dumped to the
standard output.

Following is the full list of options accepted by backup
script:

`-l level'
`--level=level'
Do backup level level (default 0).

`-f'
`--force'
Force backup even if today's log file already exists.

`-v[level]'
`--verbose[=level]'
Set verbosity level.  The higher the level is, the more debugging
information will be output during execution.  Devault level
is 100, which means the highest debugging level.

`-t start-time'
`--time=start-time'
Wait till time, then do backup.

`-h'
`--help'
Display short help message and exit.

`-V'
`--version'
Display information about the program's name, version, origin and legal
status, all on standard output, and then exit successfully.


5.6 Using the Restore Script
============================

To restore files that were archived using a scripted backup, use the
restore script.  Its usage is quite straightforward.  In the
simplest form, invoke restore --all, it will
then restore all the file systems and files specified in
`backup-specs' (see BACKUP_DIRS).

You may select the file systems (and/or files) to restore by
giving restore list of patterns in its command
line.  For example, running

restore 'albert:*'

will restore all file systems on the machine `albert'.  A more
complicated example:

restore 'albert:*' '*:/var'

This command will restore all file systems on the machine `albert'
as well as `/var' file system on all machines.

By default restore will start restoring files from the lowest
available dump level (usually zero) and will continue through
all available dump levels.  There may be situations where such a
thorough restore is not necessary.  For example, you may wish to
restore only files from the recent level one backup.  To do so,
use `--level' option, as shown in the example below:

restore --level=1

The full list of options accepted by restore follows:

`-a'
`--all'
Restore all file systems and files specified in `backup-specs'

`-l level'
`--level=level'
Start restoring from the given backup level, instead of the default 0.

`-v[level]'
`--verbose[=level]'
Set verbosity level.  The higher the level is, the more debugging
information will be output during execution.  Devault level
is 100, which means the highest debugging level.

`-h'
`--help'
Display short help message and exit.

`-V'
`--version'
Display information about the program's name, version, origin and legal
status, all on standard output, and then exit successfully.

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.  See Tape Positioning, for a discussion of tape
positioning.

Warning: The script will delete files from the active file
system if they were not in the file system when the archive was made.

See Incremental Dumps, for an explanation of how the script makes
that determination.

6 Choosing Files and Names for tar
**********************************
(This message will disappear, once this node revised.)

Certain options to tar enable you to specify a name for your
archive.  Other options let you decide which files to include or exclude
from the archive, based on when or whether files were modified, whether
the file names do or don't match specified patterns, or whether files
are in specified directories.

This chapter discusses these options in detail.

* file::                        Choosing the Archive's Name
* Selecting Archive Members::
* files::                       Reading Names from a File
* exclude::                     Excluding Some Files
* wildcards::                   Wildcards Patterns and Matching
* quoting styles::              Ways of Quoting Special Characters in Names
* transform::                   Modifying File and Member Names
* after::                       Operating Only on New Files
* recurse::                     Descending into Directories
* one::                         Crossing File System Boundaries

6.1 Choosing and Naming Archive Files
=====================================
(This message will disappear, once this node revised.)

By default, tar uses an archive file name that was compiled when
it was built on the system; usually this name refers to some physical
tape drive on the machine.  However, the person who installed tar
on the system may not have set the default to a meaningful value as far as
most users are concerned.  As a result, you will usually want to tell
tar where to find (or create) the archive.  The
`--file=archive-name' (`-f archive-name')
option allows you to either specify or name a file to use as the archive
instead of the default archive file location.

`--file=archive-name'
`-f archive-name'
Name the archive to create or operate on.  Use in conjunction with
any operation.

For example, in this tar command,

$ tar -cvf collection.tar blues folk jazz

`collection.tar' is the name of the archive.  It must directly
follow the `-f' option, since whatever directly follows `-f'
will end up naming the archive.  If you neglect to specify an
archive name, you may end up overwriting a file in the working directory
with the archive you create since tar will use this file's name
for the archive name.

An archive can be saved as a file in the file system, sent through a
pipe or over a network, or written to an I/O device such as a tape,
floppy disk, or CD write drive.

If you do not name the archive, tar uses the value of the
environment variable TAPE as the file name for the archive.  If
that is not available, tar uses a default, compiled-in archive
name, usually that for tape unit zero (i.e.  `/dev/tu00').

If you use `-' as an archive-name, 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
`-' as an archive-name when modifying an archive,
tar reads the original archive from its standard input and
writes the entire new archive to its standard output.

The following example is a convenient way of copying directory
hierarchy from `sourcedir' to `targetdir'.

$ (cd sourcedir; tar -cf - .) | (cd targetdir; tar -xpf -)

The `-C' option allows to avoid using subshells:

$ tar -C sourcedir -cf - . | tar -C targetdir -xpf -

In both examples above, the leftmost tar invocation archives
the contents of `sourcedir' to the standard output, while the
rightmost one reads this archive from its standard input and
extracts it.  The `-p' option tells it to restore permissions
of the extracted files.


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

--file=hostname:/dev/file-name

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


If the archive file name includes a colon (`:'), then it is assumed
to be a file on another machine.  If the archive file is
`user@host:file', then file is used on the
host host.  The remote host is accessed using the rsh
program, with a username of user.  If the username is omitted
(along with the `@' sign), then your user name will be used.
(This is the normal rsh behavior.)  It is necessary for the
remote machine, in addition to permitting your rsh access, to
have the `rmt' program installed (This command is included in
the GNU tar distribution and by default is installed under
`prefix/libexec/rmt', were prefix means your
installation prefix).  If you need to use a file whose name includes a
colon, then the remote tape drive behavior
can be inhibited by using the `--force-local' option.

When the archive is being created to `/dev/null', GNU tar
tries to minimize input and output operations.  The Amanda backup
system, when used with GNU tar, has an initial sizing pass which
uses this feature. 

6.2 Selecting Archive Members
=============================

File Name arguments specify which files in the file system
tar operates on, when creating or adding to an archive, or which
archive members tar operates on, when reading or deleting from
an archive.  See Operations.

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

If a file name begins with dash (`-'), precede it with
`--add-file' option to prevent it from being treated as an
option.


By default GNU tar attempts to unquote each file or member
name, replacing escape sequences according to the following
table:

 Escape Replaced with
 \a Audible bell (ASCII 7)
 \b Backspace (ASCII 8)
 \f Form feed (ASCII 12)
 \n New line (ASCII 10)
 \r Carriage return (ASCII 13)
 \t Horizontal tabulation (ASCII 9)
 \v Vertical tabulation (ASCII 11)
 \? ASCII 127
 \n ASCII n (n should be an octal number
                 of up to 3 digits)

A backslash followed by any other symbol is retained.

This default behavior is controlled by the following command line
option:

`--unquote'
Enable unquoting input file or member names (default).

`--no-unquote'
Disable unquoting input file or member names.

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

If you do not specify files, tar behavior differs depending
on the operation mode as described below:

When tar is invoked with `--create' (`-c'),
tar will stop immediately, reporting the following:

$ tar cf a.tar
tar: Cowardly refusing to create an empty archive
Try `tar --help' or `tar --usage' for more information.

If you specify either `--list' (`-t') or
`--extract' (`--get', `-x'), tar
operates on all the archive members in the archive.

If run with `--diff' option, tar will compare the archive with
the contents of the current working directory.

If you specify any other operation, tar does nothing.

By default, tar takes file names from the command line.  However,
there are other ways to specify file or member names, or to modify the
manner in which tar selects the files or members upon which to
operate.  In general, these methods work both for specifying the names
of files and archive members. 

6.3 Reading Names from a File
=============================

Instead of giving the names of files or archive members on the command
line, you can put the names into a file, and then use the
`--files-from=file-of-names' (`-T
file-of-names') option to tar.  Give the name of the
file which contains the list of files to include as the argument to 
`--files-from'.  In the list, the file names should be separated by
newlines.  You will frequently use this option when you have generated
the list of files to archive with the find utility.

`--files-from=file-name'
`-T file-name'
Get names to extract or create from file file-name.

If you give a single dash as a file name for `--files-from', (i.e.,
you specify either --files-from=- or -T -), then the file
names are read from standard input.

Unless you are running tar with `--create', you can not use
both --files-from=- and --file=- (-f -) in the same
command.

Any number of `-T' options can be given in the command line.

The following example shows how to use find to generate a list of
files smaller than 400K in length and put that list into a file
called `small-files'.  You can then use the `-T' option to
tar to specify the files from that file, `small-files', to
create the archive `little.tgz'.  (The `-z' option to
tar compresses the archive with gzip; see gzip for
more information.)

$ find .  -size -400 -print > small-files
$ tar -c -v -z -T small-files -f little.tgz

In the file list given by `-T' option, any file name beginning
with `-' character is considered a tar option and is
processed accordingly.(15) For example,
the common use of this feature is to change to another directory by
specifying `-C' option:

$ cat list
-C/etc
passwd
hosts
-C/lib
libc.a
$ tar -c -f foo.tar --files-from list

In this example, tar will first switch to `/etc'
directory and add files `passwd' and `hosts' to the
archive.  Then it will change to `/lib' directory and will archive
the file `libc.a'.  Thus, the resulting archive `foo.tar' will
contain:

$ tar tf foo.tar
passwd
hosts
libc.a

Notice that the option parsing algorithm used with `-T' is
stricter than the one used by shell.  Namely, when specifying option
arguments, you should observe the following rules:

* When using short (single-letter) option form, its argument must
immediately follow the option letter, without any intervening
whitespace.  For example: -Cdir.

* When using long option form, the option argument must be separated
from the option by a single equal sign.  No whitespace is allowed on
any side of the equal sign.  For example: --directory=dir.

* For both short and long option forms, the option argument can be given
on the next line after the option name, e.g.:

--directory
dir

and

-C
dir

If you happen to have a file whose name starts with `-',
precede it with `--add-file' option to prevent it from
being recognized as an option.  For example: --add-file=--my-file.

* nul::

6.3.1 NUL Terminated File Names
-------------------------------

The `--null' option causes
`--files-from=file-of-names' (`-T file-of-names')
to read file names terminated by a NUL instead of a newline, so
files whose names contain newlines can be archived using
`--files-from'. 

`--null'
Only consider NUL terminated file names, instead of files that
terminate in a newline.

The `--null' option is just like the one in GNU
xargs and cpio, and is useful with the
`-print0' predicate of GNU find.  In
tar, `--null' also disables special handling for
file names that begin with dash.

This example shows how to use find to generate a list of files
larger than 800K in length and put that list into a file called
`long-files'.  The `-print0' option to find is just
like `-print', except that it separates files with a NUL
rather than with a newline.  You can then run tar with both the
`--null' and `-T' options to specify that tar get the
files from that file, `long-files', to create the archive
`big.tgz'.  The `--null' option to tar will cause
tar to recognize the NUL separator between files.

$ find .  -size +800 -print0 > long-files
$ tar -c -v --null --files-from=long-files --file=big.tar



6.4 Excluding Some Files
========================
(This message will disappear, once this node revised.)

To avoid operating on files whose names match a particular pattern,
use the `--exclude' or `--exclude-from' options.

`--exclude=pattern'
Causes tar to ignore files that match the pattern.

The `--exclude=pattern' option prevents any file or
member whose name matches the shell wildcard (pattern) from
being operated on. 
For example, to create an archive with all the contents of the directory
`src' except for files whose names end in `.o', use the
command `tar -cf src.tar --exclude='*.o' src'.

You may give multiple `--exclude' options.

`--exclude-from=file'
`-X file'
Causes tar to ignore files that match the patterns listed in
file.

Use the `--exclude-from' option to read a
list of patterns, one per line, from file; tar will
ignore files matching those patterns.  Thus if tar is
called as `tar -c -X foo .' and the file `foo' contains a
single line `*.o', no files whose names end in `.o' will be
added to the archive.

`--exclude-caches'
Causes tar to ignore directories containing a cache directory tag.

When creating an archive, the `--exclude-caches' option causes
tar to exclude all directories that contain a cache
directory tag. A cache directory tag is a short file with the
well-known name `CACHEDIR.TAG' and having a standard header
specified in <http://www.brynosaurus.com/cachedir/spec.html>.
Various applications write cache directory tags into directories they
use to hold regenerable, non-precious data, so that such data can be
more easily excluded from backups.

* problems with exclude::

Problems with Using the exclude Options
---------------------------------------

Some users find `exclude' options confusing.  Here are some common
pitfalls:

* The main operating mode of tar does not act on a path name
explicitly listed on the command line if one of its file name
components is excluded.  In the example above, if
you create an archive and exclude files that end with `*.o', but
explicitly name the file `dir.o/foo' after all the options have been
listed, `dir.o/foo' will be excluded from the archive.

* You can sometimes confuse the meanings of `--exclude' and
`--exclude-from'.  Be careful: use `--exclude' when files
to be excluded are given as a pattern on the command line.  Use
`--exclude-from' to introduce the name of a file which contains
a list of patterns, one per line; each of these patterns can exclude
zero, one, or many files.

* When you use `--exclude=pattern', be sure to quote the
pattern parameter, so GNU tar sees wildcard characters
like `*'.  If you do not do this, the shell might expand the
`*' itself using files at hand, so tar might receive a
list of files instead of one pattern, or none at all, making the
command somewhat illegal.  This might not correspond to what you want.

For example, write:

$ tar -c -f archive.tar --exclude '*.o' directory

rather than:

# Wrong!
$ tar -c -f archive.tar --exclude *.o directory

* You must use use shell syntax, or globbing, rather than regexp
syntax, when using exclude options in tar.  If you try to use
regexp syntax to describe files to be excluded, your command
might fail.

*  
In earlier versions of tar, what is now the
`--exclude-from' option was called `--exclude' instead.
Now, `--exclude' applies to patterns listed on the command
line and `--exclude-from' applies to patterns listed in a
file.


6.5 Wildcards Patterns and Matching
===================================

Globbing is the operation by which wildcard characters,
`*' or `?' for example, are replaced and expanded into all
existing files matching the given pattern.  GNU tar can use wildcard
patterns for matching (or globbing) archive members when extracting
from or listing an archive.  Wildcard patterns are also used for
verifying volume labels of tar archives.  This section has the
purpose of explaining wildcard syntax for tar.



A pattern should be written according to shell syntax, using wildcard
characters to effect globbing.  Most characters in the pattern stand
for themselves in the matched string, and case is significant: `a'
will match only `a', and not `A'.  The character `?' in the
pattern matches any single character in the matched string.  The character
`*' in the pattern matches zero, one, or more single characters in
the matched string.  The character `\' says to take the following
character of the pattern literally; it is useful when one needs to
match the `?', `*', `[' or `\' characters, themselves.

The character `[', up to the matching `]', introduces a character
class.  A character class is a list of acceptable characters
for the next single character of the matched string.  For example,
`[abcde]' would match any of the first five letters of the alphabet.
Note that within a character class, all of the "special characters"
listed above other than `\' lose their special meaning; for example,
`[-\\[*?]]' would match any of the characters, `-', `\',
`[', `*', `?', or `]'.  (Due to parsing constraints,
the characters `-' and `]' must either come first or
last in a character class.)

If the first character of the class after the opening `['
is `!' or `^', then the meaning of the class is reversed.
Rather than listing character to match, it lists those characters which
are forbidden as the next single character of the matched string.

Other characters of the class stand for themselves.  The special
construction `[a-e]', using an hyphen between two
letters, is meant to represent all characters between a and
e, inclusive.



Periods (`.') or forward slashes (`/') are not considered
special for wildcard matches.  However, if a pattern completely matches
a directory prefix of a matched string, then it matches the full matched
string: thus, excluding a directory also excludes all the files beneath it.

* controlling pattern-matching::

Controlling Pattern-Matching
----------------------------

For the purposes of this section, we call exclusion members all
member names obtained while processing `--exclude' and
`--exclude-from' options, and inclusion members those
member names that were given in the command line or read from the file
specified with `--files-from' option.

These two pairs of member lists are used in the following operations:
`--diff', `--extract', `--list',
`--update'.

There are no inclusion members in create mode (`--create' and
`--append'), since in this mode the names obtained from the
command line refer to files, not archive members.

By default, inclusion members are compared with archive members
literally (16) and exclusion members are
treated as globbing patterns.  For example:

$ tar tf foo.tar
a.c
b.c
a.txt
[remarks]
# Member names are used verbatim:
$ tar -xf foo.tar -v '[remarks]'
[remarks]
# Exclude member names are globbed:
$ tar -xf foo.tar -v --exclude '*.c'
a.txt
[remarks]

This behavior can be altered by using the following options:

`--wildcards'
Treat all member names as wildcards. 

`--no-wildcards'
Treat all member names as literal strings.

Thus, to extract files whose names end in `.c', you can use:

$ tar -xf foo.tar -v --wildcards '*.c'
a.c
b.c

Notice quoting of the pattern to prevent the shell from interpreting
it.

The effect of `--wildcards' option is cancelled by
`--no-wildcards'.  This can be used to pass part of
the command line arguments verbatim and other part as globbing
patterns.  For example, the following invocation:

$ tar -xf foo.tar --wildcards '*.txt' --no-wildcards '[remarks]'

instructs tar to extract from `foo.tar' all files whose
names end in `.txt' and the file named `[remarks]'.

Normally, a pattern matches a name if an initial subsequence of the
name's components matches the pattern, where `*', `?', and
`[...]' are the usual shell wildcards, `\' escapes wildcards,
and wildcards can match `/'.

Other than optionally stripping leading `/' from names
(see absolute), patterns and names are used as-is.  For
example, trailing `/' is not trimmed from a user-specified name
before deciding whether to exclude it.

However, this matching procedure can be altered by the options listed
below.  These options accumulate.  For example:

--ignore-case --exclude='makefile' --no-ignore-case ---exclude='readme'

ignores case when excluding `makefile', but not when excluding
`readme'.

`--anchored'
`--no-anchored'
If anchored, a pattern must match an initial subsequence
of the name's components.  Otherwise, the pattern can match any
subsequence.  Default is `--no-anchored' for exclusion members
and `--anchored' inclusion members.

`--ignore-case'
`--no-ignore-case'
When ignoring case, upper-case patterns match lower-case names and vice versa.
When not ignoring case (the default), matching is case-sensitive.

`--wildcards-match-slash'
`--no-wildcards-match-slash'
When wildcards match slash (the default for exclusion members), a
wildcard like `*' in the pattern can match a `/' in the
name.  Otherwise, `/' is matched only by `/'.


The `--recursion' and `--no-recursion' options
(see recurse) also affect how member patterns are interpreted.  If
recursion is in effect, a pattern matches a name if it matches any of
the name's parent directories.

The following table summarizes pattern-matching default values:

 Members Default settings
 Inclusion `--no-wildcards --anchored --no-wildcards-match-slash'
 Exclusion `--wildcards --no-anchored --wildcards-match-slash'

6.6 Quoting Member Names
========================

When displaying member names, tar takes care to avoid
ambiguities caused by certain characters.  This is called name
quoting.  The characters in question are:

* Non-printable control characters:

 Character ASCII Character name
 \a 7 Audible bell
 \b 8 Backspace
 \f 12 Form feed
 \n 10 New line
 \r 13 Carriage return
 \t 9 Horizontal tabulation
 \v 11 Vertical tabulation

* Space (ASCII 32)

* Single and double quotes (`'' and `"')

* Backslash (`\')

The exact way tar uses to quote these characters depends on
the quoting style.  The default quoting style, called
escape (see below), uses backslash notation to represent control
characters, space and backslash.  Using this quoting style, control
characters are represented as listed in column `Character' in the
above table, a space is printed as `\ ' and a backslash as `\\'.

GNU tar offers seven distinct quoting styles, which can be selected
using `--quoting-style' option:

`--quoting-style=style'

Sets quoting style.  Valid values for style argument are:
literal, shell, shell-always, c, escape, locale, clocale.

These styles are described in detail below.  To illustrate their
effect, we will use an imaginary tar archive `arch.tar'
containing the following members:

# 1. Contains horizontal tabulation character.
a       tab
# 2. Contains newline character
a
newline
# 3. Contains a space
a space
# 4. Contains double quotes
a"double"quote
# 5. Contains single quotes
a'single'quote
# 6. Contains a backslash character:
a\backslash

Here is how usual ls command would have listed them, if they
had existed in the current working directory:

$ ls
a\ttab
a\nnewline
a\ space
a"double"quote
a'single'quote
a\\backslash

Quoting styles:

`literal'
No quoting, display each character as is:

$ tar tf arch.tar --quoting-style=literal
./
./a space
./a'single'quote
./a"double"quote
./a\backslash
./a	tab
./a
newline

`shell'
Display characters the same way Bourne shell does:
control characters, except `\t' and `\n', are printed using
backslash escapes, `\t' and `\n' are printed as is, and a
single quote is printed as `\''.  If a name contains any quoted
characters, it is enclosed in single quotes.  In particular, if a name
contains single quotes, it is printed as several single-quoted strings:

$ tar tf arch.tar --quoting-style=shell
./
'./a space'
'./a'\''single'\''quote'
'./a"double"quote'
'./a\backslash'
'./a	tab'
'./a
newline'

`shell-always'
Same as `shell', but the names are always enclosed in single
quotes:

$ tar tf arch.tar --quoting-style=shell-always
'./'
'./a space'
'./a'\''single'\''quote'
'./a"double"quote'
'./a\backslash'
'./a	tab'
'./a
newline'

`c'
Use the notation of the C programming language.  All names are
enclosed in double quotes.  Control characters are quoted using
backslash notations, double quotes are represented as `\"',
backslash characters are represented as `\\'.  Single quotes and
spaces are not quoted:

$ tar tf arch.tar --quoting-style=c
"./"
"./a space"
"./a'single'quote"
"./a\"double\"quote"
"./a\\backslash"
"./a\ttab"
"./a\nnewline"

`escape'
Control characters are printed using backslash notation, a space is
printed as `\ ' and a backslash as `\\'.  This is the
default quoting style, unless it was changed when configured the
package.

$ tar tf arch.tar --quoting-style=escape
./
./a space
./a'single'quote
./a"double"quote
./a\\backslash
./a\ttab
./a\nnewline

`locale'
Control characters, single quote and backslash are printed using
backslash notation.  All names are quoted using left and right
quotation marks, appropriate to the current locale.  If it does not
define quotation marks, use ``' as left and `'' as right
quotation marks.  Any occurrences of the right quotation mark in a
name are escaped with `\', for example:

For example:

$ tar tf arch.tar --quoting-style=locale
`./'
`./a space'
`./a\'single\'quote'
`./a"double"quote'
`./a\\backslash'
`./a\ttab'
`./a\nnewline'

`clocale'
Same as `locale', but `"' is used for both left and right
quotation marks, if not provided by the currently selected locale:

$ tar tf arch.tar --quoting-style=clocale
"./"
"./a space"
"./a'single'quote"
"./a\"double\"quote"
"./a\\backslash"
"./a\ttab"
"./a\nnewline"

You can specify which characters should be quoted in addition to those
implied by the current quoting style:

`--quote-chars=string'
Always quote characters from string, even if the selected
quoting style would not quote them.

For example, using `escape' quoting (compare with the usual
escape listing above): 

$ tar tf arch.tar --quoting-style=escape --quote-chars=' "'
./
./a\ space
./a'single'quote
./a\"double\"quote
./a\\backslash
./a\ttab
./a\nnewline

To disable quoting of such additional characters, use the following
option:

`--no-quote-chars=string'
Remove characters listed in string from the list of quoted
characters set by the previous `--quote-chars' option.

This option is particularly useful if you have added
`--quote-chars' to your TAR_OPTIONS (see TAR_OPTIONS)
and wish to disable it for the current invocation.

Note, that `--no-quote-chars' does not disable those
characters that are quoted by default in the selected quoting style.

6.7 Modifying File and Member Names
===================================

Tar archives contain detailed information about files stored
in them and full file names are part of that information.  When
storing file to an archive, its file name is recorded in the archive
along with the actual file contents.  When restoring from an archive,
a file is created on disk with exactly the same name as that stored
in the archive.  In the majority of cases this is the desired behavior
of a file archiver.  However, there are some cases when it is not.

First of all, it is often unsafe to extract archive members with
absolute file names or those that begin with a `../'.  GNU tar
takes special precautions when extracting such names and provides a
special option for handling them, which is described in
absolute.

Secondly, you may wish to extract file names without some leading
directory components, or with otherwise modified names.  In other
cases it is desirable to store files under differing names in the
archive.

GNU tar provides two options for these needs.

`--strip-components=number'
Strip given number of leading components from file names before
extraction.

For example, suppose you have archived whole `/usr' hierarchy to
a tar archive named `usr.tar'.  Among other files, this archive
contains `usr/include/stdlib.h', which you wish to extract to
the current working directory.  To do so, you type:

$ tar -xf usr.tar --strip=2 usr/include/stdlib.h

The option `--strip=2' instructs tar to strip the
two leading components (`usr/' and `include/') off the file
name. 

If you add to the above invocation `--verbose' (`-v')
option, you will note that the verbose listing still contains the
full file name, with the two removed components still in place.  This
can be inconvenient, so tar provides a special option for
altering this behavior:

`--show-transformed-names'
Display file or member names with all requested transformations
applied.

For example:

$ tar -xf usr.tar -v --strip=2 usr/include/stdlib.h
usr/include/stdlib.h
$ tar -xf usr.tar -v --strip=2 --show-transformed usr/include/stdlib.h
stdlib.h

Notice that in both cases the file is `stdlib.h' extracted to the
current working directory, `--show-transformed-names' affects
only the way its name is displayed.

This option is especially useful for verifying whether the invocation
will have the desired effect.  Thus, before running

$ tar -x --strip=n

it is often advisable to run

$ tar -t -v --show-transformed --strip=n

to make sure the command will produce the intended results.

In case you need to apply more complex modifications to the file name,
GNU tar provides a general-purpose transformation option:

`--transform=expression'
Modify file names using supplied expression.

The expression is a sed-like replace expression of the
form:

s/regexp/replace/[flags]

where regexp is a regular expression, replace is a
replacement for each file name part that matches regexp.  Both
regexp and replace are described in detail in
The `s' Command in GNU sed.

Supported flags are:

`g'
Apply the replacement to all matches to the regexp, not
just the first.

`i'
Use case-insensitive matching

`x'
regexp is an extended regular expression (see Extended regular expressions in GNU sed).

`number'
Only replace the numberth match of the regexp.

Note: the posix standard does not specify what should happen
when you mix the `g' and number modifiers.  GNU tar
follows the GNU sed implementation in this regard, so
the the interaction is defined to be: ignore matches before the
numberth, and then match and replace all matches from the
numberth on.
                                   

Any delimiter can be used in lieue of `/', the only requirement being
that it be used consistently throughout the expression. For example,
the following two expressions are equivalent:

s/one/two/
s,one,two,

Changing delimiters is often useful when the regex contains
slashes.  For example, it is more convenient to write s,/,-, than
s/\//-/.

Here are several examples of `--transform' usage:

1. Extract `usr/' hierarchy into `usr/local/':

$ tar --transform='s,usr/,usr/local/,' -x -f arch.tar

2. Strip two leading directory components (equivalent to
`--strip-components=2'):

$ tar --transform='s,/*[^/]*/[^/]*/,,' -x -f arch.tar

3. Prepend `/prefix/'  to each file name:

$ tar --transform 's,^,/prefix/,' -x -f arch.tar

4. Convert each file name to lower case:

$ tar --transform 's/.*/\L&/' -x -f arch.tar


Unlike `--strip-components', `--transform' can be used
in any GNU tar operation mode.  For example, the following command
adds files to the archive while replacing the leading `usr/'
component with `var/':

$ tar -cf arch.tar --transform='s,^usr/,var/,' /

To test `--transform' effect we suggest using
`--show-transformed-names' option:

$ tar -cf arch.tar --transform='s,^usr/,var/,' \
       --verbose --show-transformed-names /

If both `--strip-components' and `--transform' are used
together, then `--transform' is applied first, and the required
number of components is then stripped from its result.
    
6.8 Operating Only on New Files
===============================
(This message will disappear, once this node revised.)

The `--after-date=date' (`--newer=date',
`-N date') option causes tar to only work on
files whose data modification or status change times are newer than
the date given.  If date starts with `/' or `.',
it is taken to be a file name; the data modification time of that file
is used as the date. If you use this option when creating or appending
to an archive, the archive will only include new files.  If you use
`--after-date' when extracting an archive, tar will
only extract files newer than the date you specify.

If you only want tar to make the date comparison based on
modification of the file's data (rather than status
changes), then use the `--newer-mtime=date' option.

You may use these options with any operation.  Note that these options
differ from the `--update' (`-u') operation in that they
allow you to specify a particular date against which tar can
compare when deciding whether or not to archive the files.

`--after-date=date'
`--newer=date'
`-N date'
Only store files newer than date.

Acts on files only if their data modification or status change times are
later than date.  Use in conjunction with any operation.

If date starts with `/' or `.', it is taken to be a file
name; the data modification time of that file is used as the date.

`--newer-mtime=date'
Acts like `--after-date', but only looks at data modification times.

These options limit tar to operate only on files which have
been modified after the date specified.  A file's status is considered to have
changed if its contents have been modified, or if its owner,
permissions, and so forth, have been changed.  (For more information on
how to specify a date, see Date input formats; remember that the
entire date argument must be quoted if it contains any spaces.)

Gurus would say that `--after-date' tests both the data
modification time (mtime, the time the contents of the file
were last modified) and the status change time (ctime, the time
the file's status was last changed: owner, permissions, etc.)
fields, while `--newer-mtime' tests only the mtime
field.

To be precise, `--after-date' checks both mtime and
ctime and processes the file if either one is more recent than
date, while `--newer-mtime' only checks mtime and
disregards ctime.  Neither does it use atime (the last time the
contents of the file were looked at).

Date specifiers can have embedded spaces.  Because of this, you may need
to quote date arguments to keep the shell from parsing them as separate
arguments.  For example, the following command will add to the archive
all the files modified less than two days ago:

$ tar -cf foo.tar --newer-mtime '2 days ago'

When any of these options is used with the option `--verbose'
(see verbose tutorial) GNU tar will try to convert the specified
date back to its textual representation and compare that with the
one given with the option.  If the two dates differ, tar will
print a warning saying what date it will use.  This is to help user
ensure he is using the right date.  For example:

$ tar -c -f archive.tar --after-date='10 days ago' .
tar: Option --after-date: Treating date `10 days ago' as 2006-06-11
13:19:37.232434

Please Note: `--after-date' and `--newer-mtime'
should not be used for incremental backups.  See Incremental Dumps,
for proper way of creating incremental backups.

6.9 Descending into Directories
===============================
(This message will disappear, once this node revised.)



Usually, tar will recursively explore all directories (either
those given on the command line or through the `--files-from'
option) for the various files they contain.  However, you may not always
want tar to act this way.

The `--no-recursion' option inhibits tar's recursive descent
into specified directories.  If you specify `--no-recursion', you can
use the find utility for hunting through levels of directories to
construct a list of file names which you could then pass to tar.
find allows you to be more selective when choosing which files to
archive; see files, for more information on using find with
tar, or look.

`--no-recursion'
Prevents tar from recursively descending directories.

`--recursion'
Requires tar to recursively descend directories.
This is the default.

When you use `--no-recursion', GNU tar grabs
directory entries themselves, but does not descend on them
recursively.  Many people use find for locating files they
want to back up, and since tar usually recursively
descends on directories, they have to use the `-not -type d'
test in their find invocation (see Type test in Finding Files), as they usually do not want all the files in a
directory. They then use the `--files-from' option to archive
the files located via find.

The problem when restoring files archived in this manner is that the
directories themselves are not in the archive; so the
`--same-permissions' (`--preserve-permissions',
`-p') option does not affect them--while users might really
like it to.  Specifying `--no-recursion' is a way to tell
tar to grab only the directory entries given to it, adding
no new files on its own.  To summarize, if you use find to
create a list of files to be stored in an archive, use it as follows:

$ find dir tests | \
  tar -cf archive -T - --no-recursion

The `--no-recursion' option also applies when extracting: it
causes tar to extract only the matched directory entries, not
the files under those directories.

The `--no-recursion' option also affects how globbing patterns
are interpreted (see controlling pattern-matching).

The `--no-recursion' and `--recursion' options apply to
later options and operands, and can be overridden by later occurrences
of `--no-recursion' and `--recursion'.  For example:

$ tar -cf jams.tar --no-recursion grape --recursion grape/concord

creates an archive with one entry for `grape', and the recursive
contents of `grape/concord', but no entries under `grape'
other than `grape/concord'.

6.10 Crossing File System Boundaries
====================================
(This message will disappear, once this node revised.)

tar will normally automatically cross file system boundaries in
order to archive files which are part of a directory tree.  You can
change this behavior by running tar and specifying
`--one-file-system'.  This option only affects files that are
archived because they are in a directory that is being archived;
tar will still archive files explicitly named on the command line
or through `--files-from', regardless of where they reside.

`--one-file-system'
Prevents tar from crossing file system boundaries when
archiving.  Use in conjunction with any write operation.

The `--one-file-system' option causes tar to modify its
normal behavior in archiving the contents of directories.  If a file in
a directory is not on the same file system as the directory itself, then
tar will not archive that file.  If the file is a directory
itself, tar will not archive anything beneath it; in other words,
tar will not cross mount points.

This option is useful for making full or incremental archival backups of
a file system.  If this option is used in conjunction with
`--verbose' (`-v'), files that are excluded are
mentioned by name on the standard error.

* directory::                   Changing Directory
* absolute::                    Absolute File Names

6.10.1 Changing the Working Directory
-------------------------------------
(This message will disappear, once this node revised.)



To change the working directory in the middle of a list of file names,
either on the command line or in a file specified using
`--files-from' (`-T'), use `--directory' (`-C').
This will change the working directory to the specified directory
after that point in the list.

`--directory=directory'
`-C directory'
Changes the working directory in the middle of a command line.

For example,

$ tar -c -f jams.tar grape prune -C food cherry

will place the files `grape' and `prune' from the current
directory into the archive `jams.tar', followed by the file
`cherry' from the directory `food'.  This option is especially
useful when you have several widely separated files that you want to
store in the same archive.

Note that the file `cherry' is recorded in the archive under the
precise name `cherry', not `food/cherry'.  Thus, the
archive will contain three files that all appear to have come from the
same directory; if the archive is extracted with plain `tar
--extract', all three files will be written in the current directory.

Contrast this with the command,

$ tar -c -f jams.tar grape prune -C food red/cherry

which records the third file in the archive under the name
`red/cherry' so that, if the archive is extracted using
`tar --extract', the third file will be written in a subdirectory
named `orange-colored'.

You can use the `--directory' option to make the archive
independent of the original name of the directory holding the files.
The following command places the files `/etc/passwd',
`/etc/hosts', and `/lib/libc.a' into the archive
`foo.tar':

$ tar -c -f foo.tar -C /etc passwd hosts -C /lib libc.a

However, the names of the archive members will be exactly what they were
on the command line: `passwd', `hosts', and `libc.a'.
They will not appear to be related by file name to the original
directories where those files were located.

Note that `--directory' options are interpreted consecutively.  If
`--directory' specifies a relative file name, it is interpreted
relative to the then current directory, which might not be the same as
the original current working directory of tar, due to a previous
`--directory' option.

When using `--files-from' (see files), you can put various
tar options (including `-C') in the file list.  Notice,
however, that in this case the option and its argument may not be
separated by whitespace.  If you use short option, its argument must
either follow the option letter immediately, without any intervening
whitespace, or occupy the next line.  Otherwise, if you use long
option, separate its argument by an equal sign.

For instance, the file list for the above example will be:

-C
/etc
passwd
hosts
-C
/lib
libc.a

To use it, you would invoke tar as follows:

$ tar -c -f foo.tar --files-from list

Notice also that you can only use the short option variant in the file
list, i.e., always use `-C', not `--directory'.

The interpretation of `--directory' is disabled by
`--null' option.

6.10.2 Absolute File Names
--------------------------
(This message will disappear, once this node revised.)

`--absolute-names'
`-P'
Do not strip leading slashes from file names, and permit file names
containing a `..' file name component.

By default, GNU tar drops a leading `/' on
input or output, and complains about file names containing a `..'
component.  This option turns off this behavior.

When tar extracts archive members from an archive, it strips any
leading slashes (`/') from the member name.  This causes absolute
member names in the archive to be treated as relative file names.  This
allows you to have such members extracted wherever you want, instead of
being restricted to extracting the member in the exact directory named
in the archive.  For example, if the archive member has the name
`/etc/passwd', tar will extract it as if the name were
really `etc/passwd'.

File names containing `..' can cause problems when extracting, so
tar normally warns you about such files when creating an
archive, and rejects attempts to extracts such files.

Other tar programs do not do this.  As a result, if you
create an archive whose member names start with a slash, they will be
difficult for other people with a non-GNU tar
program to use.  Therefore, GNU tar also strips
leading slashes from member names when putting members into the
archive.  For example, if you ask tar to add the file
`/bin/ls' to an archive, it will do so, but the member name will
be `bin/ls'.(17)

If you use the `--absolute-names' (`-P') option,
tar will do none of these transformations.

To archive or extract files relative to the root directory, specify
the `--absolute-names' (`-P') option.

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

When you specify `--absolute-names' (`-P'),
tar stores file names including all superior directory
names, and preserves leading slashes.  If you only invoked
tar from the root directory you would never need the
`--absolute-names' option, but using this option
may be more convenient than switching to root.





`--absolute-names'
Preserves full file names (including superior directory names) when
archiving files.  Preserves leading slash when extracting files.




tar prints out a message about removing the `/' from
file names.  This message appears once per GNU tar
invocation.  It represents something which ought to be told; ignoring
what it means can cause very serious surprises, later.

Some people, nevertheless, do not want to see this message.  Wanting to
play really dangerously, one may of course redirect tar standard
error to the sink.  For example, under sh:

$ tar -c -f archive.tar /home 2> /dev/null

Another solution, both nicer and simpler, would be to change to
the `/' directory first, and then avoid absolute notation.
For example:

$ (cd / && tar -c -f archive.tar home)
# or:
$ tar -c -f archive.tar -C  / home




7 Date input formats
********************


First, a quote:

Our units of temporal measurement, from seconds on up to months, are so
complicated, asymmetrical and disjunctive so as to make coherent mental
reckoning in time all but impossible.  Indeed, had some tyrannical god
contrived to enslave our minds to time, to make it all but impossible
for us to escape subjection to sodden routines and unpleasant surprises,
he could hardly have done better than handing down our present system.
It is like a set of trapezoidal building blocks, with no vertical or
horizontal surfaces, like a language in which the simplest thought
demands ornate constructions, useless particles and lengthy
circumlocutions.  Unlike the more successful patterns of language and
science, which enable us to face experience boldly or at least
level-headedly, our system of temporal calculation silently and
persistently encourages our terror of time.

...  It is as though architects had to measure length in feet, width
in meters and height in ells; as though basic instruction manuals
demanded a knowledge of five different languages.  It is no wonder then
that we often look into our own immediate past or future, last Tuesday
or a week from Sunday, with feelings of helpless confusion.  ...

-- Robert Grudin, Time and the Art of Living.

This section describes the textual date representations that GNU
programs accept.  These are the strings you, as a user, can supply as
arguments to the various programs.  The C interface (via the
get_date function) is not described here.

* General date syntax::            Common rules.
* Calendar date items::            19 Dec 1994.
* Time of day items::              9:20pm.
* Time zone items::                EST, PDT, GMT.
* Day of week items::              Monday and others.
* Relative items in date strings:: next tuesday, 2 years ago.
* Pure numbers in date strings::   19931219, 1440.
* Seconds since the Epoch::        @1078100502.
* Specifying time zone rules::     TZ="America/New_York", TZ="UTC0".
* Authors of get_date::            Bellovin, Eggert, Salz, Berets, et al.


7.1 General date syntax
=======================


A date is a string, possibly empty, containing many items
separated by whitespace.  The whitespace may be omitted when no
ambiguity arises.  The empty string means the beginning of today (i.e.,
midnight).  Order of the items is immaterial.  A date string may contain
many flavors of items:

* calendar date items
* time of day items
* time zone items
* day of the week items
* relative items
* pure numbers.

We describe each of these item types in turn, below.

A few ordinal numbers may be written out in words in some contexts.  This is
most useful for specifying day of the week items or relative items (see
below).  Among the most commonly used ordinal numbers, the word
`last' stands for -1, `this' stands for 0, and
`first' and `next' both stand for 1.  Because the word
`second' stands for the unit of time there is no way to write the
ordinal number 2, but for convenience `third' stands for 3,
`fourth' for 4, `fifth' for 5,
`sixth' for 6, `seventh' for 7, `eighth' for 8,
`ninth' for 9, `tenth' for 10, `eleventh' for 11 and
`twelfth' for 12.

When a month is written this way, it is still considered to be written
numerically, instead of being "spelled in full"; this changes the
allowed strings.

In the current implementation, only English is supported for words and
abbreviations like `AM', `DST', `EST', `first',
`January', `Sunday', `tomorrow', and `year'.

The output of the date command
is not always acceptable as a date string,
not only because of the language problem, but also because there is no
standard meaning for time zone items like `IST'.  When using
date to generate a date string intended to be parsed later,
specify a date format that is independent of language and that does not
use time zone items other than `UTC' and `Z'.  Here are some
ways to do this:

$ LC_ALL=C TZ=UTC0 date
Mon Mar  1 00:21:42 UTC 2004
$ TZ=UTC0 date +'%Y-%m-%d %H:%M:%SZ'
2004-03-01 00:21:42Z
$ date --iso-8601=ns | tr T ' '  # --iso-8601 is a GNU extension.
2004-02-29 16:21:42,692722128-0800
$ date --rfc-2822  # a GNU extension
Sun, 29 Feb 2004 16:21:42 -0800
$ date +'%Y-%m-%d %H:%M:%S %z'  # %z is a GNU extension.
2004-02-29 16:21:42 -0800
$ date +'@%s.%N'  # %s and %N are GNU extensions.
@1078100502.692722128

Alphabetic case is completely ignored in dates.  Comments may be introduced
between round parentheses, as long as included parentheses are properly
nested.  Hyphens not followed by a digit are currently ignored.  Leading
zeros on numbers are ignored.

Invalid dates like `2005-02-29' or times like `24:00' are
rejected.  In the typical case of a host that does not support leap
seconds, a time like `23:59:60' is rejected even if it
corresponds to a valid leap second.


7.2 Calendar date items
=======================


A calendar date item specifies a day of the year.  It is
specified differently, depending on whether the month is specified
numerically or literally.  All these strings specify the same calendar date:

1972-09-24     # ISO 8601.
72-9-24        # Assume 19xx for 69 through 99,
               # 20xx for 00 through 68.
72-09-24       # Leading zeros are ignored.
9/24/72        # Common U.S. writing.
24 September 1972
24 Sept 72     # September has a special abbreviation.
24 Sep 72      # Three-letter abbreviations always allowed.
Sep 24, 1972
24-sep-72
24sep72

The year can also be omitted.  In this case, the last specified year is
used, or the current year if none.  For example:

9/24
sep 24

Here are the rules.

For numeric months, the ISO 8601 format
`year-month-day' is allowed, where year is
any positive number, month is a number between 01 and 12, and
day is a number between 01 and 31.  A leading zero must be present
if a number is less than ten.  If year is 68 or smaller, then 2000
is added to it; otherwise, if year is less than 100,
then 1900 is added to it.  The construct
`month/day/year', popular in the United States,
is accepted.  Also `month/day', omitting the year.

Literal months may be spelled out in full: `January',
`February', `March', `April', `May', `June',
`July', `August', `September', `October',
`November' or `December'.  Literal months may be abbreviated
to their first three letters, possibly followed by an abbreviating dot.
It is also permitted to write `Sept' instead of `September'.

When months are written literally, the calendar date may be given as any
of the following:

day month year
day month
month day year
day-month-year

Or, omitting the year:

month day


7.3 Time of day items
=====================


A time of day item in date strings specifies the time on a given
day.  Here are some examples, all of which represent the same time:

20:02:00.000000
20:02
8:02pm
20:02-0500      # In EST (U.S. Eastern Standard Time).

More generally, the time of day may be given as
`hour:minute:second', where hour is
a number between 0 and 23, minute is a number between 0 and
59, and second is a number between 0 and 59 possibly followed by
`.' or `,' and a fraction containing one or more digits.
Alternatively,
`:second' can be omitted, in which case it is taken to
be zero.  On the rare hosts that support leap seconds, second
may be 60.

If the time is followed by `am' or `pm' (or `a.m.'
or `p.m.'), hour is restricted to run from 1 to 12, and
`:minute' may be omitted (taken to be zero).  `am'
indicates the first half of the day, `pm' indicates the second
half of the day.  In this notation, 12 is the predecessor of 1:
midnight is `12am' while noon is `12pm'.
(This is the zero-oriented interpretation of `12am' and `12pm',
as opposed to the old tradition derived from Latin
which uses `12m' for noon and `12pm' for midnight.)

The time may alternatively be followed by a time zone correction,
expressed as `shhmm', where s is `+'
or `-', hh is a number of zone hours and mm is a number
of zone minutes.  You can also separate hh from mm with a colon.
When a time zone correction is given this way, it
forces interpretation of the time relative to
Coordinated Universal Time (UTC), overriding any previous
specification for the time zone or the local time zone.  For example,
`+0530' and `+05:30' both stand for the time zone 5.5 hours
ahead of UTC (e.g., India).  The minute
part of the time of day may not be elided when a time zone correction
is used.  This is the best way to specify a time zone correction by
fractional parts of an hour.

Either `am'/`pm' or a time zone correction may be specified,
but not both.


7.4 Time zone items
===================


A time zone item specifies an international time zone, indicated
by a small set of letters, e.g., `UTC' or `Z'
for Coordinated Universal
Time.  Any included periods are ignored.  By following a
non-daylight-saving time zone by the string `DST' in a separate
word (that is, separated by some white space), the corresponding
daylight saving time zone may be specified.
Alternatively, a non-daylight-saving time zone can be followed by a
time zone correction, to add the two values.  This is normally done
only for `UTC'; for example, `UTC+05:30' is equivalent to
`+05:30'.

Time zone items other than `UTC' and `Z'
are obsolescent and are not recommended, because they
are ambiguous; for example, `EST' has a different meaning in
Australia than in the United States.  Instead, it's better to use
unambiguous numeric time zone corrections like `-0500', as
described in the previous section.

If neither a time zone item nor a time zone correction is supplied,
time stamps are interpreted using the rules of the default time zone
(see Specifying time zone rules).


7.5 Day of week items
=====================


The explicit mention of a day of the week will forward the date
(only if necessary) to reach that day of the week in the future.

Days of the week may be spelled out in full: `Sunday',
`Monday', `Tuesday', `Wednesday', `Thursday',
`Friday' or `Saturday'.  Days may be abbreviated to their
first three letters, optionally followed by a period.  The special
abbreviations `Tues' for `Tuesday', `Wednes' for
`Wednesday' and `Thur' or `Thurs' for `Thursday' are
also allowed.

A number may precede a day of the week item to move forward
supplementary weeks.  It is best used in expression like `third
monday'.  In this context, `last day' or `next
day' is also acceptable; they move one week before or after
the day that day by itself would represent.

A comma following a day of the week item is ignored.


7.6 Relative items in date strings
==================================


Relative items adjust a date (or the current date if none) forward
or backward.  The effects of relative items accumulate.  Here are some
examples:

1 year
1 year ago
3 years
2 days

The unit of time displacement may be selected by the string `year'
or `month' for moving by whole years or months.  These are fuzzy
units, as years and months are not all of equal duration.  More precise
units are `fortnight' which is worth 14 days, `week' worth 7
days, `day' worth 24 hours, `hour' worth 60 minutes,
`minute' or `min' worth 60 seconds, and `second' or
`sec' worth one second.  An `s' suffix on these units is
accepted and ignored.

The unit of time may be preceded by a multiplier, given as an optionally
signed number.  Unsigned numbers are taken as positively signed.  No
number at all implies 1 for a multiplier.  Following a relative item by
the string `ago' is equivalent to preceding the unit by a
multiplier with value -1.

The string `tomorrow' is worth one day in the future (equivalent
to `day'), the string `yesterday' is worth
one day in the past (equivalent to `day ago').

The strings `now' or `today' are relative items corresponding
to zero-valued time displacement, these strings come from the fact
a zero-valued time displacement represents the current time when not
otherwise changed by previous items.  They may be used to stress other
items, like in `12:00 today'.  The string `this' also has
the meaning of a zero-valued time displacement, but is preferred in
date strings like `this thursday'.

When a relative item causes the resulting date to cross a boundary
where the clocks were adjusted, typically for daylight saving time,
the resulting date and time are adjusted accordingly.

The fuzz in units can cause problems with relative items.  For
example, `2003-07-31 -1 month' might evaluate to 2003-07-01,
because 2003-06-31 is an invalid date.  To determine the previous
month more reliably, you can ask for the month before the 15th of the
current month.  For example:

$ date -R
Thu, 31 Jul 2003 13:02:39 -0700
$ date --date='-1 month' +'Last month was %B?'
Last month was July?
$ date --date="$(date +%Y-%m-15) -1 month" +'Last month was %B!'
Last month was June!

Also, take care when manipulating dates around clock changes such as
daylight saving leaps.  In a few cases these have added or subtracted
as much as 24 hours from the clock, so it is often wise to adopt
universal time by setting the TZ environment variable to
`UTC0' before embarking on calendrical calculations.

7.7 Pure numbers in date strings
================================


The precise interpretation of a pure decimal number depends
on the context in the date string.

If the decimal number is of the form yyyymmdd and no
other calendar date item (see Calendar date items) appears before it
in the date string, then yyyy is read as the year, mm as the
month number and dd as the day of the month, for the specified
calendar date.

If the decimal number is of the form hhmm and no other time
of day item appears before it in the date string, then hh is read
as the hour of the day and mm as the minute of the hour, for the
specified time of day.  mm can also be omitted.

If both a calendar date and a time of day appear to the left of a number
in the date string, but no relative item, then the number overrides the
year.


7.8 Seconds since the Epoch
===========================

If you precede a number with `@', it represents an internal time
stamp as a count of seconds.  The number can contain an internal
decimal point (either `.' or `,'); any excess precision not
supported by the internal representation is truncated toward minus
infinity.  Such a number cannot be combined with any other date
item, as it specifies a complete time stamp.

Internally, computer times are represented as a count of seconds since
an epoch--a well-defined point of time.  On GNU and
POSIX systems, the epoch is 1970-01-01 00:00:00 UTC, so
`@0' represents this time, `@1' represents 1970-01-01
00:00:01 UTC, and so forth.  GNU and most other
POSIX-compliant systems support such times as an extension
to POSIX, using negative counts, so that `@-1'
represents 1969-12-31 23:59:59 UTC.

Traditional Unix systems count seconds with 32-bit two's-complement
integers and can represent times from 1901-12-13 20:45:52 through
2038-01-19 03:14:07 UTC.  More modern systems use 64-bit counts
of seconds with nanosecond subcounts, and can represent all the times
in the known lifetime of the universe to a resolution of 1 nanosecond.

On most hosts, these counts ignore the presence of leap seconds.
For example, on most hosts `@915148799' represents 1998-12-31
23:59:59 UTC, `@915148800' represents 1999-01-01 00:00:00
UTC, and there is no way to represent the intervening leap second
1998-12-31 23:59:60 UTC.

7.9 Specifying time zone rules
==============================

Normally, dates are interpreted using the rules of the current time
zone, which in turn are specified by the TZ environment
variable, or by a system default if TZ is not set.  To specify a
different set of default time zone rules that apply just to one date,
start the date with a string of the form `TZ="rule"'.  The
two quote characters (`"') must be present in the date, and any
quotes or backslashes within rule must be escaped by a
backslash.

For example, with the GNU date command you can
answer the question "What time is it in New York when a Paris clock
shows 6:30am on October 31, 2004?" by using a date beginning with
`TZ="Europe/Paris"' as shown in the following shell transcript:

$ export TZ="America/New_York"
$ date --date='TZ="Europe/Paris" 2004-10-31 06:30'
Sun Oct 31 01:30:00 EDT 2004

In this example, the `--date' operand begins with its own
TZ setting, so the rest of that operand is processed according
to `Europe/Paris' rules, treating the string `2004-10-31
06:30' as if it were in Paris.  However, since the output of the
date command is processed according to the overall time zone
rules, it uses New York time.  (Paris was normally six hours ahead of
New York in 2004, but this example refers to a brief Halloween period
when the gap was five hours.)

A TZ value is a rule that typically names a location in the
`tz' database <http://www.twinsun.com/tz/tz-link.htm>.
A recent catalog of location names appears in the
TWiki Date and Time Gateway <http://twiki.org/cgi-bin/xtra/tzdate>.  A few non-GNU hosts require a colon before a
location name in a TZ setting, e.g.,
`TZ=":America/New_York"'.

The `tz' database includes a wide variety of locations ranging
from `Arctic/Longyearbyen' to `Antarctica/South_Pole', but
if you are at sea and have your own private time zone, or if you are
using a non-GNU host that does not support the `tz'
database, you may need to use a POSIX rule instead.  Simple
POSIX rules like `UTC0' specify a time zone without
daylight saving time; other rules can specify simple daylight saving
regimes.  See Specifying the Time Zone with TZ in The GNU C Library.

7.10 Authors of get_date
========================


get_date was originally implemented by Steven M. Bellovin
(<mailto:smb@research.att.com>) while at the University of North Carolina
at Chapel Hill.  The code was later tweaked by a couple of people on
Usenet, then completely overhauled by Rich $alz (<mailto:rsalz@bbn.com>)
and Jim Berets (<mailto:jberets@bbn.com>) in August, 1990.  Various
revisions for the GNU system were made by David MacKenzie, Jim Meyering,
Paul Eggert and others.

This chapter was originally produced by Franc,ois Pinard
(<mailto:pinard@iro.umontreal.ca>) from the `getdate.y' source code,
and then edited by K. Berry (<mailto:kb@cs.umb.edu>).

8 Controlling the Archive Format
********************************

Due to historical reasons, there are several formats of tar archives.
All of them are based on the same principles, but have some subtle
differences that often make them incompatible with each other.

GNU tar is able to create and handle archives in a variety of formats.
The most frequently used formats are (in alphabetical order):

gnu
Format used by GNU tar versions up to 1.13.25.  This format derived
from an early POSIX standard, adding some improvements such as
sparse file handling and incremental archives.  Unfortunately these
features were implemented in a way incompatible with other archive
formats.

Archives in `gnu' format are able to hold pathnames of unlimited
length.

oldgnu
Format used by GNU tar of versions prior to 1.12.

v7
Archive format, compatible with the V7 implementation of tar.  This
format imposes a number of limitations.  The most important of them
are:

1. The maximum length of a file name is limited to 99 characters.
2. The maximum length of a symbolic link is limited to 99 characters.
3. It is impossible to store special files (block and character
devices, fifos etc.)
4. Maximum value of user or group ID is limited to 2097151 (7777777
octal)
5. V7 archives do not contain symbolic ownership information (user
and group name of the file owner).

This format has traditionally been used by Automake when producing
Makefiles.  This practice will change in the future, in the meantime,
however this means that projects containing filenames more than 99
characters long will not be able to use GNU tar 1.15.92 and
Automake prior to 1.9.

ustar
Archive format defined by POSIX.1-1988 specification.  It stores
symbolic ownership information.  It is also able to store
special files.  However, it imposes several restrictions as well:

1. The maximum length of a file name is limited to 256 characters,
provided that the filename can be split at directory separator in
two parts, first of them being at most 155 bytes long.  So, in most
cases the maximum file name length will be shorter than 256
characters.
2. The maximum length of a symbolic link name is limited to
100 characters.
3. Maximum size of a file the archive is able to accomodate
is 8GB
4. Maximum value of UID/GID is 2097151.
5. Maximum number of bits in device major and minor numbers is 21.

star
Format used by Jo"rg Schilling star
implementation.  GNU tar is able to read `star' archives but
currently does not produce them.

posix
Archive format defined by POSIX.1-2001 specification.  This is the
most flexible and feature-rich format.  It does not impose any
restrictions on file sizes or filename lengths.  This format is quite
recent, so not all tar implementations are able to handle it properly.
However, this format is designed in such a way that any tar
implementation able to read `ustar' archives will be able to read
most `posix' archives as well, with the only exception that any
additional information (such as long file names etc.) will in such
case be extracted as plain text files along with the files it refers to.

This archive format will be the default format for future versions
of GNU tar.


The following table summarizes the limitations of each of these
formats:

 Format UID File Size Path Name Devn
 gnu 1.8e19 Unlimited Unlimited 63
 oldgnu 1.8e19 Unlimited Unlimited 63
 v7 2097151 8GB 99 n/a
 ustar 2097151 8GB 256 21
 posix Unlimited Unlimited Unlimited Unlimited

The default format for GNU tar is defined at compilation
time.  You may check it by running tar --help, and examining
the last lines of its output.  Usually, GNU tar is configured
to create archives in `gnu' format, however, future version will
switch to `posix'.

* Compression::                 Using Less Space through Compression
* Attributes::                  Handling File Attributes
* Portability::                 Making tar Archives More Portable
* cpio::                        Comparison of tar and cpio

8.1 Using Less Space through Compression
========================================

* gzip::                        Creating and Reading Compressed Archives
* sparse::                      Archiving Sparse Files

8.1.1 Creating and Reading Compressed Archives
----------------------------------------------

GNU tar is able to create and read compressed archives.  It supports
gzip and bzip2 compression programs.  For backward
compatibilty, it also supports compress command, although
we strongly recommend against using it, since there is a patent
covering the algorithm it uses and you could be sued for patent
infringement merely by running compress!  Besides, it is less
effective than gzip and bzip2.

Creating a compressed archive is simple: you just specify a
compression option along with the usual archive creation
commands.  The compression option is `-z' (`--gzip') to
create a gzip compressed archive, `-j'
(`--bzip2') to create a bzip2 compressed archive, and
`-Z' (`--compress') to use compress program.
For example:

$ tar cfz archive.tar.gz .

Reading compressed archive is even simpler: you don't need to specify
any additional options as GNU tar recognizes its format
automatically.  Thus, the following commands will list and extract the
archive created in previous example:

# List the compressed archive
$ tar tf archive.tar.gz
# Extract the compressed archive
$ tar xf archive.tar.gz

The only case when you have to specify a decompression option while
reading the archive is when reading from a pipe or from a tape drive
that does not support random access.  However, in this case GNU tar
will indicate which option you should use.  For example:

$ cat archive.tar.gz | tar tf -
tar: Archive is compressed.  Use -z option
tar: Error is not recoverable: exiting now

If you see such diagnostics, just add the suggested option to the
invocation of GNU tar:

$ cat archive.tar.gz | tar tfz -

Notice also, that there are several restrictions on operations on
compressed archives.  First of all, compressed archives cannot be
modified, i.e., you cannot update (`--update' (`-u')) them or delete
(`--delete') members from them.  Likewise, you cannot append
another tar archive to a compressed archive using
`--append' (`-r')).  Secondly, multi-volume archives cannot be
compressed.

The following table summarizes compression options used by GNU tar.

`-z'
`--gzip'
`--ungzip'
Filter the archive through gzip.

You can use `--gzip' and `--gunzip' on physical devices
(tape drives, etc.) and remote files as well as on normal files; data
to or from such devices or remote files is reblocked by another copy
of the tar program to enforce the specified (or default) record
size.  The default compression parameters are used; if you need to
override them, set GZIP environment variable, e.g.:

$ GZIP=--best tar cfz archive.tar.gz subdir

Another way would be to avoid the `--gzip' (`--gunzip', `--ungzip', `-z') option and run
gzip explicitly:

$ tar cf - subdir | gzip --best -c - > archive.tar.gz

About corrupted compressed archives: gzip'ed files have no
redundancy, for maximum compression.  The adaptive nature of the
compression scheme means that the compression tables are implicitly
spread all over the archive.  If you lose a few blocks, the dynamic
construction of the compression tables becomes unsynchronized, and there
is little chance that you could recover later in the archive.

There are pending suggestions for having a per-volume or per-file
compression in GNU tar.  This would allow for viewing the
contents without decompression, and for resynchronizing decompression at
every volume or file, in case of corrupted archives.  Doing so, we might
lose some compressibility.  But this would have make recovering easier.
So, there are pros and cons.  We'll see!

`-j'
`--bzip2'
Filter the archive through bzip2.  Otherwise like `--gzip'.

`-Z'
`--compress'
`--uncompress'
Filter the archive through compress.  Otherwise like `--gzip'.

The GNU Project recommends you not use
compress, because there is a patent covering the algorithm it
uses.  You could be sued for patent infringement merely by running
compress.

`--use-compress-program=prog'
Use external compression program prog.  Use this option if you
have a compression program that GNU tar does not support.  There
are two requirements to which prog should comply:

First, when called without options, it should read data from standard
input, compress it and output it on standard output.

Secondly, if called with `-d' argument, it should do exactly
the opposite, i.e., read the compressed data from the standard input
and produce uncompressed data on the standard output.

The `--use-compress-program' option, in particular, lets you
implement your own filters, not necessarily dealing with
compression/decomression.  For example, suppose you wish to implement
PGP encryption on top of compression, using gpg (see gpg --- encryption and signing tool in GNU Privacy Guard Manual).  The following script does that:  

#! /bin/sh
case $1 in
-d) gpg --decrypt - | gzip -d -c;;
'') gzip -c | gpg -s ;;
*)  echo "Unknown option $1">&2; exit 1;;
esac

Suppose you name it `gpgz' and save it somewhere in your
PATH.  Then the following command will create a commpressed
archive signed with your private key:

$ tar -cf foo.tar.gpgz --use-compress=gpgz .

Likewise, the following command will list its contents:

$ tar -tf foo.tar.gpgz --use-compress=gpgz .


8.1.2 Archiving Sparse Files
----------------------------

Files in the file system occasionally have holes.  A hole
in a file is a section of the file's contents which was never written.
The contents of a hole reads as all zeros.  On many operating systems,
actual disk storage is not allocated for holes, but they are counted
in the length of the file.  If you archive such a file, tar
could create an archive longer than the original.  To have tar
attempt to recognize the holes in a file, use `--sparse'
(`-S').  When you use this option, then, for any file using
less disk space than would be expected from its length, tar
searches the file for consecutive stretches of zeros.  It then records
in the archive for the file where the consecutive stretches of zeros
are, and only archives the "real contents" of the file.  On
extraction (using `--sparse' is not needed on extraction) any
such files have holes created wherever the continuous stretches of zeros
were found.  Thus, if you use `--sparse', tar archives
won't take more space than the original.

`-S'
`--sparse'
This option istructs tar to test each file for sparseness
before attempting to archive it.  If the file is found to be sparse it
is treated specially, thus allowing to decrease the amount of space
used by its image in the archive.

This option is meaningful only when creating or updating archives.  It
has no effect on extraction.

Consider using `--sparse' when performing file system backups,
to avoid archiving the expanded forms of files stored sparsely in the
system. 

Even if your system has no sparse files currently, some may be
created in the future.  If you use `--sparse' while making file
system backups as a matter of course, you can be assured the archive
will never take more space on the media than the files take on disk
(otherwise, archiving a disk filled with sparse files might take
hundreds of tapes).  See Incremental Dumps.

However, be aware that `--sparse' option presents a serious
drawback.  Namely, in order to determine if the file is sparse
tar has to read it before trying to archive it, so in total
the file is read twice.  So, always bear in mind that the
time needed to process all files with this option is roughly twice
the time needed to archive them without it.


When using `POSIX' archive format, GNU tar is able to store
sparse files using in three distinct ways, called sparse
formats.  A sparse format is identified by its number,
consisting, as usual of two decimal numbers, delimited by a dot.  By
default, format `1.0' is used.  If, for some reason, you wish to
use an earlier format, you can select it using
`--sparse-version' option. 

`--sparse-version=version'

Select the format to store sparse files in.  Valid version values
are: `0.0', `0.1' and `1.0'.  See Sparse Formats,
for a detailed description of each format.

Using `--sparse-format' option implies `--sparse'.

8.2 Handling File Attributes
============================
(This message will disappear, once this node revised.)

When tar reads files, it updates their access times.  To
avoid this, use the `--atime-preserve[=METHOD]' option, which can either
reset the access time retroactively or avoid changing it in the first
place.

Handling of file attributes

`--atime-preserve'
`--atime-preserve=replace'
`--atime-preserve=system'
Preserve the access times of files that are read.  This works only for
files that you own, unless you have superuser privileges.

`--atime-preserve=replace' works on most systems, but it also
restores the data modification time and updates the status change
time.  Hence it doesn't interact with incremental dumps nicely
(see Incremental Dumps), and it can set access or data modification times
incorrectly if other programs access the file while tar is
running.

`--atime-preserve=system' avoids changing the access time in
the first place, if the operating system supports this.
Unfortunately, this may or may not work on any given operating system
or file system.  If tar knows for sure it won't work, it
complains right away.

Currently `--atime-preserve' with no operand defaults to
`--atime-preserve=replace', but this is intended to change to
`--atime-preserve=system' when the latter is better-supported.

`-m'
`--touch'
Do not extract data modification time.

When this option is used, tar leaves the data modification times
of the files it extracts as the times when the files were extracted,
instead of setting it to the times recorded in the archive.

This option is meaningless with `--list' (`-t').

`--same-owner'
Create extracted files with the same ownership they have in the
archive.

This is the default behavior for the superuser,
so this option is meaningful only for non-root users, when tar
is executed on those systems able to give files away.  This is
considered as a security flaw by many people, at least because it
makes quite difficult to correctly account users for the disk space
they occupy.  Also, the suid or sgid attributes of
files are easily and silently lost when files are given away.

When writing an archive, tar writes the user id and user name
separately.  If it can't find a user name (because the user id is not
in `/etc/passwd'), then it does not write one.  When restoring,
it tries to look the name (if one was written) up in
`/etc/passwd'.  If it fails, then it uses the user id stored in
the archive instead. 

`--no-same-owner'
`-o'
Do not attempt to restore ownership when extracting.  This is the
default behavior for ordinary users, so this option has an effect
only for the superuser.

`--numeric-owner'
The `--numeric-owner' option allows (ANSI) archives to be written
without user/group name information or such information to be ignored
when extracting.  It effectively disables the generation and/or use
of user/group name information.  This option forces extraction using
the numeric ids from the archive, ignoring the names.

This is useful in certain circumstances, when restoring a backup from
an emergency floppy with different passwd/group files for example.
It is otherwise impossible to extract files with the right ownerships
if the password file in use during the extraction does not match the
one belonging to the file system(s) being extracted.  This occurs,
for example, if you are restoring your files after a major crash and
had booted from an emergency floppy with no password file or put your
disk into another machine to do the restore.

The numeric ids are always saved into tar archives.
The identifying names are added at create time when provided by the
system, unless `--old-archive' (`-o') is used.  Numeric ids could be
used when moving archives between a collection of machines using
a centralized management for attribution of numeric ids to users
and groups.  This is often made through using the NIS capabilities.

When making a tar file for distribution to other sites, it
is sometimes cleaner to use a single owner for all files in the
distribution, and nicer to specify the write permission bits of the
files as stored in the archive independently of their actual value on
the file system.  The way to prepare a clean distribution is usually
to have some Makefile rule creating a directory, copying all needed
files in that directory, then setting ownership and permissions as
wanted (there are a lot of possible schemes), and only then making a
tar archive out of this directory, before cleaning
everything out.  Of course, we could add a lot of options to
GNU tar for fine tuning permissions and ownership.
This is not the good way, I think.  GNU tar is
already crowded with options and moreover, the approach just explained
gives you a great deal of control already.

`-p'
`--same-permissions'
`--preserve-permissions'
Extract all protection information.

This option causes tar to set the modes (access permissions) of
extracted files exactly as recorded in the archive.  If this option
is not used, the current umask setting limits the permissions
on extracted files.  This option is by default enabled when
tar is executed by a superuser.


This option is meaningless with `--list' (`-t').

`--preserve'
Same as both `--same-permissions' and `--same-order'.

The `--preserve' option has no equivalent short option name.
It is equivalent to `--same-permissions' plus `--same-order'.




8.3 Making tar Archives More Portable
=====================================

Creating a tar archive on a particular system that is meant to be
useful later on many other machines and with other versions of tar
is more challenging than you might think.  tar archive formats
have been evolving since the first versions of Unix.  Many such formats
are around, and are not always compatible with each other.  This section
discusses a few problems, and gives some advice about making tar
archives more portable.

One golden rule is simplicity.  For example, limit your tar
archives to contain only regular files and directories, avoiding
other kind of special files.  Do not attempt to save sparse files or
contiguous files as such.  Let's discuss a few more problems, in turn.



* Portable Names::              Portable Names
* dereference::                 Symbolic Links
* old::                         Old V7 Archives
* ustar::                       Ustar Archives
* gnu::                         GNU and old GNU format archives.
* posix::                       POSIX archives
* Checksumming::                Checksumming Problems
* Large or Negative Values::    Large files, negative time stamps, etc.
* Other Tars::                  How to Extract GNU-Specific Data Using
                                Other tar Implementations

8.3.1 Portable Names
--------------------

Use portable file and member names.  A name is portable if it contains
only ASCII letters and digits, `/', `.', `_', and
`-'; it cannot be empty, start with `-' or `//', or
contain `/-'.  Avoid deep directory nesting.  For portability to
old Unix hosts, limit your file name components to 14 characters or
less.

If you intend to have your tar archives to be read under
MSDOS, you should not rely on case distinction for file names, and you
might use the GNU doschk program for helping you
further diagnosing illegal MSDOS names, which are even more limited
than System V's.

8.3.2 Symbolic Links
--------------------

Normally, when tar archives a symbolic link, it writes a
block to the archive naming the target of the link.  In that way, the
tar archive is a faithful record of the file system contents.
`--dereference' (`-h') is used with `--create' (`-c'), and causes
tar to archive the files symbolic links point to, instead of
the links themselves.  When this option is used, when tar
encounters a symbolic link, it will archive the linked-to file,
instead of simply recording the presence of a symbolic link.

The name under which the file is stored in the file system is not
recorded in the archive.  To record both the symbolic link name and
the file name in the system, archive the file under both names.  If
all links were recorded automatically by tar, an extracted file
might be linked to a file name that no longer exists in the file
system.

If a linked-to file is encountered again by tar while creating
the same archive, an entire second copy of it will be stored.  (This
might be considered a bug.)

So, for portable archives, do not archive symbolic links as such,
and use `--dereference' (`-h'): many systems do not support
symbolic links, and moreover, your distribution might be unusable if
it contains unresolved symbolic links.

8.3.3 Old V7 Archives
---------------------

Certain old versions of tar cannot handle additional
information recorded by newer tar programs.  To create an
archive in V7 format (not ANSI), which can be read by these old
versions, specify the `--format=v7' option in
conjunction with the `--create' (`-c') (tar also
accepts `--portability' or `--old-archive' for this
option).  When you specify it,
tar leaves out information about directories, pipes, fifos,
contiguous files, and device files, and specifies file ownership by
group and user IDs instead of group and user names.

When updating an archive, do not use `--format=v7'
unless the archive was created using this option.

In most cases, a new format archive can be read by an old
tar program without serious trouble, so this option should
seldom be needed.  On the other hand, most modern tars are
able to read old format archives, so it might be safer for you to
always use `--format=v7' for your distributions.  Notice,
however, that `ustar' format is a better alternative, as it is
free from many of `v7''s drawbacks.

8.3.4 Ustar Archive Format
--------------------------

Archive format defined by POSIX.1-1988 specification is called
ustar.  Although it is more flexible than the V7 format, it
still has many restrictions (See ustar, for the detailed
description of ustar format).  Along with V7 format,
ustar format is a good choice for archives intended to be read
with other implementations of tar.

To create archive in ustar format, use `--format=ustar'
option in conjunction with the `--create' (`-c').

8.3.5 GNU and old GNU tar format
--------------------------------

GNU tar was based on an early draft of the
POSIX 1003.1 ustar standard.  GNU extensions to
tar, such as the support for file names longer than 100
characters, use portions of the tar header record which were
specified in that POSIX draft as unused.  Subsequent changes in
POSIX have allocated the same parts of the header record for
other purposes.  As a result, GNU tar format is
incompatible with the current POSIX specification, and with
tar programs that follow it.

In the majority of cases, tar will be configured to create
this format by default.  This will change in the future releases, since
we plan to make `POSIX' format the default.

To force creation a GNU tar archive, use option
`--format=gnu'.

8.3.6 GNU tar and POSIX tar
---------------------------

Starting from version 1.14 GNU tar features full support for
POSIX.1-2001 archives.

A POSIX conformant archive will be created if tar
was given `--format=posix' (`--format=pax') option.  No
special option is required to read and extract from a POSIX
archive. 

* PAX keywords:: Controlling Extended Header Keywords.

8.3.6.1 Controlling Extended Header Keywords
............................................

`--pax-option=keyword-list'
Handle keywords in PAX extended headers.  This option is
equivalent to `-o' option of the pax utility.

Keyword-list is a comma-separated
list of keyword options, each keyword option taking one of
the following forms:

delete=pattern
When used with one of archive-creation commands,
this option instructs tar to omit from extended header records
that it produces any keywords matching the string pattern.

When used in extract or list mode, this option instructs tar
to ignore any keywords matching the given pattern in the extended
header records.  In both cases, matching is performed using the pattern
matching notation described in POSIX 1003.2, 3.13
(see wildcards). For example:

--pax-option delete=security.*

would suppress security-related information.

exthdr.name=string

This keyword allows user control over the name that is written into the
ustar header blocks for the extended headers.  The name is obtained
from string after making the following substitutions:

 Meta-character Replaced By
 %d The directory name of the file, equivalent to the
result of the dirname utility on the translated pathname.
 %f The filename of the file, equivalent to the result
of the basename utility on the translated pathname.
 %p The process ID of the tar process.
 %% A `%' character.

Any other `%' characters in string produce undefined
results.

If no option `exthdr.name=string' is specified, tar
will use the following default value:

%d/PaxHeaders.%p/%f

globexthdr.name=string
This keyword allows user control over the name that is written into
the ustar header blocks for global extended header records.  The name
is obtained from the contents of string, after making
the following substitutions:

 Meta-character Replaced By
 %n An integer that represents the
sequence number of the global extended header record in the archive,
starting at 1.
 %p The process ID of the tar process.
 %% A `%' character.

Any other `%' characters in string produce undefined results.

If no option `globexthdr.name=string' is specified, tar
will use the following default value:

$TMPDIR/GlobalHead.%p.%n

where `$TMPDIR' represents the value of the TMPDIR
environment variable.  If TMPDIR is not set, tar
uses `/tmp'.

keyword=value
When used with one of archive-creation commands, these keyword/value pairs
will be included at the beginning of the archive in a global extended
header record.  When used with one of archive-reading commands,
tar will behave as if it has encountered these keyword/value
pairs at the beginning of the archive in a global extended header
record.

keyword:=value
When used with one of archive-creation commands, these keyword/value pairs
will be included as records at the beginning of an extended header for
each file.  This is effectively equivalent to keyword=value
form except that it creates no global extended header records.

When used with one of archive-reading commands, tar will
behave as if these keyword/value pairs were included as records at the
end of each extended header; thus, they will override any global or
file-specific extended header record keywords of the same names.
For example, in the command:

tar --format=posix --create \
    --file archive --pax-option gname:=user .

the group name will be forced to a new value for all files
stored in the archive.

8.3.7 Checksumming Problems
---------------------------

SunOS and HP-UX tar fail to accept archives created using
GNU tar and containing non-ASCII file names, that
is, file names having characters with the eight bit set, because they
use signed checksums, while GNU tar uses unsigned
checksums while creating archives, as per POSIX standards.  On
reading, GNU tar computes both checksums and
accept any.  It is somewhat worrying that a lot of people may go
around doing backup of their files using faulty (or at least
non-standard) software, not learning about it until it's time to
restore their missing files with an incompatible file extractor, or
vice versa.

GNU tar compute checksums both ways, and accept
any on read, so GNU tar can read Sun tapes even with their
wrong checksums.  GNU tar produces the standard
checksum, however, raising incompatibilities with Sun.  That is to
say, GNU tar has not been modified to
produce incorrect archives to be read by buggy tar's.
I've been told that more recent Sun tar now read standard
archives, so maybe Sun did a similar patch, after all?

The story seems to be that when Sun first imported tar
sources on their system, they recompiled it without realizing that
the checksums were computed differently, because of a change in
the default signing of char's in their compiler.  So they
started computing checksums wrongly.  When they later realized their
mistake, they merely decided to stay compatible with it, and with
themselves afterwards.  Presumably, but I do not really know, HP-UX
has chosen that their tar archives to be compatible with Sun's.
The current standards do not favor Sun tar format.  In any
case, it now falls on the shoulders of SunOS and HP-UX users to get
a tar able to read the good archives they receive.

8.3.8 Large or Negative Values
------------------------------
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The above sections suggest to use `oldest possible' archive
format if in doubt.  However, sometimes it is not possible.  If you
attempt to archive a file whose metadata cannot be represented using
required format, GNU tar will print error message and ignore such a
file.  You will than have to switch to a format that is able to
handle such values.  The format summary table (see Formats) will
help you to do so.

In particular, when trying to archive files larger than 8GB or with
timestamps not in the range 1970-01-01 00:00:00 through 2242-03-16
12:56:31 UTC, you will have to chose between GNU and
POSIX archive formats.  When considering which format to
choose, bear in mind that the GNU format uses
two's-complement base-256 notation to store values that do not fit
into standard ustar range.  Such archives can generally be
read only by a GNU tar implementation.  Moreover, they sometimes
cannot be correctly restored on another hosts even by GNU tar. For
example, using two's complement representation for negative time
stamps that assumes a signed 32-bit time_t generates archives
that are not portable to hosts with differing time_t
representations.

On the other hand, POSIX archives, generally speaking, can
be extracted by any tar implementation that understands older
ustar format.  The only exception are files larger than 8GB.



8.3.9 How to Extract GNU-Specific Data Using Other tar Implementations
----------------------------------------------------------------------

In previous sections you became acquainted with various quircks
necessary to make your archives portable.  Sometimes you may need to
extract archives containing GNU-specific members using some
third-party tar implementation or an older version of
GNU tar.  Of course your best bet is to have GNU tar installed,
but if it is for some reason impossible, this section will explain
how to cope without it. 

When we speak about GNU-specific members we mean two classes of
them: members split between the volumes of a multi-volume archive and
sparse members.  You will be able to always recover such members if
the archive is in PAX format.  In addition split members can be
recovered from archives in old GNU format.  The following subsections
describe the required procedures in detail.

* Split Recovery::       Members Split Between Volumes
* Sparse Recovery::      Sparse Members

8.3.9.1 Extracting Members Split Between Volumes
................................................

If a member is split between several volumes of an old GNU format archive
most third party tar implementation will fail to extract
it.  To extract it, use tarcat program (see Tarcat).
This program is available from
GNU tar home page <http://www.gnu.org/software/tar/utils/tarcat.html>.  It concatenates several archive volumes into a single
valid archive.  For example, if you have three volumes named from
`vol-1.tar' to `vol-2.tar', you can do the following to
extract them using a third-party tar:

$ tarcat vol-1.tar vol-2.tar vol-3.tar | tar xf -

You could use this approach for many (although not all) PAX
format archives as well.  However, extracting split members from a PAX
archive is a much easier task, because PAX volumes are constructed in
such a way that each part of a split member is extracted as a
different file by tar implementations that are not aware of
GNU extensions.  More specifically, the very first part retains its
original name, and all subsequent parts are named using the pattern:

%d/GNUFileParts.%p/%f.%n

where symbols preceeded by `%' are macro characters that
have the following meaning:

 Meta-character Replaced By
 %d The directory name of the file, equivalent to the
result of the dirname utility on its full name.
 %f The file name of the file, equivalent to the result
of the basename utility on its full name.
 %p The process ID of the tar process that
created the archive.
 %n Ordinal number of this particular part.

For example, if, a file `var/longfile' was split during archive
creation between three volumes, and the creator tar process
had process ID `27962', then the member names will be:

var/longfile
var/GNUFileParts.27962/longfile.1
var/GNUFileParts.27962/longfile.2

When you extract your archive using a third-party tar, these
files will be created on your disk, and the only thing you will need
to do to restore your file in its original form is concatenate them in
the proper order, for example:

$ cd var
$ cat GNUFileParts.27962/longfile.1 \
  GNUFileParts.27962/longfile.2 >> longfile
$ rm -f GNUFileParts.27962

Notice, that if the tar implementation you use supports PAX
format archives, it will probably emit warnings about unknown keywords
during extraction.  They will lool like this:

Tar file too small
Unknown extended header keyword 'GNU.volume.filename' ignored.
Unknown extended header keyword 'GNU.volume.size' ignored.
Unknown extended header keyword 'GNU.volume.offset' ignored.

You can safely ignore these warnings.

If your tar implementation is not PAX-aware, you will get
more warnigns and more files generated on your disk, e.g.:

$ tar xf vol-1.tar
var/PaxHeaders.27962/longfile: Unknown file type 'x', extracted as
normal file 
Unexpected EOF in archive
$ tar xf vol-2.tar
tmp/GlobalHead.27962.1: Unknown file type 'g', extracted as normal file
GNUFileParts.27962/PaxHeaders.27962/sparsefile.1: Unknown file type
'x', extracted as normal file

Ignore these warnings.  The `PaxHeaders.*' directories created
will contain files with extended header keywords describing the
extracted files.  You can delete them, unless they describe sparse
members.  Read further to learn more about them.

8.3.9.2 Extracting Sparse Members
.................................

Any tar implementation will be able to extract sparse members from a
PAX archive.  However, the extracted files will be condensed,
i.e. any zero blocks will be removed from them.  When we restore such
a condensed file to its original form, by adding zero bloks (or
holes) back to their original locations, we call this process
expanding a compressed sparse file.

To expand a file, you will need a simple auxiliary program called
xsparse.  It is available in source form from
GNU tar home page <http://www.gnu.org/software/tar/utils/xsparse.html>.

Let's begin with archive members in sparse format
version 1.0(18), which are the easiest to expand.
The condensed file will contain both file map and file data, so no
additional data will be needed to restore it.  If the original file
name was `dir/name', then the condensed file will be
named `dir/GNUSparseFile.n/name', where 
n is a decimal number(19).

To expand a version 1.0 file, run xsparse as follows:

$ xsparse `cond-file'

where `cond-file' is the name of the condensed file.  The utility
will deduce the name for the resulting expanded file using the
following algorithm:

1. If `cond-file' does not contain any directories,
`../cond-file' will be used;

2. If `cond-file' has the form
`dir/t/name', where both t and name
are simple names, with no `/' characters in them, the output file
name will be `dir/name'.

3. Otherwise, if `cond-file' has the form
`dir/name', the output file name will be
`name'.

In the unlikely case when this algorithm does not suite your needs,
you can explicitely specify output file name as a second argument to
the command:

$ xsparse `cond-file'

It is often a good idea to run xsparse in dry run mode
first.  In this mode, the command does not actually expand the file,
but verbosely lists all actions it would be taking to do so.  The dry
run mode is enabled by `-n' command line argument:

$ xsparse -n /home/gray/GNUSparseFile.6058/sparsefile
Reading v.1.0 sparse map
Expanding file `/home/gray/GNUSparseFile.6058/sparsefile' to
`/home/gray/sparsefile'
Finished dry run

To actually expand the file, you would run:

$ xsparse /home/gray/GNUSparseFile.6058/sparsefile

The program behaves the same way all UNIX utilities do: it will keep
quiet unless it has simething important to tell you (e.g. an error
condition or something).  If you wish it to produce verbose output,
similar to that from the dry run mode, give it `-v' option:

$ xsparse -v /home/gray/GNUSparseFile.6058/sparsefile
Reading v.1.0 sparse map
Expanding file `/home/gray/GNUSparseFile.6058/sparsefile' to
`/home/gray/sparsefile'
Done

Additionally, if your tar implementation has extracted the
extended headers for this file, you can instruct xstar
to use them in order to verify the integrity of the expanded file.
The option `-x' sets the name of the extended header file to
use.  Continuing our example:

$ xsparse -v -x /home/gray/PaxHeaders.6058/sparsefile \
  /home/gray/GNUSparseFile.6058/sparsefile
Reading extended header file
Found variable GNU.sparse.major = 1
Found variable GNU.sparse.minor = 0
Found variable GNU.sparse.name = sparsefile
Found variable GNU.sparse.realsize = 217481216
Reading v.1.0 sparse map
Expanding file `/home/gray/GNUSparseFile.6058/sparsefile' to
`/home/gray/sparsefile'
Done


An extended header is a special tar archive header
that precedes an archive member and contains a set of
variables, describing the member properties that cannot be
stored in the standard ustar header.  While optional for
expanding sparse version 1.0 members, use of extended headers is
mandatory when expanding sparse members in older sparse formats: v.0.0
and v.0.1 (The sparse formats are described in detail in see Sparse Formats).  So, for this format, the question is: how to obtain
extended headers from the archive? 

If you use a tar implementation that does not support PAX
format, extended headers for each member will be extracted as a 
separate file.  If we represent the member name as
`dir/name', then the extended header file will be
named `dir/PaxHeaders.n/name', where
n is an integer number.

Things become more difficult if your tar implementation
does support PAX headers, because in this case you will have to
manually extract the headers.  We recommend the following algorithm:

1. Consult the documentation for your tar implementation for an
option that will print block numbers along with the archive
listing (analogous to GNU tar's `-R' option).  For example,
star has `-block-number'.

2. Obtain the verbose listing using the `block number' option, and
find the position of the sparse member in question and the member
immediately following it.  For example, running star on our
archive we obtain:

$ star -t -v -block-number -f arc.tar
...
star: Unknown extended header keyword 'GNU.sparse.size' ignored.
star: Unknown extended header keyword 'GNU.sparse.numblocks' ignored.
star: Unknown extended header keyword 'GNU.sparse.name' ignored.
star: Unknown extended header keyword 'GNU.sparse.map' ignored.
block        56:  425984 -rw-r--r--  gray/users Jun 25 14:46 2006 GNUSparseFile.28124/sparsefile
block       897:   65391 -rw-r--r--  gray/users Jun 24 20:06 2006 README
...

(as usual, ignore the warnings about unknown keywords.)

3. Let size be the size of the sparse member, Bs be its block number
and Bn be the block number of the next member.
Compute: 

N = Bs - Bn - size/512 - 2

This number gives the size of the extended header part in tar blocks.
In our example, this formula gives: 897 - 56 - 425984 / 512 - 2
= 7.

4. Use dd to extract the headers:

dd if=archive of=hname bs=512 skip=Bs count=N

where archive is the archive name, hname is a name of the
file to store the extended header in, Bs and N are
computed in previous steps.

In our example, this command will be

$ dd if=arc.tar of=xhdr bs=512 skip=56 count=7

Finally, you can expand the condensed file, using the obtained header:

$ xsparse -v -x xhdr GNUSparseFile.6058/sparsefile
Reading extended header file
Found variable GNU.sparse.size = 217481216
Found variable GNU.sparse.numblocks = 208
Found variable GNU.sparse.name = sparsefile
Found variable GNU.sparse.map = 0,2048,1050624,2048,...
Expanding file `GNUSparseFile.28124/sparsefile' to `sparsefile'
Done

8.4 Comparison of tar and cpio
==============================
(This message will disappear, once this node revised.)



The cpio archive formats, like tar, do have maximum
pathname lengths.  The binary and old ASCII formats have a max path
length of 256, and the new ASCII and CRC ASCII formats have a max
path length of 1024.  GNU cpio can read and write archives
with arbitrary pathname lengths, but other cpio implementations
may crash unexplainedly trying to read them.

tar handles symbolic links in the form in which it comes in BSD;
cpio doesn't handle symbolic links in the form in which it comes
in System V prior to SVR4, and some vendors may have added symlinks
to their system without enhancing cpio to know about them.
Others may have enhanced it in a way other than the way I did it
at Sun, and which was adopted by AT&T (and which is, I think, also
present in the cpio that Berkeley picked up from AT&T and put
into a later BSD release--I think I gave them my changes).

(SVR4 does some funny stuff with tar; basically, its cpio
can handle tar format input, and write it on output, and it
probably handles symbolic links.  They may not have bothered doing
anything to enhance tar as a result.)

cpio handles special files; traditional tar doesn't.

tar comes with V7, System III, System V, and BSD source;
cpio comes only with System III, System V, and later BSD
(4.3-tahoe and later).

tar's way of handling multiple hard links to a file can handle
file systems that support 32-bit inumbers (e.g., the BSD file system);
cpios way requires you to play some games (in its "binary"
format, i-numbers are only 16 bits, and in its "portable ASCII" format,
they're 18 bits--it would have to play games with the "file system ID"
field of the header to make sure that the file system ID/i-number pairs
of different files were always different), and I don't know which
cpios, if any, play those games.  Those that don't might get
confused and think two files are the same file when they're not, and
make hard links between them.

tars way of handling multiple hard links to a file places only
one copy of the link on the tape, but the name attached to that copy
is the only one you can use to retrieve the file; cpios
way puts one copy for every link, but you can retrieve it using any
of the names.

What type of check sum (if any) is used, and how is this calculated.

See the attached manual pages for tar and cpio format.
tar uses a checksum which is the sum of all the bytes in the
tar header for a file; cpio uses no checksum.

If anyone knows why cpio was made when tar was present
at the unix scene,

It wasn't.  cpio first showed up in PWB/UNIX 1.0; no
generally-available version of UNIX had tar at the time.  I don't
know whether any version that was generally available within AT&T
had tar, or, if so, whether the people within AT&T who did
cpio knew about it.

On restore, if there is a corruption on a tape tar will stop at
that point, while cpio will skip over it and try to restore the
rest of the files.

The main difference is just in the command syntax and header format.

tar is a little more tape-oriented in that everything is blocked
to start on a record boundary.

Is there any differences between the ability to recover crashed
archives between the two of them.  (Is there any chance of recovering
crashed archives at all.)

Theoretically it should be easier under tar since the blocking
lets you find a header with some variation of `dd skip=nn'.
However, modern cpio's and variations have an option to just
search for the next file header after an error with a reasonable chance
of resyncing.  However, lots of tape driver software won't allow you to
continue past a media error which should be the only reason for getting
out of sync unless a file changed sizes while you were writing the
archive.

If anyone knows why cpio was made when tar was present
at the unix scene, please tell me about this too.

Probably because it is more media efficient (by not blocking everything
and using only the space needed for the headers where tar
always uses 512 bytes per file header) and it knows how to archive
special files.

You might want to look at the freely available alternatives.  The
major ones are afio, GNU tar, and
pax, each of which have their own extensions with some
backwards compatibility.

Sparse files were tarred as sparse files (which you can
easily test, because the resulting archive gets smaller, and
GNU cpio can no longer read it).

9 Tapes and Other Archive Media
*******************************
(This message will disappear, once this node revised.)

A few special cases about tape handling warrant more detailed
description.  These special cases are discussed below.

Many complexities surround the use of tar on tape drives.  Since
the creation and manipulation of archives located on magnetic tape was
the original purpose of tar, it contains many features making
such manipulation easier.

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

The amount of data a tape or disk holds depends not only on its size,
but also on how it is formatted.  A 2400 foot long reel of mag tape
holds 40 megabytes of data when formatted 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 discarded when they begin to produce data errors.  EXABYTE
tape cartridges should be discarded when they generate an 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.

* Device::                      Device selection and switching
* Remote Tape Server::
* Common Problems and Solutions::
* Blocking::                    Blocking
* Many::                        Many archives on one tape
* Using Multiple Tapes::        Using Multiple Tapes
* label::                       Including a Label in the Archive
* verify::
* Write Protection::

9.1 Device Selection and Switching
==================================
(This message will disappear, once this node revised.)

`-f [hostname:]file'
`--file=[hostname:]file'
Use archive file or device file on hostname.

This option is used to specify the file name of the archive tar
works on.

If the file name is `-', tar reads the archive from standard
input (when listing or extracting), or writes it to standard output
(when creating).  If the `-' file name is given when updating an
archive, tar will read the original archive from its standard
input, and will write the entire new archive to its standard output.

If the file name contains a `:', it is interpreted as
`hostname:file name'.  If the hostname contains an at
sign (`@'), it is treated as `user@hostname:file name'.  In
either case, tar will invoke the command rsh (or
remsh) to start up an /usr/libexec/rmt on the remote
machine.  If you give an alternate login name, it will be given to the
rsh.
Naturally, the remote machine must have an executable
/usr/libexec/rmt.  This program is free software from the
University of California, and a copy of the source code can be found
with the sources for tar; it's compiled and installed by default.
The exact path to this utility is determined when configuring the package.
It is `prefix/libexec/rmt', where prefix stands for
your installation prefix.  This location may also be overridden at
runtime by using `rmt-command=command' option (See --rmt-command, for detailed description of this option.  See Remote Tape Server, for the description of rmt command).

If this option is not given, but the environment variable TAPE
is set, its value is used; otherwise, old versions of tar
used a default archive name (which was picked when tar was
compiled).  The default is normally set up to be the first tape
drive or other transportable I/O medium on the system.

Starting with version 1.11.5, GNU tar uses
standard input and standard output as the default device, and I will
not try anymore supporting automatic device detection at installation
time.  This was failing really in too many cases, it was hopeless.
This is now completely left to the installer to override standard
input and standard output for default device, if this seems
preferable.  Further, I think most actual usages of
tar are done with pipes or disks, not really tapes,
cartridges or diskettes.

Some users think that using standard input and output is running
after trouble.  This could lead to a nasty surprise on your screen if
you forget to specify an output file name--especially if you are going
through a network or terminal server capable of buffering large amounts
of output.  We had so many bug reports in that area of configuring
default tapes automatically, and so many contradicting requests, that
we finally consider the problem to be portably intractable.  We could
of course use something like `/dev/tape' as a default, but this
is also running after various kind of trouble, going from hung
processes to accidental destruction of real tapes.  After having seen
all this mess, using standard input and output as a default really
sounds like the only clean choice left, and a very useful one too.

GNU tar reads and writes archive in records, I
suspect this is the main reason why block devices are preferred over
character devices.  Most probably, block devices are more efficient
too.  The installer could also check for `DEFTAPE' in
`<sys/mtio.h>'.

`--force-local'
Archive file is local even if it contains a colon.

`--rsh-command=command'
Use remote command instead of rsh.  This option exists
so that people who use something other than the standard rsh
(e.g., a Kerberized rsh) can access a remote device.

When this command is not used, the shell command found when
the tar program was installed is used instead.  This is
the first found of `/usr/ucb/rsh', `/usr/bin/remsh',
`/usr/bin/rsh', `/usr/bsd/rsh' or `/usr/bin/nsh'.
The installer may have overridden this by defining the environment
variable RSH at installation time.

`-[0-7][lmh]'
Specify drive and density.

`-M'
`--multi-volume'
Create/list/extract multi-volume archive.

This option causes tar to write a multi-volume archive--one
that may be larger than will fit on the medium used to hold it.
See Multi-Volume Archives.

`-L num'
`--tape-length=num'
Change tape after writing num x 1024 bytes.

This option might be useful when your tape drivers do not properly
detect end of physical tapes.  By being slightly conservative on the
maximum tape length, you might avoid the problem entirely.

`-F file'
`--info-script=file'
`--new-volume-script=file'
Execute `file' at end of each tape.  This implies
`--multi-volume' (`-M').  See info-script, for a detailed
description of this option.

9.2 The Remote Tape Server
==========================

In order to access the tape drive on a remote machine, tar
uses the remote tape server written at the University of California at
Berkeley.  The remote tape server must be installed as
`prefix/libexec/rmt' on any machine whose tape drive you
want to use.  tar calls rmt by running an
rsh or remsh to the remote machine, optionally
using a different login name if one is supplied.

A copy of the source for the remote tape server is provided.  It is
Copyright (C) 1983 by the Regents of the University of
California, but can be freely distributed.  It is compiled and
installed by default.

Unless you use the `--absolute-names' (`-P') option,
GNU tar will not allow you to create an archive that contains
absolute file names (a file name beginning with `/'.) If you try,
tar will automatically remove the leading `/' from the
file names it stores in the archive.  It will also type a warning
message telling you what it is doing.

When reading an archive that was created with a different
tar program, GNU tar automatically
extracts entries in the archive which have absolute file names as if
the file names were not absolute.  This is an important feature.  A
visitor here once gave a tar tape to an operator to restore;
the operator used Sun tar instead of GNU tar,
and the result was that it replaced large portions of
our `/bin' and friends with versions from the tape; needless to
say, we were unhappy about having to recover the file system from
backup tapes.

For example, if the archive contained a file `/usr/bin/computoy',
GNU tar would extract the file to `usr/bin/computoy',
relative to the current directory.  If you want to extract the files in
an archive to the same absolute names that they had when the archive
was created, you should do a `cd /' before extracting the files
from the archive, or you should either use the `--absolute-names'
option, or use the command `tar -C / ...'.

Some versions of Unix (Ultrix 3.1 is known to have this problem),
can claim that a short write near the end of a tape succeeded,
when it actually failed.  This will result in the -M option not
working correctly.  The best workaround at the moment is to use a
significantly larger blocking factor than the default 20.

In order to update an archive, tar must be able to backspace the
archive in order to reread or rewrite a record that was just read (or
written).  This is currently possible only on two kinds of files: normal
disk files (or any other file that can be backspaced with `lseek'),
and industry-standard 9-track magnetic tape (or any other kind of tape
that can be backspaced with the MTIOCTOP ioctl.

This means that the `--append', `--concatenate', and
`--delete' commands will not work on any other kind of file.
Some media simply cannot be backspaced, which means these commands and
options will never be able to work on them. These non-backspacing
media include pipes and cartridge tape drives.

Some other media can be backspaced, and tar will work on them
once tar is modified to do so.

Archives created with the `--multi-volume', `--label', and
`--incremental' (`-G') options may not be readable by other version
of tar.  In particular, restoring a file that was split over
a volume boundary will require some careful work with dd, if
it can be done at all.  Other versions of tar may also create
an empty file whose name is that of the volume header.  Some versions
of tar may create normal files instead of directories archived
with the `--incremental' (`-G') option.

9.3 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


9.4 Blocking
============
(This message will disappear, once this node revised.)

Block and record terminology is rather confused, and it
is also confusing to the expert reader.  On the other hand, readers
who are new to the field have a fresh mind, and they may safely skip
the next two paragraphs, as the remainder of this manual uses those
two terms in a quite consistent way.

John Gilmore, the writer of the public domain tar from which
GNU tar was originally derived, wrote (June 1995):

The nomenclature of tape drives comes from IBM, where I believe
they were invented for the IBM 650 or so.  On IBM mainframes, what
is recorded on tape are tape blocks.  The logical organization of
data is into records.  There are various ways of putting records into
blocks, including F (fixed sized records), V (variable
sized records), FB (fixed blocked: fixed size records, n
to a block), VB (variable size records, n to a block),
VSB (variable spanned blocked: variable sized records that can
occupy more than one block), etc.  The JCL `DD RECFORM='
parameter specified this to the operating system.

The Unix man page on tar was totally confused about this.
When I wrote PD TAR, I used the historically correct terminology
(tar writes data records, which are grouped into blocks).
It appears that the bogus terminology made it into POSIX (no surprise
here), and now Franc,ois has migrated that terminology back
into the source code too.

The term physical block means the basic transfer chunk from or
to a device, after which reading or writing may stop without anything
being lost.  In this manual, the term block usually refers to
a disk physical block, assuming that each disk block is 512
bytes in length.  It is true that some disk devices have different
physical blocks, but tar ignore these differences in its own
format, which is meant to be portable, so a tar block is always
512 bytes in length, and block always mean a tar block.
The term logical block often represents the basic chunk of
allocation of many disk blocks as a single entity, which the operating
system treats somewhat atomically; this concept is only barely used
in GNU tar.

The term physical record is another way to speak of a physical
block, those two terms are somewhat interchangeable.  In this manual,
the term record usually refers to a tape physical block,
assuming that the tar archive is kept on magnetic tape.
It is true that archives may be put on disk or used with pipes,
but nevertheless, tar tries to read and write the archive one
record at a time, whatever the medium in use.  One record is made
up of an integral number of blocks, and this operation of putting many
disk blocks into a single tape block is called reblocking, or
more simply, blocking.  The term logical record refers to
the logical organization of many characters into something meaningful
to the application.  The term unit record describes a small set
of characters which are transmitted whole to or by the application,
and often refers to a line of text.  Those two last terms are unrelated
to what we call a record in GNU tar.

When writing to tapes, tar writes the contents of the archive
in chunks known as records.  To change the default blocking
factor, use the `--blocking-factor=512-size' (`-b
512-size') option.  Each record will then be composed of
512-size blocks.  (Each tar block is 512 bytes.
See Standard.)  Each file written to the archive uses at least one
full record.  As a result, using a larger record size can result in
more wasted space for small files.  On the other hand, a larger record
size can often be read and written much more efficiently.

Further complicating the problem is that some tape drives ignore the
blocking entirely.  For these, a larger record size can still improve
performance (because the software layers above the tape drive still
honor the blocking), but not as dramatically as on tape drives that
honor blocking.

When reading an archive, tar can usually figure out the
record size on itself.  When this is the case, and a non-standard
record size was used when the archive was created, tar will
print a message about a non-standard blocking factor, and then operate
normally.  On some tape devices, however, tar cannot figure
out the record size itself.  On most of those, you can specify a
blocking factor (with `--blocking-factor') larger than the
actual blocking factor, and then use the `--read-full-records'
(`-B') option.  (If you specify a blocking factor with
`--blocking-factor' and don't use the
`--read-full-records' option, then tar will not
attempt to figure out the recording size itself.)  On some devices,
you must always specify the record size exactly with
`--blocking-factor' when reading, because tar cannot
figure it out.  In any case, use `--list' (`-t') before
doing any extractions to see whether tar is reading the archive
correctly.

tar blocks are all fixed size (512 bytes), and its scheme for
putting them into records is to put a whole number of them (one or
more) into each record.  tar records are all the same size;
at the end of the file there's a block containing all zeros, which
is how you tell that the remainder of the last record(s) are garbage.

In a standard tar file (no options), the block size is 512
and the record size is 10240, for a blocking factor of 20.  What the
`--blocking-factor' option does is sets the blocking factor,
changing the record size while leaving the block size at 512 bytes.
20 was fine for ancient 800 or 1600 bpi reel-to-reel tape drives;
most tape drives these days prefer much bigger records in order to
stream and not waste tape.  When writing tapes for myself, some tend
to use a factor of the order of 2048, say, giving a record size of
around one megabyte.

If you use a blocking factor larger than 20, older tar
programs might not be able to read the archive, so we recommend this
as a limit to use in practice.  GNU tar, however,
will support arbitrarily large record sizes, limited only by the
amount of virtual memory or the physical characteristics of the tape
device.

* Format Variations::           Format Variations
* Blocking Factor::             The Blocking Factor of an Archive

9.4.1 Format Variations
-----------------------
(This message will disappear, once this node revised.)

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, tar uses
default parameters.  You cannot modify a compressed archive.
If you create an archive with the `--blocking-factor' option
specified (see Blocking Factor), you must specify that
blocking-factor when operating on the archive.  See Formats, for other
examples of format parameter considerations.

9.4.2 The Blocking Factor of an Archive
---------------------------------------
(This message will disappear, once this node revised.)

The data in an archive is grouped into blocks, which are 512 bytes.
Blocks are read and written in whole number multiples called
records.  The number of blocks in a record (i.e.  the size of a
record in units of 512 bytes) is called the blocking factor.
The `--blocking-factor=512-size' (`-b
512-size') option specifies the blocking factor of an archive.
The default blocking factor is typically 20 (i.e., 10240 bytes), but
can be specified at installation.  To find out the blocking factor of
an existing archive, use `tar --list --file=archive-name'.
This may not work on some devices.

Records are separated by gaps, which waste space on the archive media.
If you are archiving on magnetic tape, using a larger blocking factor
(and therefore larger records) provides faster throughput and allows you
to fit more data on a tape (because there are fewer gaps).  If you are
archiving on cartridge, a very large blocking factor (say 126 or more)
greatly increases performance.  A smaller blocking factor, on the other
hand, may be useful when archiving small files, to avoid archiving lots
of nulls as tar fills out the archive to the end of the record.
In general, the ideal record size depends on the size of the
inter-record gaps on the tape you are using, and the average size of the
files you are archiving.  See create, for information on
writing archives.



Archives with blocking factors larger than 20 cannot be read
by very old versions of tar, or by some newer versions
of tar running on old machines with small address spaces.
With GNU tar, the blocking factor of an archive is limited
only by the maximum record size of the device containing the archive,
or by the amount of available virtual memory.

Also, on some systems, not using adequate blocking factors, as sometimes
imposed by the device drivers, may yield unexpected diagnostics.  For
example, this has been reported:

Cannot write to /dev/dlt: Invalid argument

In such cases, it sometimes happen that the tar bundled by
the system is aware of block size idiosyncrasies, while GNU tar
requires an explicit specification for the block size,
which it cannot guess.  This yields some people to consider
GNU tar is misbehaving, because by comparison,
the bundle tar works OK.  Adding -b 256,
for example, might resolve the problem.

If you use a non-default blocking factor when you create an archive, you
must specify the same blocking factor when you modify that archive.  Some
archive devices will also require you to specify the blocking factor when
reading that archive, however this is not typically the case.  Usually, you
can use `--list' (`-t') without specifying a blocking factor--tar
reports a non-default record size and then lists the archive members as
it would normally.  To extract files from an archive with a non-standard
blocking factor (particularly if you're not sure what the blocking factor
is), you can usually use the `--read-full-records' (`-B') option while
specifying a blocking factor larger then the blocking factor of the archive
(i.e.  `tar --extract --read-full-records --blocking-factor=300'.
See list, for more information on the `--list' (`-t')
operation.  See Reading, for a more detailed explanation of that option.

`--blocking-factor=number'
`-b number'
Specifies the blocking factor of an archive.  Can be used with any
operation, but is usually not necessary with `--list' (`-t').

Device blocking

`-b blocks'
`--blocking-factor=blocks'
Set record size to blocks * 512 bytes.

This option is used to specify a blocking factor for the archive.
When reading or writing the archive, tar, will do reads and writes
of the archive in records of block*512 bytes.  This is true
even when the archive is compressed.  Some devices requires that all
write operations be a multiple of a certain size, and so, tar
pads the archive out to the next record boundary.

The default blocking factor is set when tar is compiled, and is
typically 20.  Blocking factors larger than 20 cannot be read by very
old versions of tar, or by some newer versions of tar
running on old machines with small address spaces.

With a magnetic tape, larger records give faster throughput and fit
more data on a tape (because there are fewer inter-record gaps).
If the archive is in a disk file or a pipe, you may want to specify
a smaller blocking factor, since a large one will result in a large
number of null bytes at the end of the archive.

When writing cartridge or other streaming tapes, a much larger
blocking factor (say 126 or more) will greatly increase performance.
However, you must specify the same blocking factor when reading or
updating the archive.

Apparently, Exabyte drives have a physical block size of 8K bytes.
If we choose our blocksize as a multiple of 8k bytes, then the problem
seems to disappear.  Id est, we are using block size of 112 right
now, and we haven't had the problem since we switched...

With GNU tar the blocking factor is limited only
by the maximum record size of the device containing the archive, or by
the amount of available virtual memory.

However, deblocking or reblocking is virtually avoided in a special
case which often occurs in practice, but which requires all the
following conditions to be simultaneously true:
* the archive is subject to a compression option,
* the archive is not handled through standard input or output, nor
redirected nor piped,
* the archive is directly handled to a local disk, instead of any special
device,
* `--blocking-factor' is not explicitly specified on the tar
invocation.

If the output goes directly to a local disk, and not through
stdout, then the last write is not extended to a full record size.
Otherwise, reblocking occurs.  Here are a few other remarks on this
topic:

* gzip will complain about trailing garbage if asked to
uncompress a compressed archive on tape, there is an option to turn
the message off, but it breaks the regularity of simply having to use
`prog -d' for decompression.  It would be nice if gzip was
silently ignoring any number of trailing zeros.  I'll ask Jean-loup
Gailly, by sending a copy of this message to him.

* compress does not show this problem, but as Jean-loup pointed
out to Michael, `compress -d' silently adds garbage after
the result of decompression, which tar ignores because it already
recognized its end-of-file indicator.  So this bug may be safely
ignored.

* `gzip -d -q' will be silent about the trailing zeros indeed,
but will still return an exit status of 2 which tar reports in turn.
tar might ignore the exit status returned, but I hate doing
that, as it weakens the protection tar offers users against
other possible problems at decompression time.  If gzip was
silently skipping trailing zeros and also avoiding setting the
exit status in this innocuous case, that would solve this situation.

* tar should become more solid at not stopping to read a pipe at
the first null block encountered.  This inelegantly breaks the pipe.
tar should rather drain the pipe out before exiting itself.

`-i'
`--ignore-zeros'
Ignore blocks of zeros in archive (means EOF).

The `--ignore-zeros' (`-i') option causes tar to ignore blocks
of zeros in the archive.  Normally a block of zeros indicates the
end of the archive, but when reading a damaged archive, or one which
was created by concatenating several archives together, this option
allows tar to read the entire archive.  This option is not on
by default because many versions of tar write garbage after
the zeroed blocks.

Note that this option causes tar to read to the end of the
archive file, which may sometimes avoid problems when multiple files
are stored on a single physical tape.

`-B'
`--read-full-records'
Reblock as we read (for reading 4.2BSD pipes).

If `--read-full-records' is used, tar
will not panic if an attempt to read a record from the archive does
not return a full record. Instead, tar will keep reading
until it has obtained a full
record.

This option is turned on by default when tar is reading
an archive from standard input, or from a remote machine.  This is
because on BSD Unix systems, a read of a pipe will return however
much happens to be in the pipe, even if it is less than tar
requested.  If this option was not used, tar would fail as
soon as it read an incomplete record from the pipe.

This option is also useful with the commands for updating an archive.


Tape blocking




When handling various tapes or cartridges, you have to take care of
selecting a proper blocking, that is, the number of disk blocks you
put together as a single tape block on the tape, without intervening
tape gaps.  A tape gap is a small landing area on the tape
with no information on it, used for decelerating the tape to a
full stop, and for later regaining the reading or writing speed.
When the tape driver starts reading a record, the record has to
be read whole without stopping, as a tape gap is needed to stop the
tape motion without loosing information.

Using higher blocking (putting more disk blocks per tape block) will use
the tape more efficiently as there will be less tape gaps.  But reading
such tapes may be more difficult for the system, as more memory will be
required to receive at once the whole record.  Further, if there is a
reading error on a huge record, this is less likely that the system will
succeed in recovering the information.  So, blocking should not be too
low, nor it should be too high.  tar uses by default a blocking of
20 for historical reasons, and it does not really matter when reading or
writing to disk.  Current tape technology would easily accommodate higher
blockings.  Sun recommends a blocking of 126 for Exabytes and 96 for DATs.
We were told that for some DLT drives, the blocking should be a multiple
of 4Kb, preferably 64Kb (-b 128) or 256 for decent performance.
Other manufacturers may use different recommendations for the same tapes.
This might also depends of the buffering techniques used inside modern
tape controllers.  Some imposes a minimum blocking, or a maximum blocking.
Others request blocking to be some exponent of two.

So, there is no fixed rule for blocking.  But blocking at read time
should ideally be the same as blocking used at write time.  At one place
I know, with a wide variety of equipment, they found it best to use a
blocking of 32 to guarantee that their tapes are fully interchangeable.

I was also told that, for recycled tapes, prior erasure (by the same
drive unit that will be used to create the archives) sometimes lowers
the error rates observed at rewriting time.

I might also use `--number-blocks' instead of
`--block-number', so `--block' will then expand to
`--blocking-factor' unambiguously.

9.5 Many Archives on One Tape
=============================



Most tape devices have two entries in the `/dev' directory, or
entries that come in pairs, which differ only in the minor number for
this device.  Let's take for example `/dev/tape', which often
points to the only or usual tape device of a given system.  There might
be a corresponding `/dev/nrtape' or `/dev/ntape'.  The simpler
name is the rewinding version of the device, while the name
having `nr' in it is the no rewinding version of the same
device.

A rewinding tape device will bring back the tape to its beginning point
automatically when this device is opened or closed.  Since tar
opens the archive file before using it and closes it afterwards, this
means that a simple:

$ tar cf /dev/tape directory

will reposition the tape to its beginning both prior and after saving
directory contents to it, thus erasing prior tape contents and
making it so that any subsequent write operation will destroy what has
just been saved.

So, a rewinding device is normally meant to hold one and only one file.
If you want to put more than one tar archive on a given tape, you
will need to avoid using the rewinding version of the tape device.  You
will also have to pay special attention to tape positioning.  Errors in
positioning may overwrite the valuable data already on your tape.  Many
people, burnt by past experiences, will only use rewinding devices and
limit themselves to one file per tape, precisely to avoid the risk of
such errors.  Be fully aware that writing at the wrong position on a
tape loses all information past this point and most probably until the
end of the tape, and this destroyed information cannot be
recovered.

To save directory-1 as a first archive at the beginning of a
tape, and leave that tape ready for a second archive, you should use:

$ mt -f /dev/nrtape rewind
$ tar cf /dev/nrtape directory-1

Tape marks are special magnetic patterns written on the tape
media, which are later recognizable by the reading hardware.  These
marks are used after each file, when there are many on a single tape.
An empty file (that is to say, two tape marks in a row) signal the
logical end of the tape, after which no file exist.  Usually,
non-rewinding tape device drivers will react to the close request issued
by tar by first writing two tape marks after your archive, and by
backspacing over one of these.  So, if you remove the tape at that time
from the tape drive, it is properly terminated.  But if you write
another file at the current position, the second tape mark will be
erased by the new information, leaving only one tape mark between files.

So, you may now save directory-2 as a second archive after the
first on the same tape by issuing the command:

$ tar cf /dev/nrtape directory-2

and so on for all the archives you want to put on the same tape.

Another usual case is that you do not write all the archives the same
day, and you need to remove and store the tape between two archive
sessions.  In general, you must remember how many files are already
saved on your tape.  Suppose your tape already has 16 files on it, and
that you are ready to write the 17th.  You have to take care of skipping
the first 16 tape marks before saving directory-17, say, by using
these commands:

$ mt -f /dev/nrtape rewind
$ mt -f /dev/nrtape fsf 16
$ tar cf /dev/nrtape directory-17

In all the previous examples, we put aside blocking considerations, but
you should do the proper things for that as well.  See Blocking.

* Tape Positioning::            Tape Positions and Tape Marks
* mt::                          The mt Utility

9.5.1 Tape Positions and Tape Marks
-----------------------------------
(This message will disappear, once this node revised.)

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 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 records "rrrr"'s, and tape marks as
"*"'s, a tape might look like the following:

rrrr*rrrrrr*rrrrr*rr*rrrrr**-------------------------

Tape devices read and write tapes using a read/write tape
head--a physical part of the device which can only access one
point on the tape at a time.  When you use 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.  You can do it manually
via mt utility (see mt).  The restore script does
that automatically (see Scripted Restoration).

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:

rrrr*rrrrrr*rrrrr*rr*rrrrr*rrr*rrrr**----------------

9.5.2 The mt Utility
--------------------
(This message will disappear, once this node revised.)


See Blocking Factor.

You can use the 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.


The syntax of the mt command is:

mt [-f tapename] operation [number]

where tapename is the name of the tape device, number is
the number of times an operation is performed (with a default of one),
and operation is one of the following:



`eof'
`weof'
Writes number tape marks at the current position on the tape.

`fsf'
Moves tape position forward number files.

`bsf'
Moves tape position back number files.

`rewind'
Rewinds the tape.  (Ignores number).

`offline'
`rewoff1'
Rewinds the tape and takes the tape device off-line.  (Ignores number).

`status'
Prints status information about the tape unit.




If you don't specify a tapename, mt uses the environment
variable TAPE; if TAPE is not set, mt will use
the default device specified in your `sys/mtio.h' file
(DEFTAPE variable).  If this is not defined, the program will
display a descriptive error message and exit with code 1.

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

9.6 Using Multiple Tapes
========================

Often you might want to write a large archive, one larger than will fit
on the actual tape you are using.  In such a case, you can run multiple
tar commands, but this can be inconvenient, particularly if you
are using options like `--exclude=pattern' or dumping entire file systems.
Therefore, tar provides a special mode for creating
multi-volume archives.

Multi-volume archive is a single tar archive, stored
on several media volumes of fixed size.  Although in this section we will
often call `volume' a tape, there is absolutely no
requirement for multi-volume archives to be stored on tapes.  Instead,
they can use whatever media type the user finds convenient, they can
even be located on files.  

When creating a multi-volume arvhive, GNU tar continues to fill
current volume until it runs out of space, then it switches to
next volume (usually the operator is queried to replace the tape on
this point), and continues working on the new volume.  This operation
continues untill all requested files are dumped.  If GNU tar detects
end of media while dumping a file, such a file is archived in split
form.  Some very big files can even be split across several volumes. 

Each volume is itself a valid GNU tar archive, so it can be read
without any special options.  Consequently any file member residing
entirely on one volume can be extracted or otherwise operated upon
without needing the other volume.  Sure enough, to extract a split
member you would need all volumes its parts reside on.

Multi-volume archives suffer from several limitations.  In particular,
they cannot be compressed.

GNU tar is able to create multi-volume archives of two formats
(see Formats): `GNU' and `POSIX'.

* Multi-Volume Archives::       Archives Longer than One Tape or Disk
* Tape Files::                  Tape Files
* Tarcat::                      Concatenate Volumes into a Single Archive


9.6.1 Archives Longer than One Tape or Disk
-------------------------------------------

To create an archive that is larger than will fit on a single unit of
the media, use the `--multi-volume' (`-M') option in conjunction with
the `--create' option (see create).  A multi-volume
archive can be manipulated like any other archive (provided the
`--multi-volume' option is specified), but is stored on more
than one tape or disk.

When you specify `--multi-volume', 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.

`--multi-volume'
`-M'
Creates a multi-volume archive, when used in conjunction with
`--create' (`-c').  To perform any other operation on a multi-volume
archive, specify `--multi-volume' in conjunction with that
operation.
For example:

$ tar --create --multi-volume --file=/dev/tape files

The method tar uses to detect end of tape is not perfect, and
fails on some operating systems or on some devices.  If tar
cannot detect the end of the tape itself, you can use
`--tape-length' option to inform it about the capacity of the
tape:

`--tape-length=size'
`-L size'
Set maximum length of a volume.  The size argument should then
be the usable size of the tape in units of 1024 bytes.  This option
selects `--multi-volume' automatically.  For example:

$ tar --create --tape-length=41943040 --file=/dev/tape files


When GNU tar comes to the end of a storage media, it asks you to
change the volume.  The built-in prompt for POSIX locale
is(20): 

Prepare volume #n for `archive' and hit return:

where n is the ordinal number of the volume to be created and
archive is archive file or device name.

When prompting for a new tape, tar accepts any of the following
responses:

?
Request tar to explain possible responses
q
Request tar to exit immediately.
n file-name
Request tar to write the next volume on the file file-name.
!
Request tar to run a subshell.  This option can be disabled
by giving `--restrict' command line option to
tar(21).
y
Request tar to begin writing the next volume.

(You should only type `y' after you have changed the tape;
otherwise tar will write over the volume it just finished.)


The volume number used by tar in its tape-changing prompt
can be changed; if you give the
`--volno-file=file-of-number' option, then
file-of-number should be an unexisting file to be created, or
else, a file already containing a decimal number.  That number will be
used as the volume number of the first volume written.  When
tar is finished, it will rewrite the file with the
now-current volume number. (This does not change the volume number
written on a tape label, as per label, it only affects
the number used in the prompt.)


If you want more elaborate behavior than this, you can write a special
new volume script, that will be responsible for changing the
volume, and instruct tar to use it instead of its normal
prompting procedure: 

`--info-script=script-name'
`--new-volume-script=script-name'
`-F script-name'
Specify the full name of the volume script to use.  The script can be
used to eject cassettes, or to broadcast messages such as
`Someone please come change my tape' when performing unattended
backups.

The script-name is executed without any command line
arguments.  It inherits tar's shell environment.
Additional data is passed to it via the following
environment variables:

TAR_VERSION
GNU tar version number.

TAR_ARCHIVE
The name of the archive tar is processing.

TAR_VOLUME
Ordinal number of the volume tar is about to start.

TAR_SUBCOMMAND
Short option describing the operation tar is executing
See Operations, for a complete list of subcommand options.

TAR_FORMAT
Format of the archive being processed. See Formats, for a complete
list of archive format names.

The volume script can instruct tar to use new archive name,
by writing in to file descriptor 3 (see below for an example).

If the info script fails, tar exits; otherwise, it begins
writing the next volume.

If you want tar to cycle through a series of files or tape
drives, there are three approaches to choose from.  First of all, you
can give tar multiple `--file' options.  In this case
the specified files will be used, in sequence, as the successive
volumes of the archive.  Only when the first one in the sequence needs
to be used again will tar prompt for a tape change (or run
the info script).  For example, suppose someone has two tape drives on
a system named `/dev/tape0' and `/dev/tape1'.  For having
GNU tar to switch to the second drive when it needs to write the
second tape, and then back to the first tape, etc., just do either of:

$ tar --create --multi-volume --file=/dev/tape0 --file=/dev/tape1 files
$ tar cMff /dev/tape0 /dev/tape1 files

The second method is to use the `n' response to the tape-change
prompt.  

Finally, the most flexible approach is to use a volume script, that
writes new archive name to the file descriptor #3.  For example, the
following volume script will create a series of archive files, named
`archive-vol', where archive is the name of the
archive being created (as given by `--file' option) and
vol is the ordinal number of the archive being created:

#! /bin/sh
echo Preparing volume $TAR_VOLUME of $TAR_ARCHIVE.

name=`expr $TAR_ARCHIVE : '\(.*\)-.*'`
case $TAR_SUBCOMMAND in
-c)       ;;
-d|-x|-t) test -r ${name:-$TAR_ARCHIVE}-$TAR_VOLUME || exit 1
	  ;;
*)        exit 1
esac

echo ${name:-$TAR_ARCHIVE}-$TAR_VOLUME >&3

The same script cant be used while listing, comparing or extracting
from the created archive.  For example:

# Create a multi-volume archive:
$ tar -c -L1024 -f archive.tar -F new-volume .
# Extract from the created archive:
$ tar -x -f archive.tar -F new-volume .

Notice, that the first command had to use `-L' option, since
otherwise GNU tar will end up writing everything to file
`archive.tar'.

You can read each individual volume of a multi-volume archive as if it
were an archive by itself.  For example, to list the contents of one
volume, use `--list', without `--multi-volume' specified.
To extract an archive member from one volume (assuming it is described
that volume), use `--extract', again without
`--multi-volume'.

If an archive member is split across volumes (i.e.  its entry begins on
one volume of the media and ends on another), you need to specify
`--multi-volume' to extract it successfully.  In this case, you
should load the volume where the archive member starts, and use
`tar --extract --multi-volume'--tar will prompt for later
volumes as it needs them.  See extracting archives, for more
information about extracting archives.

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
`--label=archive-label' (see label) when it was
created, tar will not automatically label volumes which are
added later.  To label subsequent volumes, specify
`--label=archive-label' again in conjunction with the
`--append', `--update' or `--concatenate' operation.

Notice that multi-volume support is a GNU extension and the archives
created in this mode should be read only using GNU tar.  If you
absolutely have to process such archives using a third-party tar
implementation, read Split Recovery.

9.6.2 Tape Files
----------------
(This message will disappear, once this node revised.)

To give the archive a name which will be recorded in it, use the
`--label=volume-label' (`-V volume-label')
option.  This will write a special block identifying
volume-label as the name of the archive to the front of the
archive which will be displayed when the archive is listed with
`--list'.  If you are creating a multi-volume archive with
`--multi-volume' (see Using Multiple Tapes), then the
volume label will have `Volume nnn' appended to the name
you give, where nnn is the number of the volume of the archive.
(If you use the `--label=volume-label') option when
reading an archive, it checks to make sure the label on the tape
matches the one you give. See label.

When tar writes an archive to tape, it creates a single
tape file.  If multiple archives are written to the same tape, one
after the other, they each get written as separate tape files.  When
extracting, it is necessary to position the tape at the right place
before running tar.  To do this, use the mt command.
For more information on the mt command and on the organization
of tapes into a sequence of tape files, see mt.

People seem to often do:

--label="some-prefix `date +some-format`"

or such, for pushing a common date in all volumes or an archive set.

9.6.3 Concatenate Volumes into a Single Archive
-----------------------------------------------

  Sometimes it is necessary to convert existing GNU tar multi-volume
archive to a single tar archive.  Simply concatenating all
volumes into one will not work, since each volume carries an additional
information at the beginning.  GNU tar is shipped with the shell
script tarcat designed for this purpose.

  The script takes a list of files comprising a multi-volume archive
and creates the resulting archive at the standard output.  For example:

tarcat vol.1 vol.2 vol.3 | tar tf -

  The script implements a simple heuristics to determine the format of
the first volume file and to decide how to process the rest of the
files.  However, it makes no attempt to verify whether the files are
given in order or even if they are valid tar archives.
It uses dd and does not filter its standard error, so you
will usually see lots of spurious messages.



9.7 Including a Label in the Archive
====================================
(This message will disappear, once this node revised.)

  To avoid problems caused by misplaced paper labels on the archive
media, you can include a label entry--an archive member which
contains the name of the archive--in the archive itself.  Use the
`--label=archive-label' (`-V archive-label')
option in conjunction with the `--create' operation to include
a label entry in the archive as it is being created.

`--label=archive-label'
`-V archive-label'
Includes an archive-label at the beginning of the archive when
the archive is being created, when used in conjunction with the
`--create' operation.  Checks to make sure the archive label
matches the one specified (when used in conjunction with any other
operation.

  If you create an archive using both
`--label=archive-label' (`-V archive-label')
and `--multi-volume' (`-M'), each volume of the archive
will have an archive label of the form `archive-label
Volume n', where n is 1 for the first volume, 2 for the
next, and so on. See Using Multiple Tapes, for information on
creating multiple volume archives.

  The volume label will be displayed by `--list' along with
the file contents.  If verbose display is requested, it will also be
explicitely marked as in the example below:

$ tar --verbose --list --file=iamanarchive
V--------- 0 0        0 1992-03-07 12:01 iamalabel--Volume Header--
-rw-r--r-- ringo user 40 1990-05-21 13:30 iamafilename


  However, `--list' option will cause listing entire
contents of the archive, which may be undesirable (for example, if the
archive is stored on a tape).  You can request checking only the volume
by specifying `--test-label' option.  This option reads only the
first block of an archive, so it can be used with slow storage
devices.  For example:

$ tar --test-label --file=iamanarchive
iamalabel

  If `--test-label' is used with a single command line
argument, tar compares the volume label with the
argument.  It exits with code 0 if the two strings match, and with code
2 otherwise.  In this case no output is displayed.  For example:

$ tar --test-label --file=iamanarchive 'iamalable'
=> 0
$ tar --test-label --file=iamanarchive 'iamalable' alabel
=> 1

  If you request any operation, other than `--create', along
with using `--label' option, tar will first check if
the archive label matches the one specified and will refuse to proceed
if it does not.  Use this as a safety precaution to avoid accidentally
overwriting existing archives.  For example, if you wish to add files
to `archive', presumably labelled with string `My volume',
you will get:

$ tar -rf archive --label 'My volume' .
tar: Archive not labeled to match `My volume'

in case its label does not match.  This will work even if
`archive' is not labelled at all.

  Similarly, tar will refuse to list or extract the
archive if its label doesn't match the archive-label
specified.  In those cases, archive-label argument is interpreted
as a globbing-style pattern which must match the actual magnetic
volume label.  See exclude, for a precise description of how match
is attempted(22).  If the switch `--multi-volume' (`-M') is being used,
the volume label matcher will also suffix archive-label by
` Volume [1-9]*' if the initial match fails, before giving
up.  Since the volume numbering is automatically added in labels at
creation time, it sounded logical to equally help the user taking care
of it when the archive is being read.

  The `--label' was once called `--volume', but is not
available under that name anymore.

  You can also use `--label' to get a common information on
all tapes of a series.  For having this information different in each
series created through a single script used on a regular basis, just
manage to get some date string as part of the label.  For example:

$ tar cfMV /dev/tape "Daily backup for `date +%Y-%m-%d`"
$ tar --create --file=/dev/tape --multi-volume \
     --volume="Daily backup for `date +%Y-%m-%d`"

  Also note that each label has its own date and time, which corresponds
to when GNU tar initially attempted to write it,
often soon after the operator launches tar or types the
carriage return telling that the next tape is ready.  Comparing date
labels does give an idea of tape throughput only if the delays for
rewinding tapes and the operator switching them were negligible, which
is usually not the case.

9.8 Verifying Data as It is Stored
==================================

`-W'
`--verify'
Attempt to verify the archive after writing.

This option causes tar to verify the archive after writing it.
Each volume is checked after it is written, and any discrepancies
are recorded on the standard error output.

Verification requires that the archive be on a back-space-able medium.
This means pipes, some cartridge tape drives, and some other devices
cannot be verified.

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

To check for discrepancies in an archive immediately after it is
written, use the `--verify' (`-W') option in conjunction with
the `--create' operation.  When this option is
specified, tar checks archive members against their counterparts
in the file system, and reports discrepancies on the standard error.

To verify an archive, you must be able to read it from before the end
of the last written entry.  This option is useful for detecting data
errors on some tapes.  Archives written to pipes, some cartridge tape
drives, and some other devices cannot be verified.

One can explicitly compare an already made archive with the file
system by using the `--compare' (`--diff', `-d')
option, instead of using the more automatic `--verify' option.
See compare.

Note that these two options have a slightly different intent.  The
`--compare' option checks how identical are the logical contents of some
archive with what is on your disks, while the `--verify' option is
really for checking if the physical contents agree and if the recording
media itself is of dependable quality.  So, for the `--verify'
operation, tar tries to defeat all in-memory cache pertaining to
the archive, while it lets the speed optimization undisturbed for the
`--compare' option.  If you nevertheless use `--compare' for
media verification, you may have to defeat the in-memory cache yourself,
maybe by opening and reclosing the door latch of your recording unit,
forcing some doubt in your operating system about the fact this is really
the same volume as the one just written or read.

The `--verify' option would not be necessary if drivers were indeed
able to detect dependably all write failures.  This sometimes require many
magnetic heads, some able to read after the writes occurred.  One would
not say that drivers unable to detect all cases are necessarily flawed,
as long as programming is concerned.

The `--verify' (`-W') option will not work in
conjunction with the `--multi-volume' (`-M') option or
the `--append' (`-r'), `--update' (`-u')
and `--delete' operations.  See Operations, for more
information on these operations.

Also, since tar normally strips leading `/' from file
names (see absolute), a command like `tar --verify -cf
/tmp/foo.tar /etc' will work as desired only if the working directory is
`/', as tar uses the archive's relative member names
(e.g., `etc/motd') when verifying the archive.

9.9 Write Protection
====================

Almost all tapes and diskettes, and in a few rare cases, even disks can
be write protected, to protect data on them from being changed.
Once an archive is written, you should write protect the media to prevent
the archive from being accidentally overwritten or deleted.  (This will
protect the archive from being changed with a tape or floppy drive--it
will not protect it from magnet fields or other physical hazards).

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

Appendix A Changes
******************

This appendix lists some important user-visible changes between
version GNU tar 1.15.92 and previous versions. An up-to-date
version of this document is available at
the GNU tar documentation page <http://www.gnu.org/software/tar/manual/changes.html>.

Use of globbing patterns when listing and extracting.

Previous versions of GNU tar assumed shell-style globbing when
extracting from or listing an archive.  For example:

$ tar xf foo.tar '*.c'

would extract all files whose names end in `.c'.  This behavior
was not documented and was incompatible with traditional tar
implementations.  Therefore, starting from version 1.15.91, GNU tar
no longer uses globbing by default.  For example, the above invocation
is now interpreted as a request to extract from the archive the file
named `*.c'.

To facilitate transition to the new behavior for those users who got
used to the previous incorrect one, tar will print a warning
if it finds out that a requested member was not found in the archive
and its name looks like a globbing pattern.  For example:

$ tar xf foo.tar  '*.c'
tar: Pattern matching characters used in file names. Please,
tar: use --wildcards to enable pattern matching, or --no-wildcards to
tar: suppress this warning.
tar: *.c: Not found in archive
tar: Error exit delayed from previous errors

To treat member names as globbing patterns, use -wildcards option.
If you want to tar to mimic the behavior of versions prior to 1.15.91,
add this option to your TAR_OPTIONS variable.

See wildcards, for the detailed discussion of the use of globbing
patterns by GNU tar.

Use of short option `-o'.

Earlier versions of GNU tar understood `-o' command line
option as a synonym for `--old-archive'.

GNU tar starting from version 1.13.90 understands this option as
a synonym for `--no-same-owner'.  This is compatible with
UNIX98 tar implementations.

However, to facilitate transition, `-o' option retains its
old semantics when it is used with one of archive-creation commands.
Users are encouraged to use `--format=oldgnu' instead.

It is especially important, since versions of GNU Automake
up to and including 1.8.4 invoke tar with this option to produce
distribution tarballs.  See v7, for the detailed discussion
of this issue and its implications.

.
See Changing Automake's Behavior in GNU Automake, for a description on how to use various
archive formats with automake.

Future versions of GNU tar will understand `-o' only as a
synonym for `--no-same-owner'.

Use of short option `-l'

Earlier versions of GNU tar understood `-l' option as a
synonym for `--one-file-system'.  Since such usage contradicted
to UNIX98 specification and harmed compatibility with other
implementation, it was declared deprecated in version 1.14.  However,
to facilitate transition to its new semantics, it was supported by
versions 1.15 and 1.15.90.  The present use of `-l' as a short
variant of `--check-links' was introduced in version 1.15.91.

Use of options `--portability' and `--old-archive'

These options are deprecated.  Please use `--format=v7' instead.

Use of option `--posix'

This option is deprecated.  Please use `--format=posix' instead.

Appendix B Configuring Help Summary
***********************************

Running tar --help displays the short tar option
summary (see help). This summary is organised by groups of
semantically close options. The options within each group are printed
in the following order: a short option, eventually followed by a list
of corresponding long option names, followed by a short description of
the option. For example, here is an excerpt from the actual tar
--help output:

 Main operation mode:

  -A, --catenate, --concatenate   append tar files to an archive
  -c, --create               create a new archive
  -d, --diff, --compare      find differences between archive and
                             file system
      --delete               delete from the archive

The exact visual representation of the help output is configurable via
ARGP_HELP_FMT environment variable. The value of this variable
is a comma-separated list of format variable assignments. There
are two kinds of format variables. An offset variable keeps the
offset of some part of help output text from the leftmost column on
the screen. A boolean variable is a flag that toggles some
output feature on or off. Depending on the type of the corresponding
variable, there are two kinds of assignments:

Offset assignment

The assignment to an offset variable has the following syntax:

variable=value

where variable is the variable name, and value is a
numeric value to be assigned to the variable.

Boolean assignment

To assign true value to a variable, simply put this variable name. To
assign false value, prefix the variable name with `no-'. For
example:

# Assign true value:
dup-args
# Assign false value:
no-dup-args

Following variables are declared:

 -- Help Output: boolean dup-args
If true, arguments for an option are shown with both short and long
options, even when a given option has both forms, for example:

  -f ARCHIVE, --file=ARCHIVE use archive file or device ARCHIVE

If false, then if an option has both short and long forms, the
argument is only shown with the long one, for example:

  -f, --file=ARCHIVE         use archive file or device ARCHIVE

and a message indicating that the argument is applicable to both
forms is printed below the options. This message can be disabled
using dup-args-note (see below).

The default is false.

 -- Help Output: boolean dup-args-note
If this variable is true, which is the default, the following notice
is displayed at the end of the help output:

Mandatory or optional arguments to long options are also mandatory or
optional for any corresponding short options.

Setting no-dup-args-note inhibits this message. Normally, only one of
variables dup-args or dup-args-note should be set.

 -- Help Output: offset short-opt-col
Column in which short options start. Default is 2.

$ tar --help|grep ARCHIVE
  -f, --file=ARCHIVE   use archive file or device ARCHIVE
$ ARGP_HELP_FMT=short-opt-col=6 tar --help|grep ARCHIVE
      -f, --file=ARCHIVE   use archive file or device ARCHIVE

 -- Help Output: offset long-opt-col
Column in which long options start. Default is 6. For example:

$ tar --help|grep ARCHIVE
  -f, --file=ARCHIVE   use archive file or device ARCHIVE
$ ARGP_HELP_FMT=long-opt-col=16 tar --help|grep ARCHIVE
  -f,           --file=ARCHIVE   use archive file or device ARCHIVE

 -- Help Output: offset doc-opt-col
Column in which doc options start.  A doc option isn't actually
an option, but rather an arbitrary piece of documentation that is
displayed in much the same manner as the options.  For example, in
the description of `--format' option:

  -H, --format=FORMAT        create archive of the given format.

 FORMAT is one of the following:

    gnu                      GNU tar 1.13.x format
    oldgnu                   GNU format as per tar <= 1.12
    pax                      POSIX 1003.1-2001 (pax) format
    posix                    same as pax
    ustar                    POSIX 1003.1-1988 (ustar) format
    v7                       old V7 tar format

the format names are doc options. Thus, if you set
ARGP_HELP_FMT=doc-opt-col=6 the above part of the help output
will look as follows:

  -H, --format=FORMAT        create archive of the given format.

 FORMAT is one of the following:

        gnu                      GNU tar 1.13.x format
        oldgnu                   GNU format as per tar <= 1.12
        pax                      POSIX 1003.1-2001 (pax) format
        posix                    same as pax
        ustar                    POSIX 1003.1-1988 (ustar) format
        v7                       old V7 tar format

 -- Help Output: offset opt-doc-col
Column in which option description starts. Default is 29.

$ tar --help|grep ARCHIVE
  -f, --file=ARCHIVE         use archive file or device ARCHIVE
$ ARGP_HELP_FMT=opt-doc-col=19 tar --help|grep ARCHIVE
  -f, --file=ARCHIVE   use archive file or device ARCHIVE
$ ARGP_HELP_FMT=opt-doc-col=9 tar --help|grep ARCHIVE
  -f, --file=ARCHIVE
           use archive file or device ARCHIVE

Notice, that the description starts on a separate line if
opt-doc-col value is too small.

 -- Help Output: offset header-col
Column in which group headers are printed.  A group header is a
descriptive text preceding an option group.  For example, in the
following text:

 Main operation mode:

  -A, --catenate, --concatenate   append tar files to
                             an archive
  -c, --create               create a new archive
`Main operation mode:' is the group header.

The default value is 1.

 -- Help Output: offset usage-indent
Indentation of wrapped usage lines. Affects `--usage'
output. Default is 12.

 -- Help Output: offset rmargin
Right margin of the text output. Used for wrapping.

Appendix C Tar Internals
************************

* Standard::           Basic Tar Format
* Extensions::         GNU Extensions to the Archive Format
* Sparse Formats::     Storing Sparse Files
* Snapshot Files::
* Dumpdir::

Basic Tar Format
================
(This message will disappear, once this node revised.)

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 tar utility or Tar mode in
GNU Emacs.

Physically, an archive consists of a series of file entries terminated
by an end-of-archive entry, which consists of two 512 blocks of zero
bytes.  A file
entry usually describes one of the files in the archive (an
archive member), and consists of a file header and the contents
of the file.  File headers contain file names and statistics, checksum
information which tar uses to detect file corruption, and
information about file types.

Archives are permitted to have more than one member with the same
member name.  One way this situation can occur is if more than one
version of a file has been stored in the archive.  For information
about adding new versions of a file to an archive, see update.

In addition to entries describing archive members, an archive may
contain entries which tar itself uses to store information.
See label, for an example of such an archive entry.

A tar archive file contains a series of blocks.  Each block
contains BLOCKSIZE bytes.  Although this format may be thought
of as being on magnetic tape, other media are often used.

Each file archived is represented by a header block which describes
the file, followed by zero or more blocks which give the contents
of the file.  At the end of the archive file there are two 512-byte blocks
filled with binary zeros as an end-of-file marker.  A reasonable system
should write such end-of-file marker at the end of an archive, but
must not assume that such a block exists when reading an archive.  In
particular GNU tar always issues a warning if it does not encounter it.

The blocks may be blocked for physical I/O operations.
Each record of n blocks (where n is set by the
`--blocking-factor=512-size' (`-b 512-size') option to tar) is written with a single
`write ()' operation.  On magnetic tapes, the result of
such a write is a single record.  When writing an archive,
the last record of blocks should be written at the full size, with
blocks after the zero block containing all zeros.  When reading
an archive, a reasonable system should properly handle an archive
whose last record is shorter than the rest, or which contains garbage
records after a zero block.

The header block is defined in C as follows.  In the GNU tar
distribution, this is part of file `src/tar.h':


/* tar Header Block, from POSIX 1003.1-1990.  */

/* POSIX header.  */

struct posix_header
{                              /* byte offset */
  char name[100];               /*   0 */
  char mode[8];                 /* 100 */
  char uid[8];                  /* 108 */
  char gid[8];                  /* 116 */
  char size[12];                /* 124 */
  char mtime[12];               /* 136 */
  char chksum[8];               /* 148 */
  char typeflag;                /* 156 */
  char linkname[100];           /* 157 */
  char magic[6];                /* 257 */
  char version[2];              /* 263 */
  char uname[32];               /* 265 */
  char gname[32];               /* 297 */
  char devmajor[8];             /* 329 */
  char devminor[8];             /* 337 */
  char prefix[155];             /* 345 */
                                /* 500 */
};

#define TMAGIC   "ustar"        /* ustar and a null */
#define TMAGLEN  6
#define TVERSION "00"           /* 00 and no null */
#define TVERSLEN 2

/* Values used in typeflag field.  */
#define REGTYPE  '0'            /* regular file */
#define AREGTYPE '\0'           /* regular file */
#define LNKTYPE  '1'            /* link */
#define SYMTYPE  '2'            /* reserved */
#define CHRTYPE  '3'            /* character special */
#define BLKTYPE  '4'            /* block special */
#define DIRTYPE  '5'            /* directory */
#define FIFOTYPE '6'            /* FIFO special */
#define CONTTYPE '7'            /* reserved */

#define XHDTYPE  'x'            /* Extended header referring to the
                                   next file in the archive */
#define XGLTYPE  'g'            /* Global extended header */

/* Bits used in the mode field, values in octal.  */
#define TSUID    04000          /* set UID on execution */
#define TSGID    02000          /* set GID on execution */
#define TSVTX    01000          /* reserved */
                                /* file permissions */
#define TUREAD   00400          /* read by owner */
#define TUWRITE  00200          /* write by owner */
#define TUEXEC   00100          /* execute/search by owner */
#define TGREAD   00040          /* read by group */
#define TGWRITE  00020          /* write by group */
#define TGEXEC   00010          /* execute/search by group */
#define TOREAD   00004          /* read by other */
#define TOWRITE  00002          /* write by other */
#define TOEXEC   00001          /* execute/search by other */

/* tar Header Block, GNU extensions.  */

/* In GNU tar, SYMTYPE is for to symbolic links, and CONTTYPE is for
   contiguous files, so maybe disobeying the `reserved' comment in POSIX
   header description.  I suspect these were meant to be used this way, and
   should not have really been `reserved' in the published standards.  */

/* *BEWARE* *BEWARE* *BEWARE* that the following information is still
   boiling, and may change.  Even if the OLDGNU format description should be
   accurate, the so-called GNU format is not yet fully decided.  It is
   surely meant to use only extensions allowed by POSIX, but the sketch
   below repeats some ugliness from the OLDGNU format, which should rather
   go away.  Sparse files should be saved in such a way that they do *not*
   require two passes at archive creation time.  Huge files get some POSIX
   fields to overflow, alternate solutions have to be sought for this.  */

/* Descriptor for a single file hole.  */

struct sparse
{                              /* byte offset */
  char offset[12];              /*   0 */
  char numbytes[12];            /*  12 */
                                /*  24 */
};

/* Sparse files are not supported in POSIX ustar format.  For sparse files
   with a POSIX header, a GNU extra header is provided which holds overall
   sparse information and a few sparse descriptors.  When an old GNU header
   replaces both the POSIX header and the GNU extra header, it holds some
   sparse descriptors too.  Whether POSIX or not, if more sparse descriptors
   are still needed, they are put into as many successive sparse headers as
   necessary.  The following constants tell how many sparse descriptors fit
   in each kind of header able to hold them.  */

#define SPARSES_IN_EXTRA_HEADER  16
#define SPARSES_IN_OLDGNU_HEADER 4
#define SPARSES_IN_SPARSE_HEADER 21

/* Extension header for sparse files, used immediately after the GNU extra
   header, and used only if all sparse information cannot fit into that
   extra header.  There might even be many such extension headers, one after
   the other, until all sparse information has been recorded.  */

struct sparse_header
{                              /* byte offset */
  struct sparse sp[SPARSES_IN_SPARSE_HEADER];
                                /*   0 */
  char isextended;              /* 504 */
                                /* 505 */
};

/* The old GNU format header conflicts with POSIX format in such a way that
   POSIX archives may fool old GNU tar's, and POSIX tar's might well be
   fooled by old GNU tar archives.  An old GNU format header uses the space
   used by the prefix field in a POSIX header, and cumulates information
   normally found in a GNU extra header.  With an old GNU tar header, we
   never see any POSIX header nor GNU extra header.  Supplementary sparse
   headers are allowed, however.  */

struct oldgnu_header
{                              /* byte offset */
  char unused_pad1[345];        /*   0 */
  char atime[12];               /* 345 Incr. archive: atime of the file */
  char ctime[12];               /* 357 Incr. archive: ctime of the file */
  char offset[12];              /* 369 Multivolume archive: the offset of
                                   the start of this volume */
  char longnames[4];            /* 381 Not used */
  char unused_pad2;             /* 385 */
  struct sparse sp[SPARSES_IN_OLDGNU_HEADER];
                                /* 386 */
  char isextended;              /* 482 Sparse file: Extension sparse header
                                   follows */
  char realsize[12];            /* 483 Sparse file: Real size*/
                                /* 495 */
};

/* OLDGNU_MAGIC uses both magic and version fields, which are contiguous.
   Found in an archive, it indicates an old GNU header format, which will be
   hopefully become obsolescent.  With OLDGNU_MAGIC, uname and gname are
   valid, though the header is not truly POSIX conforming.  */
#define OLDGNU_MAGIC "ustar  "  /* 7 chars and a null */

/* The standards committee allows only capital A through capital Z for
   user-defined expansion.  Other letters in use include:

   'A' Solaris Access Control List
   'E' Solaris Extended Attribute File
   'I' Inode only, as in 'star'
   'X' POSIX 1003.1-2001 eXtended (VU version)  */

/* This is a dir entry that contains the names of files that were in the
   dir at the time the dump was made.  */
#define GNUTYPE_DUMPDIR 'D'

/* Identifies the *next* file on the tape as having a long linkname.  */
#define GNUTYPE_LONGLINK 'K'

/* Identifies the *next* file on the tape as having a long name.  */
#define GNUTYPE_LONGNAME 'L'

/* This is the continuation of a file that began on another volume.  */
#define GNUTYPE_MULTIVOL 'M'

/* For storing filenames that do not fit into the main header.  */
#define GNUTYPE_NAMES 'N'

/* This is for sparse files.  */
#define GNUTYPE_SPARSE 'S'

/* This file is a tape/volume header.  Ignore it on extraction.  */
#define GNUTYPE_VOLHDR 'V'

/* Solaris extended header */
#define SOLARIS_XHDTYPE 'X'

/* Jo"rg Schilling star header */

struct star_header
{                              /* byte offset */
  char name[100];               /*   0 */
  char mode[8];                 /* 100 */
  char uid[8];                  /* 108 */
  char gid[8];                  /* 116 */
  char size[12];                /* 124 */
  char mtime[12];               /* 136 */
  char chksum[8];               /* 148 */
  char typeflag;                /* 156 */
  char linkname[100];           /* 157 */
  char magic[6];                /* 257 */
  char version[2];              /* 263 */
  char uname[32];               /* 265 */
  char gname[32];               /* 297 */
  char devmajor[8];             /* 329 */
  char devminor[8];             /* 337 */
  char prefix[131];             /* 345 */
  char atime[12];               /* 476 */
  char ctime[12];               /* 488 */
                                /* 500 */
};

#define SPARSES_IN_STAR_HEADER      4
#define SPARSES_IN_STAR_EXT_HEADER  21

struct star_in_header
{
  char fill[345];       /*   0  Everything that is before t_prefix */
  char prefix[1];       /* 345  t_name prefix */
  char fill2;           /* 346  */
  char fill3[8];        /* 347  */
  char isextended;      /* 355  */
  struct sparse sp[SPARSES_IN_STAR_HEADER]; /* 356  */
  char realsize[12];    /* 452  Actual size of the file */
  char offset[12];      /* 464  Offset of multivolume contents */
  char atime[12];       /* 476  */
  char ctime[12];       /* 488  */
  char mfill[8];        /* 500  */
  char xmagic[4];       /* 508  "tar" */
};

struct star_ext_header
{
  struct sparse sp[SPARSES_IN_STAR_EXT_HEADER];
  char isextended;
};


All characters in header blocks are represented by using 8-bit
characters in the local variant of ASCII.  Each field within the
structure is contiguous; that is, there is no padding used within
the structure.  Each character on the archive medium is stored
contiguously.

Bytes representing the contents of files (after the header block
of each file) are not translated in any way and are not constrained
to represent characters in any character set.  The tar format
does not distinguish text files from binary files, and no translation
of file contents is performed.

The name, linkname, magic, uname, and
gname are null-terminated character strings.  All other fields
are zero-filled octal numbers in ASCII.  Each numeric field of width
w contains w minus 1 digits, and a null.

The name field is the file name of the file, with directory names
(if any) preceding the file name, separated by slashes.



The mode field provides nine bits specifying file permissions
and three bits to specify the Set UID, Set GID, and Save Text
(sticky) modes.  Values for these bits are defined above.
When special permissions are required to create a file with a given
mode, and the user restoring files from the archive does not hold such
permissions, the mode bit(s) specifying those special permissions
are ignored.  Modes which are not supported by the operating system
restoring files from the archive will be ignored.  Unsupported modes
should be faked up when creating or updating an archive; e.g., the
group permission could be copied from the other permission.

The uid and gid fields are the numeric user and group
ID of the file owners, respectively.  If the operating system does
not support numeric user or group IDs, these fields should be ignored.

The size field is the size of the file in bytes; linked files
are archived with this field specified as zero.  
The mtime field is the data modification time of the file at
the time it was archived.  It is the ASCII representation of the octal
value of the last time the file's contents were modified, represented
as an integer number of
seconds since January 1, 1970, 00:00 Coordinated Universal Time.

The chksum field is the ASCII representation of the octal value
of the simple sum of all bytes in the header block.  Each 8-bit
byte in the header is added to an unsigned integer, initialized to
zero, the precision of which shall be no less than seventeen bits.
When calculating the checksum, the chksum field is treated as
if it were all blanks.

The typeflag field specifies the type of file archived.  If a
particular implementation does not recognize or permit the specified
type, the file will be extracted as if it were a regular file.  As this
action occurs, tar issues a warning to the standard error.

The atime and ctime fields are used in making incremental
backups; they store, respectively, the particular file's access and
status change times.

The offset is used by the `--multi-volume' (`-M') option, when
making a multi-volume archive.  The offset is number of bytes into
the file that we need to restart at to continue the file on the next
tape, i.e., where we store the location that a continued file is
continued at.

The following fields were added to deal with sparse files.  A file
is sparse if it takes in unallocated blocks which end up being
represented as zeros, i.e., no useful data.  A test to see if a file
is sparse is to look at the number blocks allocated for it versus the
number of characters in the file; if there are fewer blocks allocated
for the file than would normally be allocated for a file of that
size, then the file is sparse.  This is the method tar uses to
detect a sparse file, and once such a file is detected, it is treated
differently from non-sparse files.

Sparse files are often dbm files, or other database-type files
which have data at some points and emptiness in the greater part of
the file.  Such files can appear to be very large when an `ls
-l' is done on them, when in truth, there may be a very small amount
of important data contained in the file.  It is thus undesirable
to have tar think that it must back up this entire file, as
great quantities of room are wasted on empty blocks, which can lead
to running out of room on a tape far earlier than is necessary.
Thus, sparse files are dealt with so that these empty blocks are
not written to the tape.  Instead, what is written to the tape is a
description, of sorts, of the sparse file: where the holes are, how
big the holes are, and how much data is found at the end of the hole.
This way, the file takes up potentially far less room on the tape,
and when the file is extracted later on, it will look exactly the way
it looked beforehand.  The following is a description of the fields
used to handle a sparse file:

The sp is an array of struct sparse.  Each struct
sparse contains two 12-character strings which represent an offset
into the file and a number of bytes to be written at that offset.
The offset is absolute, and not relative to the offset in preceding
array element.

The header can hold four of these struct sparse at the moment;
if more are needed, they are not stored in the header.

The isextended flag is set when an extended_header
is needed to deal with a file.  Note that this means that this flag
can only be set when dealing with a sparse file, and it is only set
in the event that the description of the file will not fit in the
allotted room for sparse structures in the header.  In other words,
an extended_header is needed.

The extended_header structure is used for sparse files which
need more sparse structures than can fit in the header.  The header can
fit 4 such structures; if more are needed, the flag isextended
gets set and the next block is an extended_header.

Each extended_header structure contains an array of 21
sparse structures, along with a similar isextended flag
that the header had.  There can be an indeterminate number of such
extended_headers to describe a sparse file.

REGTYPE
AREGTYPE
These flags represent a regular file.  In order to be compatible
with older versions of tar, a typeflag value of
AREGTYPE should be silently recognized as a regular file.
New archives should be created using REGTYPE.  Also, for
backward compatibility, tar treats a regular file whose name
ends with a slash as a directory.

LNKTYPE
This flag represents a file linked to another file, of any type,
previously archived.  Such files are identified in Unix by each
file having the same device and inode number.  The linked-to name is
specified in the linkname field with a trailing null.

SYMTYPE
This represents a symbolic link to another file.  The linked-to name
is specified in the linkname field with a trailing null.

CHRTYPE
BLKTYPE
These represent character special files and block special files
respectively.  In this case the devmajor and devminor
fields will contain the major and minor device numbers respectively.
Operating systems may map the device specifications to their own
local specification, or may ignore the entry.

DIRTYPE
This flag specifies a directory or sub-directory.  The directory
name in the name field should end with a slash.  On systems where
disk allocation is performed on a directory basis, the size field
will contain the maximum number of bytes (which may be rounded to
the nearest disk block allocation unit) which the directory may
hold.  A size field of zero indicates no such limiting.  Systems
which do not support limiting in this manner should ignore the
size field.

FIFOTYPE
This specifies a FIFO special file.  Note that the archiving of a
FIFO file archives the existence of this file and not its contents.

CONTTYPE
This specifies a contiguous file, which is the same as a normal
file except that, in operating systems which support it, all its
space is allocated contiguously on the disk.  Operating systems
which do not allow contiguous allocation should silently treat this
type as a normal file.

A ... Z
These are reserved for custom implementations.  Some of these are
used in the GNU modified format, as described below.


Other values are reserved for specification in future revisions of
the P1003 standard, and should not be used by any tar program.

The magic field indicates that this archive was output in
the P1003 archive format.  If this field contains TMAGIC,
the uname and 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 uid and gid fields.

For references, see ISO/IEC 9945-1:1990 or IEEE Std 1003.1-1990, pages
169-173 (section 10.1) for Archive/Interchange File Format; and
IEEE Std 1003.2-1992, pages 380-388 (section 4.48) and pages 936-940
(section E.4.48) for pax - Portable archive interchange.

GNU Extensions to the Archive Format
====================================
(This message will disappear, once this node revised.)

The GNU format uses additional file types to describe new types of
files in an archive.  These are listed below.

GNUTYPE_DUMPDIR
'D'
This represents a directory and a list of files created by the
`--incremental' (`-G') option.  The size field gives the total
size of the associated list of files.  Each file name is preceded by
either a `Y' (the file should be in this archive) or an `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.

GNUTYPE_MULTIVOL
'M'
This represents a file continued from another volume of a multi-volume
archive created with the `--multi-volume' (`-M') option.  The original
type of the file is not given here.  The 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 offset field
gives the offset from the beginning of the file where this part of
the file begins.  Thus size plus offset should equal
the original size of the file.

GNUTYPE_SPARSE
'S'
This flag indicates that we are dealing with a sparse file.  Note
that archiving a sparse file requires special operations to find
holes in the file, which mark the positions of these holes, along
with the number of bytes of data to be found after the hole.

GNUTYPE_VOLHDR
'V'
This file type is used to mark the volume header that was given with
the `--label=archive-label' (`-V archive-label') option when the archive was created.  The name
field contains the name given after the `--label=archive-label' (`-V archive-label') option.
The size field is zero.  Only the first file in each volume
of an archive should have this type.


You may have trouble reading a GNU format archive on a
non-GNU system if the options `--incremental' (`-G'),
`--multi-volume' (`-M'), `--sparse' (`-S'), or `--label=archive-label' (`-V archive-label') were
used when writing the archive.  In general, if tar does not
use the GNU-added fields of the header, other versions of
tar should be able to read the archive.  Otherwise, the
tar program will give an error, the most likely one being a
checksum error.

Storing Sparse Files
====================

The notion of sparse file, and the ways of handling it from the point
of view of GNU tar user have been described in detail in
sparse.  This chapter describes the internal format GNU tar
uses to store such files.

The support for sparse files in GNU tar has a long history.  The
earliest version featuring this support that I was able to find was 1.09,
released in November, 1990.  The format introduced back then is called
old GNU sparse format and in spite of the fact that its design
contained many flaws, it was the only format GNU tar supported 
until version 1.14 (May, 2004), which introduced initial support for
sparse archives in PAX archives (see posix).  This
format was not free from design flows, either and it was subsequently
improved in versions 1.15.2 (November, 2005) and 1.15.92 (June,
2006). 

In addition to GNU sparse format, GNU tar is able to read and
extract sparse files archived by star.

The following subsections describe each format in detail.

* Old GNU Format::
* PAX 0::                PAX Format, Versions 0.0 and 0.1
* PAX 1::                PAX Format, Version 1.0

C.0.1 Old GNU Format
--------------------

The format introduced some time around 1990 (v. 1.09).  It was
designed on top of standard ustar headers in such an
unfortunate way that some of its fields overwrote fields required by
POSIX.

An old GNU sparse header is designated by type `S'
(GNUTYPE_SPARSE) and has the following layout:

 Offset Size Name Data type Contents
 0 345  N/A Not used.
 345 12 atime Number atime of the file.
 357 12 ctime Number ctime of the file .
 369 12 offset Number For
multivolume archives: the offset of the start of this volume.
 381 4  N/A Not used.
 385 1  N/A Not used.
 386 96 sp sparse_header (4 entries) File map.
 482 1 isextended Bool 1 if an
extension sparse header follows, 0 otherwise.
 483 12 realsize Number Real size of the file.

Each of sparse_header object at offset 386 describes a single
data chunk. It has the following structure: 

 Offset Size Data type Contents
 0 12 Number Offset of the
beginning of the chunk.
 12 12 Number Size of the chunk.

If the member contains more than four chunks, the isextended
field of the header has the value 1 and the main header is
followed by one or more extension headers.  Each such header has
the following structure:

 Offset Size Name Data type Contents
 0 21 sp sparse_header (21 entires) File map.
 504 1 isextended Bool 1 if an
extension sparse header follows, or 0 otherwise.

A header with isextended=0 ends the map.

C.0.2 PAX Format, Versions 0.0 and 0.1
--------------------------------------

There are two formats available in this branch.  The version 0.0
is the initial version of sparse format used by tar
versions 1.14-1.15.1.  The sparse file map is kept in extended
(x) PAX header variables:

GNU.sparse.size
Real size of the stored file

GNU.sparse.numblocks
Number of blocks in the sparse map

GNU.sparse.offset
Offset of the data block

GNU.sparse.numbytes
Size of the data block

The latter two variables repeat for each data block, so the overall
structure is like this:

GNU.sparse.size=size      
GNU.sparse.numblocks=numblocks 
repeat numblocks times
  GNU.sparse.offset=offset    
  GNU.sparse.numbytes=numbytes  
end repeat

This format presented the following two problems:

1. Whereas the POSIX specification allows a variable to appear multiple
times in a header, it requires that only the last occurrence be
meaningful.  Thus, multiple ocurrences of GNU.sparse.offset and
GNU.sparse.numbytes are conficting with the POSIX specs.

2. Attempting to extract such archives using a third-party tars
results in extraction of sparse files in compressed form.  If
the tar implementation in question does not support POSIX
format, it will also extract a file containing extension header
attributes.  This file can be used to expand the file to its original
state.  However, posix-aware tars will usually ignore the
unknown variables, which makes restoring the file more
difficult.  See Extraction of sparse members in v.0.0 format, for the detailed description of how to
restore such members using non-GNU tars.

GNU tar 1.15.2 introduced sparse format version 0.1, which
attempted to solve these problems.  As its predecessor, this format
stores sparse map in the extended POSIX header.  It retains
GNU.sparse.size and GNU.sparse.numblocks variables, but
instead of GNU.sparse.offset/GNU.sparse.numbytes pairs
it uses a single variable:

GNU.sparse.map
Map of non-null data chunks.  It is a string consisting of
comma-separated values "offset,size[,offset-1,size-1...]" 

To address the 2nd problem, the name field in ustar
is replaced with a special name, constructed using the following pattern:

%d/GNUSparseFile.%p/%f

The real name of the sparse file is stored in the variable
GNU.sparse.name.  Thus, those tar implementations
that are not aware of GNU extensions will at least extract the files
into separate directories, giving the user a possibility to expand it
afterwards.  See Extraction of sparse members in v.0.1 format, for the detailed description of how to
restore such members using non-GNU tars.

The resulting GNU.sparse.map string can be very long.
Although POSIX does not impose any limit on the length of a x
header variable, this possibly can confuse some tars.

C.0.3 PAX Format, Version 1.0
-----------------------------

The version 1.0 of sparse format was introduced with GNU tar
1.15.92.  Its main objective was to make the resulting file
extractable with little effort even by non-posix aware tar
implementations.  Starting from this version, the extended header
preceding a sparse member always contains the following variables that
identify the format being used:

GNU.sparse.major
Major version

GNU.sparse.minor
Minor version

The name field in ustar header contains a special name,
constructed using the following pattern:

%d/GNUSparseFile.%p/%f

The real name of the sparse file is stored in the variable
GNU.sparse.name.  The real size of the file is stored in the
variable GNU.sparse.realsize.

The sparse map itself is stored in the file data block, preceding the actual
file data.  It consists of a series of octal numbers of arbitrary length, delimited 
by newlines. The map is padded with nulls to the nearest block boundary.

The first number gives the number of entries in the map. Following are map entries,
each one consisting of two numbers giving the offset and size of the
data block it describes.

The format is designed in such a way that non-posix aware tars and tars not
supporting GNU.sparse.* keywords will extract each sparse file
in its condensed form with the file map prepended and will place it
into a separate directory.  Then, using a simple program it would be
possible to expand the file to its original form even without GNU tar.
See Sparse Recovery, for the detailed information on how to extract
sparse members without GNU tar.
 

Format of the Incremental Snapshot Files
========================================

  A snapshot file (or directory file) is created during
incremental backups (see Incremental Dumps).  It
contains the status of the filesystem at the time of the dump and is
used to determine which files were modified since the last backup.

  GNU tar version 1.15.92 supports two snapshot file
formats.  The first format, called format 0, is the one used by
GNU tar versions up to 1.15.1. The second format, called format
1 is an extended version of this format, that contains more metadata
and allows for further extensions.

  `Format 0' snapshot file begins with a line containing a
decimal number that represents the UNIX timestamp of the beginning of
the last archivation. This line is followed by directory metadata
descriptions, one per line. Each description has the following format:

[nfs]dev inode name

where optional nfs is a single plus character (`+') if this
directory is located on an NFS-mounted partition, dev and
inode are the device and inode numbers of the directory, and
name is the directory name.

  `Format 1' snapshot file begins with a line specifying the
format of the file. This line has the following structure:

`GNU tar-'tar-version`-'incr-format-version

where tar-version is the version of GNU tar implementation
that created this snapshot, and incr-format-version is the
version number of the snapshot format (in this case `1').

  The following line contains two decimal numbers, representing the
time of the last backup. First number is the number of seconds, the
second one is the number of nanoseconds, since the beginning of the
epoch.

  Following lines contain directory metadate, one line per
directory. The line format is:

[nfs]mtime-sec mtime-nsec dev inode name

where mtime-sec and mtime-nsec represent the last
modification time of this directory with nanosecond precision;
nfs, dev, inode and name have the same meaning
as with `format 0'.



  

Dumpdir
=======

  Incremental archives keep information about contents of each
dumped directory in special data blocks called dumpdirs.

  Dumpdir is a sequence of entries of the following form:

C filename \0

where C is one of the control codes described below,
filename is the name of the file C operates upon, and
`\0' represents a nul character (ASCII 0).  The white space 
characters were added for readability, real dumpdirs do not contain
them.

  Each dumpdir ends with a single nul character.

  The following table describes control codes and their meanings:

`Y'
filename is contained in the archive.

`N'
filename was present in the directory at the time the archive
was made, yet it was not dumped to the archive, because it had not
changed since the last backup.

`D'
filename is a directory.

`R'
This code requests renaming of the filename to the name
specified with the following `T' command.

`T'
Specify target file name for `R' command (see below).

`X'
Specify temporary directory name for a rename operation (see below).

  Codes `Y', `N' and `D' require filename argument
to be a relative file name to the directory this dumpdir describes,
whereas codes `R', `T' and `X' require their argument
to be an absolute file name.

 The three codes `R', `T' and `X' specify a
renaming operation.  In the simplest case it is:

R`source'\0T`dest'\0

which means "rename file `source' to file `dest'".

  However, there are cases that require using a temporary
directory.  For example, consider the following scenario:

1. Previous run dumped a directory `foo' which contained the
following three directories: 

a
b
c

2. They were renamed cyclically, so that:

`a' became `b'
`b' became `c'
`c' became `a'

3. New incremental dump was made.

  This case cannot be handled by three successive renames, since
renaming `a' to `b' will destroy existing directory.
To handle such case a temporary directory is required. GNU tar
will create the following dumpdir (newlines have been added for
readability): 

Xfoo\0
Rfoo/a\0T\0
Rfoo/b\0Tfoo/c\0
Rfoo/c\0Tfoo/a\0
R\0Tfoo/a\0

  The first command, `Xfoo\0', instructs the extractor to create a
temporary directory in the directory `foo'.  Second command,
`Rfoo/aT\0', says "rename file `foo/a' to the temporary
directory that has just been created" (empty file name after a
command means use temporary directory).  Third and fourth commands
work as usual, and, finally, the last command, `R\0Tfoo/a\0'
tells tar to rename the temporary directory to `foo/a'.

  The exact placement of a dumpdir in the archive depends on the
archive format (see Formats): 

* PAX archives

In PAX archives, dumpdir is stored in the extended header of the
corresponding directory, in variable GNU.dumpdir.

* GNU and old GNU archives

These formats implement special header type `D', which is similar
to ustar header `5' (directory), except that it preceeds a data
block containing the dumpdir.



Appendix D Genfile
******************

    This appendix describes genfile, an auxiliary program
used in the GNU tar testsuite. If you are not interested in developing
GNU tar, skip this appendix.

    Initially, genfile was used to generate data files for
the testsuite, hence its name. However, new operation modes were being
implemented as the testsuite grew more sophisticated, and now
genfile is a multi-purpose instrument.

    There are three basic operation modes:

File Generation
    This is the default mode. In this mode, genfile
generates data files.

File Status
    In this mode genfile displays status of specified files.

Synchronous Execution.
    In this mode genfile executes the given program with
`--checkpoint' option and executes a set of actions when
specified checkpoints are reached.

* Generate Mode::     File Generation Mode.
* Status Mode::       File Status Mode.
* Exec Mode::         Synchronous Execution mode.

D.1 Generate Mode
=================

    In this mode genfile creates a data file for the test
suite. The size of the file is given with the `--length'
(`-l') option. By default the file contents is written to the
standard output, this can be changed using `--file'
(`-f') command line option. Thus, the following two commands
are equivalent:

genfile --length 100 > outfile
genfile --length 100 --file outfile

    If `--length' is not given, genfile will
generate an empty (zero-length) file.

    You can instruct genfile to create several files at one
go, by giving it `--files-from' (`-T') option followed
by a name of file containing a list of file names. Using dash
(`-') instead of the file name causes genfile to read
file list from the standard input. For example:

# Read file names from file `file.list'
genfile --files-from file.list
# Read file names from standard input
genfile --files-from -

    The list file is supposed to contain one file name per line. To
use file lists separated by ASCII NUL character, use `--null'
(`-0') command line option:

genfile --null --files-from file.list

    The default data pattern for filling the generated file consists
of first 256 letters of ASCII code, repeated enough times to fill the
entire file. This behavior can be changed with `--pattern'
option. This option takes a mandatory argument, specifying pattern
name to use. Currently two patterns are implemented:

`--pattern=default'
    The default pattern as described above.

`--pattern=zero'
    Fills the file with zeroes.

    If no file name was given, the program exits with the code
0.  Otherwise, it exits with 0 only if it was able to
create a file of the specified length.
    
    Special option `--sparse' (`-s') instructs
genfile to create a sparse file. Sparse files consist of
data fragments, separated by holes or blocks of zeros. On
many operating systems, actual disk storage is not allocated for
holes, but they are counted in the length of the file. To create a
sparse file, genfile should know where to put data fragments,
and what data to use to fill them. So, when `--sparse' is given
the rest of the command line specifies a so-called file map.

    The file map consists of any number of fragment
descriptors. Each descriptor is composed of two values: a number,
specifying fragment offset from the end of the previous fragment or,
for the very first fragment, from the beginning of the file, and
contents string, i.e. a string of characters, specifying the
pattern to fill the fragment with. File offset can be suffixed with
the following quantifiers:

`k'
`K'
The number is expressed in kilobytes.
`m'
`M'
The number is expressed in megabytes.
`g'
`G'
The number is expressed in gigabytes.

    For each letter in contents string genfile will generate
a block of data, filled with this letter and will write it to
the fragment. The size of block is given by `--block-size'
option. It defaults to 512. Thus, if the string consists of n
characters, the resulting file fragment will contain
n*block-size of data. 

    Last fragment descriptor can have only file offset part. In this
case genfile will create a hole at the end of the file up to
the given offset.

    For example, consider the following invocation:

genfile --sparse --file sparsefile 0 ABCD 1M EFGHI 2000K 

It will create 3101184-bytes long file of the following structure:

 Offset Length Contents
 0 4*512=2048 Four 512-byte blocks, filled with
letters `A', `B', `C' and `D'.
 2048 1046528 Zero bytes
 1050624 5*512=2560 Five blocks, filled with letters
`E', `F', `G', `H', `I'.
 1053184 2048000 Zero bytes

    The exit code of genfile --status command is 0
only if created file is actually sparse.
    
D.2 Status Mode
===============

    In status mode, genfile prints file system status for
each file specified in the command line. This mode is toggled by
`--stat' (`-S') command line option. An optional argument to this
option specifies output format: a comma-separated list of
struct stat fields to be displayed. This list can contain
following identifiers :

name
    The file name.
    
dev
st_dev
    Device number in decimal.
    
ino
st_ino
    Inode number.
    
mode[.number]
st_mode[.number]
    File mode in octal.  Optional number specifies octal mask to
be applied to the mode before outputting.  For example, --stat
mode.777 will preserve lower nine bits of it.  Notice, that you can
use any punctuation caracter in place of `.'.
    
nlink
st_nlink
    Number of hard links.
    
uid
st_uid
    User ID of owner.
    
gid
st_gid
    Group ID of owner.
    
size
st_size
    File size in decimal.
    
blksize
st_blksize
    The size in bytes of each file block.
    
blocks
st_blocks
    Number of blocks allocated.
     
atime
st_atime
    Time of last access.
    
mtime
st_mtime
    Time of last modification

ctime
st_ctime
    Time of last status change

sparse
    A boolean value indicating whether the file is `sparse'.     

    Modification times are displayed in UTC as
UNIX timestamps, unless suffixed with `H' (for
"human-readable"), as in `ctimeH', in which case usual
tar tv output format is used.

    The default output format is: `name,dev,ino,mode,
nlink,uid,gid,size,blksize,blocks,atime,mtime,ctime'. 

    For example, the following command will display file names and
corresponding times of last access for each file in the current working
directory:

genfile --stat=name,atime *

D.3 Exec Mode
=============

    This mode is designed for testing the behavior of paxutils
commands when some of the files change during archiving. It is an
experimental mode.

    The `Exec Mode' is toggled by `--run' command line
option (or its alias `-r'). The argument to this option gives
the command line to be executed. The actual command line is
constructed by inserting `--checkpoint' option between the
command name and its first argument (if any). Due to this, the
argument to `--run' may not use traditional tar
option syntax, i.e. the following is wrong: 

# Wrong!
genfile --run 'tar cf foo bar'


Use the following syntax instead:

genfile --run 'tar -cf foo bar'

    The rest of command line after `--run' or its equivalent
specifies checkpoint values and actions to be executed upon reaching
them. Checkpoint values are introduced with `--checkpoint'
command line option. Argument to this option is the number of
checkpoint in decimal.

    Any number of actions may be specified after a
checkpoint. Available actions are

`--cut file'
`--truncate file'
    Truncate file to the size specified by previous
`--length' option (or 0, if it is not given).
    
`--append file'
    Append data to file. The size of data and its pattern are
given by previous `--length' and `pattern' options.

`--touch file'
    Update the access and modification times of file. These
timestamps are changed to the current time, unless `--date'
option was given, in which case they are changed to the specified
time. Argument to `--date' option is a date specification in
an almost arbitrary format (see Date input formats).

`--exec command'
    Execute given shell command.
    

    Option `--verbose' instructs genfile to print on
standard output notifications about checkpoints being executed and to
verbosely describe exit status of the command.

    While the command is being executed its standard output remains
connected to descriptor 1. All messages it prints to file descriptor
2, except checkpoint notifications, are forwarded to standard
error.

    Genfile exits with the exit status of the executed command.

Appendix E Free Software Needs Free Documentation
*************************************************

The biggest deficiency in the free software community today is not in
the software--it is the lack of good free documentation that we can
include with the free software.  Many of our most important
programs do not come with free reference manuals and free introductory
texts.  Documentation is an essential part of any software package;
when an important free software package does not come with a free
manual and a free tutorial, that is a major gap.  We have many such
gaps today.

Consider Perl, for instance.  The tutorial manuals that people
normally use are non-free.  How did this come about?  Because the
authors of those manuals published them with restrictive terms--no
copying, no modification, source files not available--which exclude
them from the free software world.

That wasn't the first time this sort of thing happened, and it was far
from the last.  Many times we have heard a GNU user eagerly describe a
manual that he is writing, his intended contribution to the community,
only to learn that he had ruined everything by signing a publication
contract to make it non-free.

Free documentation, like free software, is a matter of freedom, not
price.  The problem with the non-free manual is not that publishers
charge a price for printed copies--that in itself is fine.  (The Free
Software Foundation sells printed copies of manuals, too.)  The
problem is the restrictions on the use of the manual.  Free manuals
are available in source code form, and give you permission to copy and
modify.  Non-free manuals do not allow this.

The criteria of freedom for a free manual are roughly the same as for
free software.  Redistribution (including the normal kinds of
commercial redistribution) must be permitted, so that the manual can
accompany every copy of the program, both on-line and on paper.

Permission for modification of the technical content is crucial too.
When people modify the software, adding or changing features, if they
are conscientious they will change the manual too--so they can
provide accurate and clear documentation for the modified program.  A
manual that leaves you no choice but to write a new manual to document
a changed version of the program is not really available to our
community.

Some kinds of limits on the way modification is handled are
acceptable.  For example, requirements to preserve the original
author's copyright notice, the distribution terms, or the list of
authors, are ok.  It is also no problem to require modified versions
to include notice that they were modified.  Even entire sections that
may not be deleted or changed are acceptable, as long as they deal
with nontechnical topics (like this one).  These kinds of restrictions
are acceptable because they don't obstruct the community's normal use
of the manual.

However, it must be possible to modify all the technical
content of the manual, and then distribute the result in all the usual
media, through all the usual channels.  Otherwise, the restrictions
obstruct the use of the manual, it is not free, and we need another
manual to replace it.

Please spread the word about this issue.  Our community continues to
lose manuals to proprietary publishing.  If we spread the word that
free software needs free reference manuals and free tutorials, perhaps
the next person who wants to contribute by writing documentation will
realize, before it is too late, that only free manuals contribute to
the free software community.

If you are writing documentation, please insist on publishing it under
the GNU Free Documentation License or another free documentation
license.  Remember that this decision requires your approval--you
don't have to let the publisher decide.  Some commercial publishers
will use a free license if you insist, but they will not propose the
option; it is up to you to raise the issue and say firmly that this is
what you want.  If the publisher you are dealing with refuses, please
try other publishers.  If you're not sure whether a proposed license
is free, write to <mailto:licensing@gnu.org>.

You can encourage commercial publishers to sell more free, copylefted
manuals and tutorials by buying them, and particularly by buying
copies from the publishers that paid for their writing or for major
improvements.  Meanwhile, try to avoid buying non-free documentation
at all.  Check the distribution terms of a manual before you buy it,
and insist that whoever seeks your business must respect your freedom.
Check the history of the book, and try reward the publishers that have
paid or pay the authors to work on it.

The Free Software Foundation maintains a list of free documentation
published by other publishers, at
<http://www.fsf.org/doc/other-free-books.html>.

Appendix F Copying This Manual
******************************

* GNU Free Documentation License::  License for copying this manual


F.1 GNU Free Documentation License
==================================

Version 1.2, November 2002

Copyright (C) 2000,2001,2002 Free Software Foundation, Inc.
51 Franklin St, Fifth Floor, Boston, MA  02110-1301, USA

Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.

0. PREAMBLE

The purpose of this License is to make a manual, textbook, or other
functional and useful document free in the sense of freedom: to
assure everyone the effective freedom to copy and redistribute it,
with or without modifying it, either commercially or noncommercially.
Secondarily, this License preserves for the author and publisher a way
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for modifications made by others.

This License is a kind of "copyleft", which means that derivative
works of the document must themselves be free in the same sense.  It
complements the GNU General Public License, which is a copyleft
license designed for free software.

We have designed this License in order to use it for manuals for free
software, because free software needs free documentation: a free
program should come with manuals providing the same freedoms that the
software does.  But this License is not limited to software manuals;
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principally for works whose purpose is instruction or reference.

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2. VERBATIM COPYING

You may copy and distribute the Document in any medium, either
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conditions whatsoever to those of this License.  You may not use
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You may also lend copies, under the same conditions stated above, and
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3. COPYING IN QUANTITY

If you publish printed copies (or copies in media that commonly have
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location until at least one year after the last time you distribute an
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4. MODIFICATIONS

You may copy and distribute a Modified Version of the Document under
the conditions of sections 2 and 3 above, provided that you release
the Modified Version under precisely this License, with the Modified
Version filling the role of the Document, thus licensing distribution
and modification of the Modified Version to whoever possesses a copy
of it.  In addition, you must do these things in the Modified Version:

A. Use in the Title Page (and on the covers, if any) a title distinct
from that of the Document, and from those of previous versions
(which should, if there were any, be listed in the History section
of the Document).  You may use the same title as a previous version
if the original publisher of that version gives permission.

B. List on the Title Page, as authors, one or more persons or entities
responsible for authorship of the modifications in the Modified
Version, together with at least five of the principal authors of the
Document (all of its principal authors, if it has fewer than five),
unless they release you from this requirement.

C. State on the Title page the name of the publisher of the
Modified Version, as the publisher.

D. Preserve all the copyright notices of the Document.

E. Add an appropriate copyright notice for your modifications
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F. Include, immediately after the copyright notices, a license notice
giving the public permission to use the Modified Version under the
terms of this License, in the form shown in the Addendum below.

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and required Cover Texts given in the Document's license notice.

H. Include an unaltered copy of this License.

I. Preserve the section Entitled "History", Preserve its Title, and add
to it an item stating at least the title, year, new authors, and
publisher of the Modified Version as given on the Title Page.  If
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stating the title, year, authors, and publisher of the Document as
given on its Title Page, then add an item describing the Modified
Version as stated in the previous sentence.

J. Preserve the network location, if any, given in the Document for
public access to a Transparent copy of the Document, and likewise
the network locations given in the Document for previous versions
it was based on.  These may be placed in the "History" section.
You may omit a network location for a work that was published at
least four years before the Document itself, or if the original
publisher of the version it refers to gives permission.

K. For any section Entitled "Acknowledgements" or "Dedications", Preserve
the Title of the section, and preserve in the section all the
substance and tone of each of the contributor acknowledgements and/or
dedications given therein.

L. Preserve all the Invariant Sections of the Document,
unaltered in their text and in their titles.  Section numbers
or the equivalent are not considered part of the section titles.

M. Delete any section Entitled "Endorsements".  Such a section
may not be included in the Modified Version.

N. Do not retitle any existing section to be Entitled "Endorsements" or
to conflict in title with any Invariant Section.

O. Preserve any Warranty Disclaimers.

If the Modified Version includes new front-matter sections or
appendices that qualify as Secondary Sections and contain no material
copied from the Document, you may at your option designate some or all
of these sections as invariant.  To do this, add their titles to the
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These titles must be distinct from any other section titles.

You may add a section Entitled "Endorsements", provided it contains
nothing but endorsements of your Modified Version by various
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been approved by an organization as the authoritative definition of a
standard.

You may add a passage of up to five words as a Front-Cover Text, and a
passage of up to 25 words as a Back-Cover Text, to the end of the list
of Cover Texts in the Modified Version.  Only one passage of
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you may not add another; but you may replace the old one, on explicit
permission from the previous publisher that added the old one.

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imply endorsement of any Modified Version.

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Invariant Sections of all of the original documents, unmodified, and
list them all as Invariant Sections of your combined work in its
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different contents, make the title of each such section unique by
adding at the end of it, in parentheses, the name of the original
author or publisher of that section if known, or else a unique number.
Make the same adjustment to the section titles in the list of
Invariant Sections in the license notice of the combined work.

In the combination, you must combine any sections Entitled "History"
in the various original documents, forming one section Entitled
"History"; likewise combine any sections Entitled "Acknowledgements",
and any sections Entitled "Dedications".  You must delete all
sections Entitled "Endorsements."

6. COLLECTIONS OF DOCUMENTS

You may make a collection consisting of the Document and other documents
released under this License, and replace the individual copies of this
License in the various documents with a single copy that is included in
the collection, provided that you follow the rules of this License for
verbatim copying of each of the documents in all other respects.

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it individually under this License, provided you insert a copy of this
License into the extracted document, and follow this License in all
other respects regarding verbatim copying of that document.

7. AGGREGATION WITH INDEPENDENT WORKS

A compilation of the Document or its derivatives with other separate
and independent documents or works, in or on a volume of a storage or
distribution medium, is called an "aggregate" if the copyright
resulting from the compilation is not used to limit the legal rights
of the compilation's users beyond what the individual works permit.
When the Document is included in an aggregate, this License does not
apply to the other works in the aggregate which are not themselves
derivative works of the Document.

If the Cover Text requirement of section 3 is applicable to these
copies of the Document, then if the Document is less than one half of
the entire aggregate, the Document's Cover Texts may be placed on
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Otherwise they must appear on printed covers that bracket the whole
aggregate.

8. TRANSLATION

Translation is considered a kind of modification, so you may
distribute translations of the Document under the terms of section 4.
Replacing Invariant Sections with translations requires special
permission from their copyright holders, but you may include
translations of some or all Invariant Sections in addition to the
original versions of these Invariant Sections.  You may include a
translation of this License, and all the license notices in the
Document, and any Warranty Disclaimers, provided that you also include
the original English version of this License and the original versions
of those notices and disclaimers.  In case of a disagreement between
the translation and the original version of this License or a notice
or disclaimer, the original version will prevail.

If a section in the Document is Entitled "Acknowledgements",
"Dedications", or "History", the requirement (section 4) to Preserve
its Title (section 1) will typically require changing the actual
title.

9. TERMINATION

You may not copy, modify, sublicense, or distribute the Document except
as expressly provided for under this License.  Any other attempt to
copy, modify, sublicense or distribute the Document is void, and will
automatically terminate your rights under this License.  However,
parties who have received copies, or rights, from you under this
License will not have their licenses terminated so long as such
parties remain in full compliance.

10. FUTURE REVISIONS OF THIS LICENSE

The Free Software Foundation may publish new, revised versions
of the GNU Free Documentation License from time to time.  Such new
versions will be similar in spirit to the present version, but may
differ in detail to address new problems or concerns.  See
<http://www.gnu.org/copyleft/>.

Each version of the License is given a distinguishing version number.
If the Document specifies that a particular numbered version of this
License "or any later version" applies to it, you have the option of
following the terms and conditions either of that specified version or
of any later version that has been published (not as a draft) by the
Free Software Foundation.  If the Document does not specify a version
number of this License, you may choose any version ever published (not
as a draft) by the Free Software Foundation.


F.1.1 ADDENDUM: How to use this License for your documents
----------------------------------------------------------

To use this License in a document you have written, include a copy of
the License in the document and put the following copyright and
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Appendix G Index of Command Line Options
****************************************

This appendix contains an index of all GNU tar long command line
options. The options are listed without the preceeding double-dash.
For a cross-reference of short command line options, Short Option Summary.

* Index:
* absolute-names: 6.10.2 Absolute File Names.
* absolute-names, summary: 3.4.2 tar Options.
* add-file: 6.3 Reading Names from a File.
* after-date: 6.8 Operating Only on New Files.
* after-date, summary: 3.4.2 tar Options.
* anchored: Controlling Pattern-Matching.
* anchored, summary: 3.4.2 tar Options.
* append: 4.2.2 How to Add Files to Existing Archives: `--append'.
* append, summary: 3.4.1 Operations.
* atime-preserve: 8.2 Handling File Attributes.
* atime-preserve, summary: 3.4.2 tar Options.
* backup: 4.5 Backup options.
* backup, summary: 3.4.2 tar Options.
* block-number: 3.7 Checking tar progress.
* block-number, summary: 3.4.2 tar Options.
* blocking-factor: 9.4.2 The Blocking Factor of an Archive.
* blocking-factor, summary: 3.4.2 tar Options.
* bzip2: 8.1.1 Creating and Reading Compressed Archives.
* bzip2, summary: 3.4.2 tar Options.
* catenate: 4.2.4 Combining Archives with `--concatenate'.
* catenate, summary: 3.4.1 Operations.
* check-links, summary: 3.4.2 tar Options.
* checkpoint: 3.7 Checking tar progress.
* checkpoint, summary: 3.4.2 tar Options.
* compare: 4.2.6 Comparing Archive Members with the File System.
* compare, summary: 3.4.1 Operations.
* compress: 8.1.1 Creating and Reading Compressed Archives.
* compress, summary: 3.4.2 tar Options.
* concatenate: 4.2.4 Combining Archives with `--concatenate'.
* concatenate, summary: 3.4.1 Operations.
* confirmation, summary: 3.4.2 tar Options.
* create, additional options: 4.3 Options Used by `--create'.
* create, complementary notes: 4.1 Basic GNU tar Operations.
* create, introduced: 2.6.2 Creating the Archive.
* create, summary: 3.4.1 Operations.
* create, using with `--verbose': 2.6.3 Running `--create' with `--verbose'.
* create, using with `--verify': 9.8 Verifying Data as It is Stored.
* delay-directory-restore: Directory Modification Times and Permissions.
* delay-directory-restore, summary: 3.4.2 tar Options.
* delete: 4.2.5 Removing Archive Members Using `--delete'.
* delete, summary: 3.4.1 Operations.
* dereference: 8.3.2 Symbolic Links.
* dereference, summary: 3.4.2 tar Options.
* diff, summary: 3.4.1 Operations.
* directory: 6.10.1 Changing the Working Directory.
* directory, summary: 3.4.2 tar Options.
* directory, using in `--files-from' argument: 6.3 Reading Names from a File.
* exclude: 6.4 Excluding Some Files.
* exclude, potential problems with: Problems with Using the exclude Options.
* exclude, summary: 3.4.2 tar Options.
* exclude-caches: 6.4 Excluding Some Files.
* exclude-caches, summary: 3.4.2 tar Options.
* exclude-from: 6.4 Excluding Some Files.
* exclude-from, summary: 3.4.2 tar Options.
* extract: 2.8 How to Extract Members from an Archive.
* extract, additional options: 4.4 Options Used by `--extract'.
* extract, complementary notes: 4.1 Basic GNU tar Operations.
* extract, summary: 3.4.1 Operations.
* extract, using with `--listed-incremental': 5.2 Using tar to Perform Incremental Dumps.
* file, short description: 6.1 Choosing and Naming Archive Files.
* file, summary: 3.4.2 tar Options.
* file, tutorial: The `--file' Option.
* files-from: 6.3 Reading Names from a File.
* files-from, summary: 3.4.2 tar Options.
* force-local, short description: 9.1 Device Selection and Switching.
* force-local, summary: 3.4.2 tar Options.
* format, summary: 3.4.2 tar Options.
* get, summary: 3.4.1 Operations.
* group: 4.3.1 Overriding File Metadata.
* group, summary: 3.4.2 tar Options.
* gunzip, summary: 3.4.2 tar Options.
* gzip: 8.1.1 Creating and Reading Compressed Archives.
* gzip, summary: 3.4.2 tar Options.
* help: Getting Help: Using the `--help' Option.
* help, introduction: 3.5 GNU tar documentation.
* help, summary: 3.4.2 tar Options.
* ignore-case: Controlling Pattern-Matching.
* ignore-case, summary: 3.4.2 tar Options.
* ignore-command-error: Writing to an External Program.
* ignore-command-error, summary: 3.4.2 tar Options.
* ignore-failed-read: 4.3.2 Ignore Fail Read.
* ignore-failed-read, summary: 3.4.2 tar Options.
* ignore-zeros: Ignoring Blocks of Zeros.
* ignore-zeros, short description: 9.4.2 The Blocking Factor of an Archive.
* ignore-zeros, summary: 3.4.2 tar Options.
* incremental, summary: 3.4.2 tar Options.
* incremental, using with `--list': 5.2 Using tar to Perform Incremental Dumps.
* index-file, summary: 3.4.2 tar Options.
* info-script: 9.6.1 Archives Longer than One Tape or Disk.
* info-script, short description: 9.1 Device Selection and Switching.
* info-script, summary: 3.4.2 tar Options.
* interactive: 3.8 Asking for Confirmation During Operations.
* interactive, summary: 3.4.2 tar Options.
* keep-newer-files: Keep Newer Files.
* keep-newer-files, summary: 3.4.2 tar Options.
* keep-old-files: Keep Old Files.
* keep-old-files, introduced: Options Controlling the Overwriting of Existing Files.
* keep-old-files, summary: 3.4.2 tar Options.
* label: 9.7 Including a Label in the Archive.
* label, summary: 3.4.2 tar Options.
* list: 2.7 How to List Archives.
* list, summary: 3.4.1 Operations.
* list, using with `--incremental': 5.2 Using tar to Perform Incremental Dumps.
* list, using with `--listed-incremental': 5.2 Using tar to Perform Incremental Dumps.
* list, using with `--verbose': 2.7 How to List Archives.
* list, using with file name arguments: 2.7 How to List Archives.
* listed-incremental: 5.2 Using tar to Perform Incremental Dumps.
* listed-incremental, summary: 3.4.2 tar Options.
* listed-incremental, using with `--extract': 5.2 Using tar to Perform Incremental Dumps.
* listed-incremental, using with `--list': 5.2 Using tar to Perform Incremental Dumps.
* mode: 4.3.1 Overriding File Metadata.
* mode, summary: 3.4.2 tar Options.
* mtime: 4.3.1 Overriding File Metadata.
* mtime, summary: 3.4.2 tar Options.
* multi-volume: 9.6.1 Archives Longer than One Tape or Disk.
* multi-volume, short description: 9.1 Device Selection and Switching.
* multi-volume, summary: 3.4.2 tar Options.
* new-volume-script: 9.6.1 Archives Longer than One Tape or Disk.
* new-volume-script, short description: 9.1 Device Selection and Switching.
* new-volume-script, summary: 3.4.2 tar Options.
* new-volume-script, summary: 3.4.2 tar Options.
* newer: 6.8 Operating Only on New Files.
* newer, summary: 3.4.2 tar Options.
* newer-mtime: 6.8 Operating Only on New Files.
* newer-mtime, summary: 3.4.2 tar Options.
* no-anchored: Controlling Pattern-Matching.
* no-anchored, summary: 3.4.2 tar Options.
* no-delay-directory-restore: Directory Modification Times and Permissions.
* no-delay-directory-restore, summary: 3.4.2 tar Options.
* no-ignore-case: Controlling Pattern-Matching.
* no-ignore-case, summary: 3.4.2 tar Options.
* no-ignore-command-error: Writing to an External Program.
* no-ignore-command-error, summary: 3.4.2 tar Options.
* no-overwrite-dir, summary: 3.4.2 tar Options.
* no-quote-chars, summary: 3.4.2 tar Options.
* no-recursion: 6.9 Descending into Directories.
* no-recursion, summary: 3.4.2 tar Options.
* no-same-owner: 8.2 Handling File Attributes.
* no-same-owner, summary: 3.4.2 tar Options.
* no-same-permissions, summary: 3.4.2 tar Options.
* no-unquote: 6.2 Selecting Archive Members.
* no-unquote, summary: 3.4.2 tar Options.
* no-wildcards: Controlling Pattern-Matching.
* no-wildcards, summary: 3.4.2 tar Options.
* no-wildcards-match-slash: Controlling Pattern-Matching.
* no-wildcards-match-slash, summary: 3.4.2 tar Options.
* null: 6.3.1 NUL Terminated File Names.
* null, summary: 3.4.2 tar Options.
* numeric-owner: 8.2 Handling File Attributes.
* numeric-owner, summary: 3.4.2 tar Options.
* occurrence, summary: 3.4.2 tar Options.
* old-archive, summary: 3.4.2 tar Options.
* one-file-system: 6.10 Crossing File System Boundaries.
* one-file-system, summary: 3.4.2 tar Options.
* overwrite: Overwrite Old Files.
* overwrite, introduced: Options Controlling the Overwriting of Existing Files.
* overwrite, summary: 3.4.2 tar Options.
* overwrite-dir: Overwrite Old Files.
* overwrite-dir, introduced: Options Controlling the Overwriting of Existing Files.
* overwrite-dir, summary: 3.4.2 tar Options.
* owner: 4.3.1 Overriding File Metadata.
* owner, summary: 3.4.2 tar Options.
* pax-option: 8.3.6.1 Controlling Extended Header Keywords.
* pax-option, summary: 3.4.2 tar Options.
* portability, summary: 3.4.2 tar Options.
* posix, summary: 3.4.2 tar Options.
* preserve: 8.2 Handling File Attributes.
* preserve, summary: 3.4.2 tar Options.
* preserve-order: Same Order.
* preserve-order, summary: 3.4.2 tar Options.
* preserve-permissions: Setting Access Permissions.
* preserve-permissions, short description: 8.2 Handling File Attributes.
* preserve-permissions, summary: 3.4.2 tar Options.
* quote-chars, summary: 3.4.2 tar Options.
* quoting-style: 6.6 Quoting Member Names.
* quoting-style, summary: 3.4.2 tar Options.
* read-full-records: 4.4.1 Options to Help Read Archives.
* read-full-records: Reading Full Records.
* read-full-records, short description: 9.4.2 The Blocking Factor of an Archive.
* read-full-records, summary: 3.4.2 tar Options.
* record-size, summary: 3.4.2 tar Options.
* recursion: 6.9 Descending into Directories.
* recursion, summary: 3.4.2 tar Options.
* recursive-unlink: Recursive Unlink.
* recursive-unlink, summary: 3.4.2 tar Options.
* remove-files: Removing Files.
* remove-files, summary: 3.4.2 tar Options.
* restrict, summary: 3.4.2 tar Options.
* rmt-command, summary: 3.4.2 tar Options.
* rsh-command: 9.1 Device Selection and Switching.
* rsh-command, summary: 3.4.2 tar Options.
* same-order: Same Order.
* same-order, summary: 3.4.2 tar Options.
* same-owner: 8.2 Handling File Attributes.
* same-owner, summary: 3.4.2 tar Options.
* same-permissions: Setting Access Permissions.
* same-permissions, short description: 8.2 Handling File Attributes.
* same-permissions, summary: 3.4.2 tar Options.
* same-permissions, summary: 3.4.2 tar Options.
* seek, summary: 3.4.2 tar Options.
* show-defaults: 3.6 Obtaining GNU tar default values.
* show-defaults, summary: 3.4.2 tar Options.
* show-omitted-dirs: 3.7 Checking tar progress.
* show-omitted-dirs, summary: 3.4.2 tar Options.
* show-stored-names: 2.7 How to List Archives.
* show-stored-names, summary: 3.4.2 tar Options.
* show-transformed-names: 6.7 Modifying File and Member Names.
* show-transformed-names, summary: 3.4.2 tar Options.
* sparse: 8.1.2 Archiving Sparse Files.
* sparse, summary: 3.4.2 tar Options.
* sparse-version: 8.1.2 Archiving Sparse Files.
* sparse-version, summary: 3.4.2 tar Options.
* starting-file: Starting File.
* starting-file, summary: 3.4.2 tar Options.
* strip-components: 6.7 Modifying File and Member Names.
* strip-components, summary: 3.4.2 tar Options.
* suffix: 4.5 Backup options.
* suffix, summary, summary: 3.4.2 tar Options.
* tape-length: 9.6.1 Archives Longer than One Tape or Disk.
* tape-length, short description: 9.1 Device Selection and Switching.
* tape-length, summary: 3.4.2 tar Options.
* test-label: 9.7 Including a Label in the Archive.
* test-label, summary: 3.4.2 tar Options.
* to-command: Writing to an External Program.
* to-command, summary: 3.4.2 tar Options.
* to-stdout: Writing to Standard Output.
* to-stdout, summary: 3.4.2 tar Options.
* totals: 3.7 Checking tar progress.
* totals, summary: 3.4.2 tar Options.
* touch: Setting Data Modification Times.
* touch: 8.2 Handling File Attributes.
* touch, summary: 3.4.2 tar Options.
* transform: 6.7 Modifying File and Member Names.
* transform, summary: 3.4.2 tar Options.
* uncompress: 8.1.1 Creating and Reading Compressed Archives.
* uncompress, summary: 3.4.2 tar Options.
* uncompress, summary: 3.4.2 tar Options.
* ungzip: 8.1.1 Creating and Reading Compressed Archives.
* ungzip, summary: 3.4.2 tar Options.
* ungzip, summary: 3.4.2 tar Options.
* unlink-first: Unlink First.
* unlink-first, introduced: Options Controlling the Overwriting of Existing Files.
* unlink-first, summary: 3.4.2 tar Options.
* unquote: 6.2 Selecting Archive Members.
* unquote, summary: 3.4.2 tar Options.
* update: 4.2.3 Updating an Archive.
* update, summary: 3.4.1 Operations.
* usage: 3.5 GNU tar documentation.
* use-compress-program: 8.1.1 Creating and Reading Compressed Archives.
* use-compress-program, summary: 3.4.2 tar Options.
* utc, summary: 3.4.2 tar Options.
* verbose: 3.7 Checking tar progress.
* verbose, introduced: The `--verbose' Option.
* verbose, summary: 3.4.2 tar Options.
* verbose, using with `--create': 2.6.3 Running `--create' with `--verbose'.
* verbose, using with `--list': 2.7 How to List Archives.
* verify, short description: 9.8 Verifying Data as It is Stored.
* verify, summary: 3.4.2 tar Options.
* verify, using with `--create': 9.8 Verifying Data as It is Stored.
* version: 3.5 GNU tar documentation.
* version, summary: 3.4.2 tar Options.
* volno-file: 9.6.1 Archives Longer than One Tape or Disk.
* volno-file, summary: 3.4.2 tar Options.
* wildcards: Controlling Pattern-Matching.
* wildcards, summary: 3.4.2 tar Options.
* wildcards-match-slash: Controlling Pattern-Matching.
* wildcards-match-slash, summary: 3.4.2 tar Options.

Appendix H Index
****************

* Index:
* abbreviations for months: 7.2 Calendar date items.
* absolute file names: 9.2 The Remote Tape Server.
* Adding archives to an archive: 4.2.4 Combining Archives with `--concatenate'.
* Adding files to an Archive: 4.2.2.1 Appending Files to an Archive.
* ADMINISTRATOR: 5.4.1 General-Purpose Variables.
* Age, excluding files by: 6.8 Operating Only on New Files.
* ago in date strings: 7.6 Relative items in date strings.
* am in date strings: 7.3 Time of day items.
* Appending files to an Archive: 4.2.2.1 Appending Files to an Archive.
* archive: 1.2 Some Definitions.
* Archive creation: 6.1 Choosing and Naming Archive Files.
* archive member: 1.2 Some Definitions.
* Archive Name: 6.1 Choosing and Naming Archive Files.
* Archive, creation of: 2.6 How to Create Archives.
* Archives, Appending files to: 4.2.2.1 Appending Files to an Archive.
* Archiving Directories: 2.6.5 Archiving Directories.
* archiving files: GNU tar: an archiver tool.
* ARGP_HELP_FMT, environment variable: Appendix B Configuring Help Summary.
* authors of get_date: 7.10 Authors of get_date.
* Avoiding recursion in directories: 6.9 Descending into Directories.
* backup options: 4.5 Backup options.
* backup suffix: 4.5 Backup options.
* backups: 4.5 Backup options.
* BACKUP_DIRS: 5.4.1 General-Purpose Variables.
* BACKUP_FILES: 5.4.1 General-Purpose Variables.
* BACKUP_HOUR: 5.4.1 General-Purpose Variables.
* beginning of time, for POSIX: 7.8 Seconds since the Epoch.
* Bellovin, Steven M.: 7.10 Authors of get_date.
* Berets, Jim: 7.10 Authors of get_date.
* Berry, K.: 7.10 Authors of get_date.
* Block number where error occurred: 3.7 Checking tar progress.
* BLOCKING: 5.4.1 General-Purpose Variables.
* Blocking Factor: 9.4.2 The Blocking Factor of an Archive.
* blocking factor: 9.4.2 The Blocking Factor of an Archive.
* Blocks per record: 9.4.2 The Blocking Factor of an Archive.
* bug reports: 1.6 Reporting bugs or suggestions.
* Bytes per record: 9.4.2 The Blocking Factor of an Archive.
* calendar date item: 7.2 Calendar date items.
* case, ignored in dates: 7.1 General date syntax.
* cat vs concatenate: 4.2.4 Combining Archives with `--concatenate'.
* Changing directory mid-stream: 6.10.1 Changing the Working Directory.
* Character class, excluding characters from: 6.5 Wildcards Patterns and Matching.
* Choosing an archive file: 6.1 Choosing and Naming Archive Files.
* comments, in dates: 7.1 General date syntax.
* Compressed archives: 8.1.1 Creating and Reading Compressed Archives.
* concatenate vs cat: 4.2.4 Combining Archives with `--concatenate'.
* Concatenating Archives: 4.2.4 Combining Archives with `--concatenate'.
* corrupted archives: 5.1 Using tar to Perform Full Dumps.
* corrupted archives: 8.1.1 Creating and Reading Compressed Archives.
* Creation of the archive: 2.6 How to Create Archives.
* DAT blocking: 9.4.2 The Blocking Factor of an Archive.
* Data Modification time, excluding files by: 6.8 Operating Only on New Files.
* Data modification times of extracted files: Setting Data Modification Times.
* date format, ISO 8601: 7.2 Calendar date items.
* date input formats: 7 Date input formats.
* day in date strings: 7.6 Relative items in date strings.
* day in date strings: 7.6 Relative items in date strings.
* day of week item: 7.5 Day of week items.
* Deleting files from an archive: 4.2.5 Removing Archive Members Using `--delete'.
* Deleting from tape archives: 4.2.5 Removing Archive Members Using `--delete'.
* Descending directories, avoiding: 6.9 Descending into Directories.
* Directories, Archiving: 2.6.5 Archiving Directories.
* Directories, avoiding recursion: 6.9 Descending into Directories.
* Directory, changing mid-stream: 6.10.1 Changing the Working Directory.
* DIRLIST: 5.4.1 General-Purpose Variables.
* displacement of dates: 7.6 Relative items in date strings.
* doc-opt-col: Appendix B Configuring Help Summary.
* Double-checking a write operation: 9.8 Verifying Data as It is Stored.
* dumps, full: 5.1 Using tar to Perform Full Dumps.
* DUMP_BEGIN: 5.4.3 User Hooks.
* DUMP_END: 5.4.3 User Hooks.
* DUMP_REMIND_SCRIPT: 5.4.1 General-Purpose Variables.
* dup-args: Appendix B Configuring Help Summary.
* dup-args-note: Appendix B Configuring Help Summary.
* Eggert, Paul: 7.10 Authors of get_date.
* End-of-archive blocks, ignoring: Ignoring Blocks of Zeros.
* End-of-archive info script: 9.6.1 Archives Longer than One Tape or Disk.
* entry: 1.4 How tar Archives are Named.
* epoch, for POSIX: 7.8 Seconds since the Epoch.
* Error message, block number of: 3.7 Checking tar progress.
* Exabyte blocking: 9.4.2 The Blocking Factor of an Archive.
* exclude: 6.4 Excluding Some Files.
* exclude-caches: 6.4 Excluding Some Files.
* exclude-from: 6.4 Excluding Some Files.
* Excluding characters from a character class: 6.5 Wildcards Patterns and Matching.
* Excluding file by age: 6.8 Operating Only on New Files.
* Excluding files by file system: 6.4 Excluding Some Files.
* Excluding files by name and pattern: 6.4 Excluding Some Files.
* Exec Mode, genfile: D.3 Exec Mode.
* existing backup method: 4.5 Backup options.
* exit status: 3.1 General Synopsis of tar.
* extraction: 1.2 Some Definitions.
* Extraction: 2.8 How to Extract Members from an Archive.
* FDL, GNU Free Documentation License: F.1 GNU Free Documentation License.
* file archival: GNU tar: an archiver tool.
* File lists separated by NUL characters: D.1 Generate Mode.
* file name: 1.2 Some Definitions.
* File Name arguments, alternatives: 6.3 Reading Names from a File.
* File name arguments, using `--list' with: 2.7 How to List Archives.
* File names, excluding files by: 6.4 Excluding Some Files.
* File names, terminated by NUL: 6.3.1 NUL Terminated File Names.
* File names, using symbolic links: 8.3.2 Symbolic Links.
* File system boundaries, not crossing: 6.10 Crossing File System Boundaries.
* FILELIST: 5.4.1 General-Purpose Variables.
* first in date strings: 7.1 General date syntax.
* Format Options: 9.4.1 Format Variations.
* Format Parameters: 9.4.1 Format Variations.
* Format, old style: 8.3.3 Old V7 Archives.
* fortnight in date strings: 7.6 Relative items in date strings.
* free documentation: Appendix E Free Software Needs Free Documentation.
* full dumps: 5.1 Using tar to Perform Full Dumps.
* future time stamps: 8.3.8 Large or Negative Values.
* general date syntax: 7.1 General date syntax.
* Generate Mode, genfile: D.1 Generate Mode.
* genfile: Appendix D Genfile.
* genfile, create file: D.1 Generate Mode.
* genfile, creating sparse files: D.1 Generate Mode.
* genfile, generate mode: D.1 Generate Mode.
* genfile, reading a list of file names: D.1 Generate Mode.
* Getting program version number: 3.5 GNU tar documentation.
* get_date: 7 Date input formats.
* GNU archive format: 8.3.5 GNU and old GNU tar format.
* GNU.sparse.major, extended header variable: C.0.3 PAX Format, Version 1.0.
* GNU.sparse.map, extended header variable: C.0.2 PAX Format, Versions 0.0 and 0.1.
* GNU.sparse.minor, extended header variable: C.0.3 PAX Format, Version 1.0.
* GNU.sparse.name, extended header variable: C.0.2 PAX Format, Versions 0.0 and 0.1.
* GNU.sparse.name, extended header variable, in v.1.0: C.0.3 PAX Format, Version 1.0.
* GNU.sparse.numblocks, extended header variable: C.0.2 PAX Format, Versions 0.0 and 0.1.
* GNU.sparse.numbytes, extended header variable: C.0.2 PAX Format, Versions 0.0 and 0.1.
* GNU.sparse.offset, extended header variable: C.0.2 PAX Format, Versions 0.0 and 0.1.
* GNU.sparse.realsize, extended header variable: C.0.3 PAX Format, Version 1.0.
* GNU.sparse.size, extended header variable: C.0.2 PAX Format, Versions 0.0 and 0.1.
* gnupg, using with tar: 8.1.1 Creating and Reading Compressed Archives.
* gpg, using with tar: 8.1.1 Creating and Reading Compressed Archives.
* header-col: Appendix B Configuring Help Summary.
* hook: 5.4.3 User Hooks.
* hour in date strings: 7.6 Relative items in date strings.
* Ignoring end-of-archive blocks: Ignoring Blocks of Zeros.
* Info script: 9.6.1 Archives Longer than One Tape or Disk.
* Interactive operation: 3.8 Asking for Confirmation During Operations.
* ISO 8601 date format: 7.2 Calendar date items.
* items in date strings: 7.1 General date syntax.
* Labeling an archive: 9.7 Including a Label in the Archive.
* Labeling multi-volume archives: 9.7 Including a Label in the Archive.
* Labels on the archive media: 9.7 Including a Label in the Archive.
* language, in dates: 7.1 General date syntax.
* language, in dates: 7.1 General date syntax.
* Large lists of file names on small machines: Same Order.
* large values: 8.3.8 Large or Negative Values.
* last day: 7.5 Day of week items.
* last in date strings: 7.1 General date syntax.
* Listing all tar options: 3.5 GNU tar documentation.
* listing member and file names: 2.7 How to List Archives.
* Listing volume label: 9.7 Including a Label in the Archive.
* Lists of file names: 6.3 Reading Names from a File.
* Local and remote archives: 6.1 Choosing and Naming Archive Files.
* long-opt-col: Appendix B Configuring Help Summary.
* MacKenzie, David: 7.10 Authors of get_date.
* member: 1.2 Some Definitions.
* member name: 1.2 Some Definitions.
* Members, replacing with other members: 4.2.2 How to Add Files to Existing Archives: `--append'.
* Meyering, Jim: 7.10 Authors of get_date.
* Middle of the archive, starting in the: Starting File.
* midnight in date strings: 7.3 Time of day items.
* minute in date strings: 7.6 Relative items in date strings.
* minutes, time zone correction by: 7.3 Time of day items.
* Modes of extracted files: Setting Access Permissions.
* Modification time, excluding files by: 6.8 Operating Only on New Files.
* Modification times of extracted files: Setting Data Modification Times.
* month in date strings: 7.6 Relative items in date strings.
* month names in date strings: 7.2 Calendar date items.
* months, written-out: 7.1 General date syntax.
* MT: 5.4.1 General-Purpose Variables.
* MT_BEGIN: 5.4.2 Magnetic Tape Control.
* MT_OFFLINE: 5.4.2 Magnetic Tape Control.
* MT_REWIND: 5.4.2 Magnetic Tape Control.
* MT_STATUS: 5.4.2 Magnetic Tape Control.
* Multi-volume archives: 9.6.1 Archives Longer than One Tape or Disk.
* Mutli-volume archives in PAX format, extracting using non-GNU tars: 8.3.9.1 Extracting Members Split Between Volumes.
* Mutli-volume archives, extracting using non-GNU tars: 8.3.9.1 Extracting Members Split Between Volumes.
* Naming an archive: 6.1 Choosing and Naming Archive Files.
* negative time stamps: 8.3.8 Large or Negative Values.
* next day: 7.5 Day of week items.
* next in date strings: 7.1 General date syntax.
* noon in date strings: 7.3 Time of day items.
* now in date strings: 7.6 Relative items in date strings.
* ntape device: 9.5 Many Archives on One Tape.
* NUL terminated file names: 6.3.1 NUL Terminated File Names.
* Number of blocks per record: 9.4.2 The Blocking Factor of an Archive.
* Number of bytes per record: 9.4.2 The Blocking Factor of an Archive.
* numbered backup method: 4.5 Backup options.
* numbers, written-out: 7.1 General date syntax.
* Obtaining help: 3.5 GNU tar documentation.
* Obtaining total status information: 3.7 Checking tar progress.
* Old GNU archive format: 8.3.5 GNU and old GNU tar format.
* Old GNU sparse format: C.0.1 Old GNU Format.
* Old style archives: 8.3.3 Old V7 Archives.
* Old style format: 8.3.3 Old V7 Archives.
* opt-doc-col: Appendix B Configuring Help Summary.
* option syntax, traditional: 3.3.3 Old Option Style.
* Options when reading archives: 4.4.1 Options to Help Read Archives.
* Options, archive format specifying: 9.4.1 Format Variations.
* Options, format specifying: 9.4.1 Format Variations.
* ordinal numbers: 7.1 General date syntax.
* Overwriting old files, prevention: Options Controlling the Overwriting of Existing Files.
* pattern, genfile: D.1 Generate Mode.
* PAX archive format: 8.3.6 GNU tar and POSIX tar.
* Permissions of extracted files: Setting Access Permissions.
* Pinard, F.: 7.10 Authors of get_date.
* pm in date strings: 7.3 Time of day items.
* POSIX archive format: 8.3.6 GNU tar and POSIX tar.
* Progress information: 3.7 Checking tar progress.
* Protecting old files: Options Controlling the Overwriting of Existing Files.
* pure numbers in date strings: 7.7 Pure numbers in date strings.
* Reading file names from a file: 6.3 Reading Names from a File.
* Reading incomplete records: 4.4.1 Options to Help Read Archives.
* Record Size: 9.4.2 The Blocking Factor of an Archive.
* Records, incomplete: 4.4.1 Options to Help Read Archives.
* Recursion in directories, avoiding: 6.9 Descending into Directories.
* relative items in date strings: 7.6 Relative items in date strings.
* Remote devices: 6.1 Choosing and Naming Archive Files.
* remote tape drive: 9.2 The Remote Tape Server.
* Removing files from an archive: 4.2.5 Removing Archive Members Using `--delete'.
* Replacing members with other members: 4.2.2 How to Add Files to Existing Archives: `--append'.
* reporting bugs: 1.6 Reporting bugs or suggestions.
* RESTORE_BEGIN: 5.4.3 User Hooks.
* RESTORE_END: 5.4.3 User Hooks.
* Resurrecting files from an archive: 2.8 How to Extract Members from an Archive.
* Retrieving files from an archive: 2.8 How to Extract Members from an Archive.
* return status: 3.1 General Synopsis of tar.
* rmargin: Appendix B Configuring Help Summary.
* rmt: 9.2 The Remote Tape Server.
* RSH: 5.4.1 General-Purpose Variables.
* RSH_COMMAND: 5.4.1 General-Purpose Variables.
* Running out of space: 4.4.3 Coping with Scarce Resources.
* Salz, Rich: 7.10 Authors of get_date.
* short-opt-col: Appendix B Configuring Help Summary.
* simple backup method: 4.5 Backup options.
* SIMPLE_BACKUP_SUFFIX: 4.5 Backup options.
* SLEEP_MESSAGE: 5.4.1 General-Purpose Variables.
* SLEEP_TIME: 5.4.1 General-Purpose Variables.
* Small memory: 4.4.3 Coping with Scarce Resources.
* Sparse Files: 8.1.2 Archiving Sparse Files.
* sparse files v.0.0, extracting with non-GNU tars: 8.3.9.2 Extracting Sparse Members.
* sparse files v.0.1, extracting with non-GNU tars: 8.3.9.2 Extracting Sparse Members.
* sparse files v.1.0, extracting with non-GNU tars: 8.3.9.2 Extracting Sparse Members.
* Sparse files, creating using genfile: D.1 Generate Mode.
* sparse files, extracting with non-GNU tars: 8.3.9.2 Extracting Sparse Members.
* sparse formats: Storing Sparse Files.
* sparse formats, defined: 8.1.2 Archiving Sparse Files.
* sparse formats, Old GNU: C.0.1 Old GNU Format.
* sparse formats, v.0.0: C.0.2 PAX Format, Versions 0.0 and 0.1.
* sparse formats, v.0.1: C.0.2 PAX Format, Versions 0.0 and 0.1.
* sparse formats, v.1.0: C.0.3 PAX Format, Version 1.0.
* sparse versions: Storing Sparse Files.
* Specifying archive members: 6.2 Selecting Archive Members.
* Specifying files to act on: 6.2 Selecting Archive Members.
* Standard input and output: 6.1 Choosing and Naming Archive Files.
* Standard output, writing extracted files to: Writing to Standard Output.
* Storing archives in compressed format: 8.1.1 Creating and Reading Compressed Archives.
* Symbolic link as file name: 8.3.2 Symbolic Links.
* TAPE: The `--file' Option.
* tape blocking: 9.4.2 The Blocking Factor of an Archive.
* tape marks: 9.5 Many Archives on One Tape.
* tape positioning: 9.5 Many Archives on One Tape.
* Tapes, using `--delete' and: 4.2.5 Removing Archive Members Using `--delete'.
* TAPE_FILE: 5.4.1 General-Purpose Variables.
* tar: 1.3 What tar Does.
* TAR: 5.4.1 General-Purpose Variables.
* tar archive: 1.2 Some Definitions.
* Tar archive formats: 8 Controlling the Archive Format.
* tar entry: 1.4 How tar Archives are Named.
* tar file: 1.4 How tar Archives are Named.
* tar to a remote device: 6.1 Choosing and Naming Archive Files.
* tar to standard input and output: 6.1 Choosing and Naming Archive Files.
* tarcat: 9.6.3 Concatenate Volumes into a Single Archive.
* TAR_ARCHIVE, info script environment variable: 9.6.1 Archives Longer than One Tape or Disk.
* TAR_ATIME, to-command environment: Writing to an External Program.
* TAR_CTIME, to-command environment: Writing to an External Program.
* TAR_FILENAME, to-command environment: Writing to an External Program.
* TAR_FILETYPE, to-command environment: Writing to an External Program.
* TAR_FORMAT, info script environment variable: 9.6.1 Archives Longer than One Tape or Disk.
* TAR_GID, to-command environment: Writing to an External Program.
* TAR_GNAME, to-command environment: Writing to an External Program.
* TAR_MODE, to-command environment: Writing to an External Program.
* TAR_MTIME, to-command environment: Writing to an External Program.
* TAR_OPTIONS, environment variable: 3.2 Using tar Options.
* TAR_REALNAME, to-command environment: Writing to an External Program.
* TAR_SIZE, to-command environment: Writing to an External Program.
* TAR_SUBCOMMAND, info script environment variable: 9.6.1 Archives Longer than One Tape or Disk.
* TAR_UID, to-command environment: Writing to an External Program.
* TAR_UNAME, to-command environment: Writing to an External Program.
* TAR_VERSION, info script environment variable: 9.6.1 Archives Longer than One Tape or Disk.
* TAR_VOLUME, info script environment variable: 9.6.1 Archives Longer than One Tape or Disk.
* this in date strings: 7.6 Relative items in date strings.
* time of day item: 7.3 Time of day items.
* time zone correction: 7.3 Time of day items.
* time zone item: 7.1 General date syntax.
* time zone item: 7.4 Time zone items.
* today in date strings: 7.6 Relative items in date strings.
* tomorrow in date strings: 7.6 Relative items in date strings.
* TZ: 7.9 Specifying time zone rules.
* Ultrix 3.1 and write failure: 9.2 The Remote Tape Server.
* unpacking: 1.2 Some Definitions.
* Updating an archive: 4.2.3 Updating an Archive.
* usage-indent: Appendix B Configuring Help Summary.
* Using encrypted archives: 8.1.1 Creating and Reading Compressed Archives.
* ustar archive format: 8.3.4 Ustar Archive Format.
* uuencode: 4.6 Notable tar Usages.
* v7 archive format: 8.3.3 Old V7 Archives.
* Verbose operation: 3.7 Checking tar progress.
* Verifying a write operation: 9.8 Verifying Data as It is Stored.
* Verifying the currency of an archive: 4.2.6 Comparing Archive Members with the File System.
* Version of the tar program: 3.5 GNU tar documentation.
* version-control Emacs variable: 4.5 Backup options.
* VERSION_CONTROL: 4.5 Backup options.
* volno file: 9.6.1 Archives Longer than One Tape or Disk.
* VOLNO_FILE: 5.4.1 General-Purpose Variables.
* Volume label, listing: 9.7 Including a Label in the Archive.
* Volume number file: 9.6.1 Archives Longer than One Tape or Disk.
* week in date strings: 7.6 Relative items in date strings.
* Where is the archive?: 6.1 Choosing and Naming Archive Files.
* Working directory, specifying: 6.10.1 Changing the Working Directory.
* Writing extracted files to standard output: Writing to Standard Output.
* Writing new archives: 6.1 Choosing and Naming Archive Files.
* XLIST: 5.4.1 General-Purpose Variables.
* xsparse: 8.3.9.2 Extracting Sparse Members.
* year in date strings: 7.6 Relative items in date strings.
* yesterday in date strings: 7.6 Relative items in date strings.

Short Table of Contents
***********************

GNU tar: an archiver tool
1 Introduction
2 Tutorial Introduction to tar
3 Invoking GNU tar
4 GNU tar Operations
5 Performing Backups and Restoring Files
6 Choosing Files and Names for tar
7 Date input formats
8 Controlling the Archive Format
9 Tapes and Other Archive Media
Appendix A Changes
Appendix B Configuring Help Summary
Appendix C Tar Internals
Appendix D Genfile
Appendix E Free Software Needs Free Documentation
Appendix F Copying This Manual
Appendix G Index of Command Line Options
Appendix H Index

Table of Contents
*****************

GNU tar: an archiver tool
1 Introduction
 1.1 What this Book Contains
 1.2 Some Definitions
 1.3 What tar Does
 1.4 How tar Archives are Named
 1.5 GNU tar Authors
 1.6 Reporting bugs or suggestions
2 Tutorial Introduction to tar
 2.1 Assumptions this Tutorial Makes
 2.2 Stylistic Conventions
 2.3 Basic tar Operations and Options
 2.4 The Three Most Frequently Used Operations
 2.5 Two Frequently Used Options
  The `--file' Option
  The `--verbose' Option
  Getting Help: Using the `--help' Option
 2.6 How to Create Archives
  2.6.1 Preparing a Practice Directory for Examples
  2.6.2 Creating the Archive
  2.6.3 Running `--create' with `--verbose'
  2.6.4 Short Forms with `create'
  2.6.5 Archiving Directories
 2.7 How to List Archives
  Listing the Contents of a Stored Directory
 2.8 How to Extract Members from an Archive
  2.8.1 Extracting an Entire Archive
  2.8.2 Extracting Specific Files
  2.8.3 Extracting Files that are Directories
  2.8.4 Extracting Archives from Untrusted Sources
  2.8.5 Commands That Will Fail
 2.9 Going Further Ahead in this Manual
3 Invoking GNU tar
 3.1 General Synopsis of tar
 3.2 Using tar Options
 3.3 The Three Option Styles
  3.3.1 Long Option Style
  3.3.2 Short Option Style
  3.3.3 Old Option Style
  3.3.4 Mixing Option Styles
 3.4 All tar Options
  3.4.1 Operations
  3.4.2 tar Options
  3.4.3 Short Options Cross Reference
 3.5 GNU tar documentation
 3.6 Obtaining GNU tar default values
 3.7 Checking tar progress
 3.8 Asking for Confirmation During Operations
4 GNU tar Operations
 4.1 Basic GNU tar Operations
 4.2 Advanced GNU tar Operations
  4.2.1 The Five Advanced tar Operations
  4.2.2 How to Add Files to Existing Archives: `--append'
   4.2.2.1 Appending Files to an Archive
   4.2.2.2 Multiple Members with the Same Name
  4.2.3 Updating an Archive
   4.2.3.1 How to Update an Archive Using `--update'
  4.2.4 Combining Archives with `--concatenate'
  4.2.5 Removing Archive Members Using `--delete'
  4.2.6 Comparing Archive Members with the File System
 4.3 Options Used by `--create'
  4.3.1 Overriding File Metadata
  4.3.2 Ignore Fail Read
 4.4 Options Used by `--extract'
  4.4.1 Options to Help Read Archives
   Reading Full Records
   Ignoring Blocks of Zeros
  4.4.2 Changing How tar Writes Files
   Options Controlling the Overwriting of Existing Files
   Overwrite Old Files
   Keep Old Files
   Keep Newer Files
   Unlink First
   Recursive Unlink
   Setting Data Modification Times
   Setting Access Permissions
   Directory Modification Times and Permissions
   Writing to Standard Output
   Writing to an External Program
   Removing Files
  4.4.3 Coping with Scarce Resources
   Starting File
   Same Order
 4.5 Backup options
 4.6 Notable tar Usages
 4.7 Looking Ahead: The Rest of this Manual
5 Performing Backups and Restoring Files
 5.1 Using tar to Perform Full Dumps
 5.2 Using tar to Perform Incremental Dumps
 5.3 Levels of Backups
 5.4 Setting Parameters for Backups and Restoration
  5.4.1 General-Purpose Variables
  5.4.2 Magnetic Tape Control
  5.4.3 User Hooks
  5.4.4 An Example Text of `Backup-specs'
 5.5 Using the Backup Scripts
 5.6 Using the Restore Script
6 Choosing Files and Names for tar
 6.1 Choosing and Naming Archive Files
 6.2 Selecting Archive Members
 6.3 Reading Names from a File
  6.3.1 NUL Terminated File Names
 6.4 Excluding Some Files
  Problems with Using the exclude Options
 6.5 Wildcards Patterns and Matching
  Controlling Pattern-Matching
 6.6 Quoting Member Names
 6.7 Modifying File and Member Names
 6.8 Operating Only on New Files
 6.9 Descending into Directories
 6.10 Crossing File System Boundaries
  6.10.1 Changing the Working Directory
  6.10.2 Absolute File Names
7 Date input formats
 7.1 General date syntax
 7.2 Calendar date items
 7.3 Time of day items
 7.4 Time zone items
 7.5 Day of week items
 7.6 Relative items in date strings
 7.7 Pure numbers in date strings
 7.8 Seconds since the Epoch
 7.9 Specifying time zone rules
 7.10 Authors of get_date
8 Controlling the Archive Format
 8.1 Using Less Space through Compression
  8.1.1 Creating and Reading Compressed Archives
  8.1.2 Archiving Sparse Files
 8.2 Handling File Attributes
 8.3 Making tar Archives More Portable
  8.3.1 Portable Names
  8.3.2 Symbolic Links
  8.3.3 Old V7 Archives
  8.3.4 Ustar Archive Format
  8.3.5 GNU and old GNU tar format
  8.3.6 GNU tar and POSIX tar
   8.3.6.1 Controlling Extended Header Keywords
  8.3.7 Checksumming Problems
  8.3.8 Large or Negative Values
  8.3.9 How to Extract GNU-Specific Data Using Other tar Implementations
   8.3.9.1 Extracting Members Split Between Volumes
   8.3.9.2 Extracting Sparse Members
 8.4 Comparison of tar and cpio
9 Tapes and Other Archive Media
 9.1 Device Selection and Switching
 9.2 The Remote Tape Server
 9.3 Some Common Problems and their Solutions
 9.4 Blocking
  9.4.1 Format Variations
  9.4.2 The Blocking Factor of an Archive
 9.5 Many Archives on One Tape
  9.5.1 Tape Positions and Tape Marks
  9.5.2 The mt Utility
 9.6 Using Multiple Tapes
  9.6.1 Archives Longer than One Tape or Disk
  9.6.2 Tape Files
  9.6.3 Concatenate Volumes into a Single Archive
 9.7 Including a Label in the Archive
 9.8 Verifying Data as It is Stored
 9.9 Write Protection
Appendix A Changes
Appendix B Configuring Help Summary
Appendix C Tar Internals
 Basic Tar Format
 GNU Extensions to the Archive Format
 Storing Sparse Files
  C.0.1 Old GNU Format
  C.0.2 PAX Format, Versions 0.0 and 0.1
  C.0.3 PAX Format, Version 1.0
 Format of the Incremental Snapshot Files
 Dumpdir
Appendix D Genfile
 D.1 Generate Mode
 D.2 Status Mode
 D.3 Exec Mode
Appendix E Free Software Needs Free Documentation
Appendix F Copying This Manual
 F.1 GNU Free Documentation License
  F.1.1 ADDENDUM: How to use this License for your documents
Appendix G Index of Command Line Options
Appendix H Index

Footnotes
*********

(1)
This is only accidentally
true, but not in general.  Whereas modification times are always
restored, in most cases, one has to be root for restoring the owner,
and use a special option for restoring permissions.  Here, it just
happens that the restoring user is also the owner of the archived
members, and that the current umask is compatible with original
permissions.

(2)
Clustering many
options, the last of which has an argument, is a rather opaque way to
write options.  Some wonder if GNU getopt should not
even be made helpful enough for considering such usages as invalid.

(3)
Beware that if you precede options
with a dash, you are announcing the short option style instead of the
old option style; short options are decoded differently.

(4)
Before GNU tar version 1.11.6,
a bug prevented intermixing old style options with long options in
some cases.

(5)
Earlier versions of GNU tar understood `-l' as a
synonym for `--one-file-system'.  The current semantics, which
complies to UNIX98, was introduced with version
1.15.91. See Changes, for more information.

(6)
Earlier versions of GNU tar understood `-l' as a
synonym for `--one-file-system'.  This has changed in version
1.15.91. See Changes, for more information.

(7)
This option was called `--strip-path' in
version 1.14.

(8)
There are plans to merge the cpio and
tar packages into a single one which would be called
paxutils.  So, who knows if, one of this days, the
`--version' would not output `tar (GNU
paxutils) 3.2'

(9)
This is well described in Unix-haters
Handbook, by Simson Garfinkel, Daniel Weise & Steven Strassmann, IDG
Books, ISBN 1-56884-203-1.

(10)
Unless you give it
`--keep-old-files' option, or the disk copy is newer than the
the one in the archive and you invoke tar with
`--keep-newer-files' option

(11)
This can cause multiple members to have the same name, for
information on how this affects reading the archive, multiple.

(12)
Notice, that since both archives
were created withouth `-P' option (see absolute), these
commands should be run from the root file system.

(13)
Two
`--verbose' options were selected to avoid breaking usual
verbose listing output (`--list --verbose') when using in
scripts.

Versions of GNU tar up to 1.15.1 used to dump verbatim binary
contents of the DUMPDIR header (with terminating nulls) when
`--incremental' or `--listed-incremental' option was
given, no matter what the verbosity level.  This behavior, and,
especially, the binary output it produced were considered incovenient
and were changed in version 1.16

(14)
For backward compatibility, the backup will also
try to deduce the requested dump level from the name of the
script itself.  If the name consists of a string `level-'
followed by a single decimal digit, that digit is taken as
the dump level number.  Thus, you may create a link from backup
to level-1 and then run level-1 whenever you need to
create a level one dump.

(15)
Versions of GNU tar up to 1.15.1
recognized only `-C' option in file lists, and only if the
option and its argument occupied two consecutive lines.

(16)
Notice that earlier GNU tar versions used 
globbing for inclusion members, which contradicted to UNIX98
specification and was not documented. See Changes, for more
information on this and other changes.

(17)
A side effect of this is that when
`--create' is used with `--verbose' the resulting output
is not, generally speaking, the same as the one you'd get running
tar --list command.  This may be important if you use some
scripts for comparing both outputs.  See listing member and file names,
for the information on how to handle this case.

(18)
See PAX 1.

(19)
technically speaking, n is a
process ID of the tar process which created the
archive (see PAX keywords).

(20)
If you run GNU tar under a different locale, the
translation to the locale's language will be used.

(21)
See --restrict, for more information about
this option

(22)
Previous versions of tar used full
regular expression matching, or before that, only exact string
matching, instead of wildcard matchers.  We decided for the sake of
simplicity to use a uniform matching device through
tar.

