This is tar.info, produced by makeinfo version 4.6 from tar.texi.

START-INFO-DIR-ENTRY
* tar: (tar).			Making tape (or disk) archives.
END-INFO-DIR-ENTRY

   This file documents GNU `tar', a utility used to store, backup, and
transport files.

   Copyright (C) 1992, 1994, 1995, 1996, 1997, 1999 Free Software
Foundation, Inc.

   Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

   Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, provided that
the entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

   Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions, except that this permission notice may be stated in a
translation approved by the Foundation.

   This file documents GNU `tar', which is a utility used to store,
backup, and transport files.  `tar' is a tape (or disk) archiver.  This
manual documents the release 1.13.


File: tar.info,  Node: Top,  Next: Introduction,  Prev: (dir),  Up: (dir)

* Menu:

* Introduction::
* Tutorial::
* tar invocation::
* operations::
* Backups::
* Choosing::
* Date input formats::
* Formats::
* Media::
* 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
* posix compliance::
* 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::
* failing commands::

Invoking GNU `tar'

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

The Three Option Styles

* Mnemonic Options::            Mnemonic 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::
* extract options::
* backup::
* Applications::
* looking ahead::

Advanced GNU `tar' Operations

* Operations::
* current state::
* 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 `--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::
* Ignore Failed Read::

Changing How `tar' Writes Files

* Prevention Overwriting::
* Keep Old Files::
* Unlink First::
* Recursive Unlink::
* Modification Times::
* Setting Access Permissions::
* Writing to Standard Output::
* remove files::

Options to Prevent Overwriting Files

* Keep Old Files::
* Unlink First::
* Recursive Unlink::

Coping with Scarce Resources

* Starting File::
* Same Order::

Performing Backups and Restoring Files

* Full Dumps::                  Using `tar' to Perform Full Dumps
* Inc Dumps::                   Using `tar' to Perform Incremental Dumps
* incremental and listed-incremental::  The Incremental Options
* Backup Levels::               Levels of Backups
* Backup Parameters::           Setting Parameters for Backups and Restoration
* Scripted Backups::            Using the Backup Scripts
* Scripted Restoration::        Using the Restore Script

Setting Parameters for Backups and Restoration

* backup-specs example::        An Example Text of `Backup-specs'
* Script Syntax::               Syntax for `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::
* after::                       Operating Only on New Files
* recurse::                     Descending into Directories
* one::                         Crossing Filesystem Boundaries

Reading Names from a File

* nul::

Excluding Some Files

* problems with exclude::

Crossing Filesystem Boundaries

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

Date input formats

* General date syntax::         Common rules.
* Calendar date item::          19 Dec 1994.
* Time of day item::            9:20pm.
* Timezone item::               EST, DST, BST, UCT, AHST, ...
* Day of week item::            Monday and others.
* Relative item in date strings::  next tuesday, 2 years ago.
* Pure numbers in date strings::  19931219, 1440.
* Authors of getdate::          Bellovin, Salz, Berets, et al.

Controlling the Archive Format

* Portability::                 Making `tar' Archives More Portable
* Compression::                 Using Less Space through Compression
* Attributes::                  Handling File Attributes
* Standard::                    The Standard Format
* Extensions::                  GNU Extensions to the Archive Format
* cpio::                        Comparison of `tar' and `cpio'

Making `tar' Archives More Portable

* Portable Names::              Portable Names
* dereference::                 Symbolic Links
* old::                         Old V7 Archives
* posix::                       POSIX archives
* Checksumming::                Checksumming Problems

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


File: tar.info,  Node: Introduction,  Next: Tutorial,  Prev: Top,  Up: Top

Introduction
************

Welcome to the GNU `tar' manual.  GNU `tar' is used to create and
manipulate files ("archives") which are actually collections of many
other files; the program provides users with an organized and
systematic method for controlling a large amount of data.

* Menu:

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


File: tar.info,  Node: Book Contents,  Next: Definitions,  Prev: Introduction,  Up: Introduction

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 (*note 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; *note 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 (*note Date input formats::) exists in its
entirety in other GNU manuals, and is mostly self-contained.  In
addition, one section of this manual (*note Standard::) contains a big
quote which is taken directly from `tar' sources.

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


File: tar.info,  Node: Definitions,  Next: What tar Does,  Prev: Book Contents,  Up: Introduction

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 last modification time.  Some archives also
record the file names in each archived directory, as well as other file
and directory information.)  You can use `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 filesystem, 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 filesystem.  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 peformed using `tar'.


File: tar.info,  Node: What tar Does,  Next: Naming tar Archives,  Prev: Definitions,  Up: Introduction

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
`t'ape `ar'chiver.  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 filesystem.

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.


File: tar.info,  Node: Naming tar Archives,  Next: posix compliance,  Prev: What tar Does,  Up: Introduction

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.


File: tar.info,  Node: posix compliance,  Next: Authors,  Prev: Naming tar Archives,  Up: Introduction

POSIX Compliance
================

We make some of our recommendations throughout this book for one reason
in addition to what we think of as "good sense".  The main additional
reason for a recommendation is to be compliant with the POSIX
standards.  If you set the shell environment variable
`POSIXLY_CORRECT', GNU `tar' will force you to adhere to these
standards.  Therefore, if this variable is set and you violate one of
the POSIX standards in the way you phrase a command, for example, GNU
`tar' will not allow the command and will signal an error message.  You
would then have to reorder the options or rephrase the command to
comply with the POSIX standards.

   There is a chance in the future that, if you set this environment
variable, your archives will be forced to comply with POSIX standards,
also.  No GNU `tar' extensions will be allowed.


File: tar.info,  Node: Authors,  Next: Reports,  Prev: posix compliance,  Up: Introduction

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, and finally Franc,ois Pinard, 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 draft has
been distributed in `tar' versions 1.04 (or even before?)   through
1.10, then 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.

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


File: tar.info,  Node: Reports,  Prev: Authors,  Up: Introduction

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

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


File: tar.info,  Node: Tutorial,  Next: tar invocation,  Prev: Introduction,  Up: Top

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.

* Menu:

* 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::


File: tar.info,  Node: assumptions,  Next: stylistic conventions,  Prev: Tutorial,  Up: Tutorial

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

   * 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.  *Note Media::,
     for complete information on using `tar' archives with tape drives.



File: tar.info,  Node: stylistic conventions,  Next: basic tar options,  Prev: assumptions,  Up: Tutorial

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'.  When we have lines which are too long to be displayed in any
other way, we will show them like this:

     This is an example of a line which would otherwise not fit in this space.


File: tar.info,  Node: basic tar options,  Next: frequent operations,  Prev: stylistic conventions,  Up: Tutorial

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'.  We present a full discussion of this way of writing options and
operations appears in *Note Old Options::, and we discuss the other two
styles of writing options in *Note Mnemonic Options:: and *Note 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 *Note 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.


File: tar.info,  Node: frequent operations,  Next: Two Frequent Options,  Prev: basic tar options,  Up: Tutorial

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.


File: tar.info,  Node: Two Frequent Options,  Next: create,  Prev: frequent operations,  Up: Tutorial

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.)

* Menu:

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


File: tar.info,  Node: file tutorial,  Next: verbose tutorial,  Prev: Two Frequent Options,  Up: Two Frequent Options

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 use a default,
usually some physical tape drive attached to your machine.  If there is
no tape drive attached, or the default is not meaningful, then `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 specfiy 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 *Note file::.


File: tar.info,  Node: verbose tutorial,  Next: help tutorial,  Prev: file tutorial,  Up: Two Frequent Options

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.

   Sometimes, a single instance of `--verbose' on the command line will
show a full, `ls' style listing of an archive or files, giving sizes,
owners, and similar information.  Other times, `--verbose' will only
show files or members that the particular operation is operating on at
the time.  In the latter case, you can use `--verbose' twice in a
command to get a listing such as that in the former case.  For example,
instead of saying

     tar -cvf afiles.tar apple angst aspic

above, you might say

     tar -cvvf afiles.tar apple angst 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'.


File: tar.info,  Node: help tutorial,  Prev: verbose tutorial,  Up: Two Frequent Options

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.


File: tar.info,  Node: create,  Next: list,  Prev: Two Frequent Options,  Up: Tutorial

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.

* Menu:

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


File: tar.info,  Node: prepare for examples,  Next: Creating the archive,  Prev: create,  Up: create

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 a file unless you
specify an option which does this .  To add files to an existing
archive, you need to use a different option, such as `--append' (`-r');
see *Note append:: for information on how to do this.


File: tar.info,  Node: Creating the archive,  Next: create verbose,  Prev: prepare for examples,  Up: create

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 part of the command which says,
`--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.
*Note 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) .  Now that they are are in the archive, they are called
_archive members_, not files .

   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.  *Note append::.


File: tar.info,  Node: create verbose,  Next: short create,  Prev: Creating the archive,  Up: create

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.


File: tar.info,  Node: short create,  Next: create dir,  Prev: create verbose,  Up: create

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 *Note 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.
(Placing options in an unusual order can also cause `tar' to report an
error if you have set the shell environment variable,
`POSIXLY_CORRECT'; *note posix compliance:: for more information on
this.)


File: tar.info,  Node: create dir,  Prev: short create,  Up: create

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; *Note 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, 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 versions of `tar' are not so
clever; they will enter an infinite loop when this happens, so you
should not depend on this behavior unless you are certain you are
running GNU `tar'.  )


File: tar.info,  Node: list,  Next: extract,  Prev: create,  Up: Tutorial

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.

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

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

   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 (ie., 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 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 are compared
using a simplistic name comparison, in which an exact match is
necessary.  *Note absolute::.

   However, `tar --list --file=collection.tar folk' would respond with
`folk', because `folk' is in the archive file `collection.tar'.  If you
are not sure of the exact file name, try listing all the files in the
archive and searching for the one you expect to find; remember that if
you use `--list' with no file names as arguments, `tar' will print the
names of all the members stored in the specified archive.

* Menu:

* list dir::


File: tar.info,  Node: list dir,  Prev: list,  Up: list

Listing the Contents of a Stored Directory
------------------------------------------

     _(This message will disappear, once this node revised.)_

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

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


File: tar.info,  Node: extract,  Next: going further,  Prev: list,  Up: Tutorial

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', `-x') operation.  As with
`--create' (`-c'), specify the name of the archive with
`--file=ARCHIVE-NAME' (`-f ARCHIVE-NAME').  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.

* Menu:

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


File: tar.info,  Node: extracting archives,  Next: extracting files,  Prev: extract,  Up: extract

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


File: tar.info,  Node: extracting files,  Next: extract dir,  Prev: extracting archives,  Up: extract

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.  It 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, creation
times, and owner.   (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'.
To find the exact member names of the members of an archive, use
`--list' (`-t') (*note list::).

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


File: tar.info,  Node: extract dir,  Next: failing commands,  Prev: extracting files,  Up: extract

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 overwrite
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.

   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

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.


File: tar.info,  Node: failing commands,  Prev: extract dir,  Up: extract

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 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.


File: tar.info,  Node: going further,  Prev: extract,  Up: Tutorial

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


File: tar.info,  Node: tar invocation,  Next: operations,  Prev: Tutorial,  Up: Top

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 (*note Synopsis::).  There are numerous options, and
many styles for writing them.  One mandatory option specifies the
operation `tar' should perform (*note Operation Summary::), other
options are meant to detail how this operation should be performed
(*note 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 (*note 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' (*note help::), `--verbose' (`-v') (*note verbose::) and
`--interactive' (`-w') options (*note interactive::).

* Menu:

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


File: tar.info,  Node: Synopsis,  Next: using tar options,  Prev: tar invocation,  Up: tar invocation

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' (`-t').  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' (`-P') option).
*Note absolute::, for more information about `--absolute-names' (`-P').

   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 filesystem 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.  *Note 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' (`-t'),
`--extract' (`--get', `-x'), `--compare' (`--diff', `-d'), and
`--update' (`-u')) 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.


File: tar.info,  Node: using tar options,  Next: Styles,  Prev: Synopsis,  Up: tar invocation

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 *Note frequent operations:: and *Note
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 *note All
Options::.)

   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.


File: tar.info,  Node: Styles,  Next: All Options,  Prev: using tar options,  Up: tar invocation

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=ARCHIVE-NAME' (`-f ARCHIVE-NAME') 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.

* Menu:

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


File: tar.info,  Node: Mnemonic Options,  Next: Short Options,  Prev: Styles,  Up: Styles

Mnemonic 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
mnemonic 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 mnemonic 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.)

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

     $ 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'.

   Mnemonic options which require arguments take those arguments
immediately following the option name; they are introduced by an equal
sign.  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 the notation `--file=archive.tar' for the mnemonic option.


File: tar.info,  Node: Short Options,  Next: Old Options,  Prev: Mnemonic Options,  Up: Styles

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' 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(1).

   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.

   ---------- Footnotes ----------

   (1) 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.


File: tar.info,  Node: Old Options,  Next: Mixing,  Prev: Short Options,  Up: Styles

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(1).  This set of
letters must be the first to appear on the command line, after the
`tar' program name and some whitespace; old options cannot appear
anywhere else.  The letter of an old option is exactly the same letter
as the corresponding short option.  For example, the old option `t' is
the same as the short option `-t', and consequently, the same as the
mnemonic 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.

   ---------- Footnotes ----------

   (1) 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.


File: tar.info,  Node: Mixing,  Prev: Old Options,  Up: Styles

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(1).  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 whitespace).
Modern options may be given only after all arguments to the old options
have been collected.  If this rule is not respected, a modern option
might be falsely interpreted as the value of the argument to one of the
old style options.

   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.

   ---------- Footnotes ----------

   (1) Before GNU `tar' version 1.11.6, a bug prevented intermixing old
style options with mnemonic options in some cases.


File: tar.info,  Node: All Options,  Next: help,  Prev: Styles,  Up: tar invocation

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.

* Menu:

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


File: tar.info,  Node: Operation Summary,  Next: Option Summary,  Prev: All Options,  Up: All Options

Operations
----------

`--append'
`-r'
     Appends files to the end of the archive.  *Note append::.

`--catenate'
`-A'
     Same as `--concatenate'.  *Note 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.  *Note compare::.

`--concatenate'
`-A'
     Appends other `tar' archives to the end of the archive.  *Note
     concatenate::.

`--create'
`-c'
     Creates a new `tar' archive.  *Note create::.

`--delete'
     Deletes members from the archive.  Don't try this on a archive on a
     tape!  *Note delete::.

`--diff'
`-d'
     Same `--compare'.  *Note compare::.

`--extract'
`-x'
     Extracts members from the archive into the file system.  *Note
     extract::.

`--get'
`-x'
     Same as `--extract'.   *Note extract::.

`--list'
`-t'
     Lists the members in an archive.  *Note 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.  *Note update::.



File: tar.info,  Node: Option Summary,  Next: Short Option Summary,  Prev: Operation Summary,  Up: All Options

`tar' Options
-------------

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

`--after-date'
     (See `--newer'; .)

`--atime-preserve'
     Tells `tar' to preserve the access time field in a file's inode
     when dumping it.  .

`--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.  .

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

`--blocking-factor=BLOCKING'
`-b BLOCKING'
     Sets the blocking factor `tar' uses to BLOCKING x 512 bytes per
     record.  .

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

`--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.  .

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

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

`--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.  .

`--exclude=PATTERN'
     When performing operations, `tar' will skip files that match
     PATTERN.  .

`--exclude-from=FILE'
`-X FILE'
     Similar to `--exclude', except `tar' will use the list of patterns
     in the file FILE.  .

`--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.
     .

`--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.  .

`--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.  .

`--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.  .

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

`--gunzip'
     (See `--gzip'; .)

`--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.  .

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

`--ignore-failed-read'
     Instructs `tar' to exit successfully if it encounters an
     unreadable file.  *Note Reading::.

`--ignore-umask'
     (See `--preserve-permissions'; *note Writing::.)

`--ignore-zeros'
`-i'
     With this option, `tar' will ignore zeroed blocks in the archive,
     which normally signals EOF.  *Note 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.  .

`--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.  .

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

`--keep-old-files'
`-k'
     When extracting files from an archive, `tar' will not overwrite
     existing files if this option is present.  *Note Writing::.

`--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.  .

`--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.  .

`--mode=PERMISSIONS'
     When adding files to an archive, `tar' will use PERMISSIONS for
     the archive members, rather than the permissions from the files.
     The program `chmod' and this `tar' option share the same syntax
     for what PERMISSIONS might be.  *Note Permissions:
     (filetutils)File permissions.  This reference also has useful
     information for those not being overly familiar with the Unix
     permission system.

     Of course, PERMISSIONS might be plainly specified as an octal
     number.  However, by using generic symbolic modifications to mode
     bits, this allows more flexibility.  For example, the value `a+rw'
     adds read and write permissions for everybody, while retaining
     executable bits on directories or on any other file already marked
     as executable.

`--multi-volume'
`-M'
     Informs `tar' that it should create or otherwise operate on a
     multi-volume `tar' archive.  .

`--new-volume-script'
     (see -info-script)

`--newer=DATE'
`--after-date=DATE'
`-N'
     When creating an archive, `tar' will only add files that have
     changed since DATE.  .

`--newer-mtime'
     In conjunction with `--newer', `tar' will only add files whose
     contents have changed (as opposed to just `--newer', which will
     also back up files for which any status information has changed).

`--no-recursion'
     With this option, `tar' will not recurse into directories unless a
     directory is explicitly named as an argument to `tar'.  .

`--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.  .

`--numeric-owner'
     This option will notify `tar' that it should use numeric user and
     group IDs when creating a `tar' file, rather than names.  .

`--old-archive'
     (See `--portability'; .)

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

`--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.  .

     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.

`--portability'
`--old-archive'
`-o'
     Tells `tar' to create an archive that is compatible with Unix V7
     `tar'.  .

`--posix'
     Instructs `tar' to create a POSIX compliant `tar' archive.  .

`--preserve'
     Synonymous with specifying both `--preserve-permissions' and
     `--same-order'.  .

`--preserve-order'
     (See `--same-order'; *note 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.  *Note Writing::.

`--read-full-records'
`-B'
     Specifies that `tar' should reblock its input, for reading from
     pipes on systems with buggy implementations.  *Note Reading::.

`--record-size=SIZE'
     Instructs `tar' to use SIZE bytes per record when accessing the
     archive.  .

`--recursive-unlink'
     Similar to the `--unlink-first' option, removing existing
     directory hierarchies before extracting directories of the same
     name from the archive.  *Note Writing::.

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

`--rsh-command=CMD'
     Notifies `tar' that is should use CMD to communicate with remote
     devices.  .

`--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.  *Note Reading::.

`--same-owner'
     When extracting an archive, `tar' will attempt to preserve the
     owner specified in the `tar' archive with this option present.  .

`--same-permissions'
     (See `--preserve-permissions'; *note Writing::.)

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

`--sparse'
`-S'
     Invokes a GNU extension when adding files to an archive that
     handles sparse files efficiently.  .

`--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.  *Note
     Scarce::.

`--suffix=SUFFIX'
     Alters the suffix `tar' uses when backing up files from the default
     `~'.  .

`--tape-length=NUM'
`-L NUM'
     Specifies the length of tapes that `tar' is writing as being
     NUM x 1024 bytes long.  .

`--to-stdout'
`-O'
     During extraction, `tar' will extract files to stdout rather than
     to the file system.  *Note Writing::.

`--totals'
     Displays the total number of bytes written after creating an
     archive.  .

`--touch'
`-m'
     Sets the modification time of extracted files to the extraction
     time, rather than the modification time stored in the archive.
     *Note Writing::.

`--uncompress'
     (See `--compress'; .)

`--ungzip'
     (See `--gzip'; .)

`--unlink-first'
`-U'
     Directs `tar' to remove the corresponding file from the file system
     before extracting it from the archive.  *Note Writing::.

`--use-compress-program=PROG'
     Instructs `tar' to access the archive through PROG, which is
     presumed to be a compression program of some sort.  .

`--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.  .

`--verify'
`-W'
     Verifies that the archive was correctly written when creating an
     archive.  .

`--version'
     `tar' will print an informational message about what version it is
     and a copyright message, some credits, and then exit.  .

`--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.  .


File: tar.info,  Node: Short Option Summary,  Prev: Option Summary,  Up: All Options

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

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

`-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'

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

`-l'
     `--one-file-system'

`-m'
     `--touch'

`-o'
     `--portability'

`-p'
     `--preserve-permissions'

`-r'
     `--append'

`-s'
     `--same-order'

`-t'
     `--list'

`-u'
     `--update'

`-v'
     `--verbose'

`-w'
     `--interactive'

`-x'
     `--extract'

`-z'
     `--gzip'



File: tar.info,  Node: help,  Next: verbose,  Prev: All Options,  Up: tar invocation

GNU `tar' documentation
=======================

Being careful, the first thing is really checking that you are using GNU
`tar', indeed.  The `--version' option will generate a message giving
confirmation that you are using GNU `tar', with the precise version of
GNU `tar' you are using.  `tar' identifies itself and prints the
version number to the standard output, then immediately exits
successfully, without doing anything else, ignoring all other options.
For example, `tar --version' might return:

     tar (GNU tar) 1.13

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(1).

   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.

   The perceptive reader would have noticed some contradiction in the
previous paragraphs.  It is written that both `--version' and `--help'
print something, and have all other options ignored.  In fact, they
cannot ignore each other, and one of them has to win.  We do not
specify which is stronger, here; experiment if you really wonder!

   The short help output is quite succint, 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 printed form, as a kind of small book.  It may
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 long to
GNU `tar', or it has not been produced by GNU.  Currently, GNU `tar'
documentation is provided in Texinfo format only, if we except, of
course, the short result of `tar --help'.

   ---------- Footnotes ----------

   (1) 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 yield `tar (GNU paxutils) 3.2'


File: tar.info,  Node: verbose,  Next: interactive,  Prev: help,  Up: tar invocation

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' (`-c') or `--extract' (`--get', `-x'), `--verbose'
(`-v') 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
(reminiscent of `ls -l') for each member.  Since `--list' (`-t')
already prints the names of the members, `--verbose' (`-v') used once
with `--list' (`-t') 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 xvv 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.

   The `--totals' option--which is only meaningful when used with
`--create' (`-c')--causes `tar' to print the total amount written to
the archive, after it has been fully created.

   The `--checkpoint' option prints an occasional message as `tar'
reads or writes the archive.  In fact, it print directory names while
reading 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.

   The `--show-omitted-dirs' option, when reading an archive--with
`--list' (`-t') or `--extract' (`--get', `-x'), 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).  .


File: tar.info,  Node: interactive,  Prev: verbose,  Up: tar invocation

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 (*note 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.


File: tar.info,  Node: operations,  Next: Backups,  Prev: tar invocation,  Up: Top

GNU `tar' Operations
********************

* Menu:

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


File: tar.info,  Node: Basic tar,  Next: Advanced tar,  Prev: operations,  Up: operations

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(1).  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 ot 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' (`-c')
     option is given, there are no arguments besides options, and
     `--files-from=FILE-OF-NAMES' (`-T FILE-OF-NAMES') 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=FILE-OF-NAMES' (`-T
     FILE-OF-NAMES') 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 `1996-11-09', while it used to show
     them as `Nov 11 1996'.  (One can revert to the old behavior by
     defining `USE_OLD_CTIME' in `src/list.c' before reinstalling.)
     But preferrably, people you should get used to ISO 8601 dates.
     Local American dates should be made available again with full date
     localisation support, once ready.  In the meantime, programs not
     being localisable for dates should prefer international dates,
     that's really the way to go.

     Look up <http://www.ft.uni-erlangen.de/~mskuhn/iso-time.html> if
     you are curious, it contains a detailed explanation of the ISO
     8601 standard.


   ---------- Footnotes ----------

   (1) This is well described in `Unix-haters Handbook', by Simson
Garfinkel, Daniel Weise & Steven Strassmann, IDG Books, ISBN
1-56884-203-1.


File: tar.info,  Node: Advanced tar,  Next: extract options,  Prev: Basic tar,  Up: operations

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.

* Menu:

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


File: tar.info,  Node: Operations,  Next: current state,  Prev: Advanced tar,  Up: Advanced tar

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'.
`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 *Note 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.


File: tar.info,  Node: current state,  Next: append,  Prev: Operations,  Up: Advanced tar

The Current State of the Practice Files
---------------------------------------

   Currently, the listing of the directory using `ls' is as follows:


The archive file `collection.tar' looks like this:

     $ tar -tvf collection.tar

The archive file `music.tar' looks like this:

     $ tar -tvf music.tar


File: tar.info,  Node: append,  Next: update,  Prev: current state,  Up: Advanced tar

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', *note update::.)

   If you use `--append' (`-r') 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 numbers 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 modification times, owners, etc.

   Other operations don't deal with these members as perfectly as you
might prefer; if you were to use `--extract' (`--get', `-x') 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
_overwrite_ a file of the same name which existed in the directory
already, and `tar' will not prompt you about this.  Thus, only the most
recently archived member will end up being extracted, as it will
overwrite the one extracted before it, and so on.

   There are a few ways to get around this.  .

   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 truely
"replace" one member with another.  (Replacing one member with another
will not work on certain types of media, such as tapes; see *Note
delete:: and *Note Media::, for more information.)

* Menu:

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


File: tar.info,  Node: appending files,  Next: multiple,  Prev: append,  Up: append

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.  *Note 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-rw-rw- me user     28 1996-10-18 16:31 jazz
     -rw-rw-rw- me user     21 1996-09-23 16:44 blues
     -rw-rw-rw- me user     20 1996-09-23 16:44 folk
     -rw-rw-rw- me user     20 1996-09-23 16:44 rock


File: tar.info,  Node: multiple,  Prev: appending files,  Up: append

Multiple Files 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'; *note
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
overwrite 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 overwritten 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-rw-rw- me user     28 1996-10-18 16:31 jazz
     -rw-rw-rw- me user     21 1996-09-23 16:44 blues
     -rw-rw-rw- me user     20 1996-09-23 16:44 folk
     -rw-rw-rw- me user     20 1996-09-23 16:44 rock
     -rw-rw-rw- me user     58 1996-10-24 18:30 blues

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 overwritten by
the newer version.  You can confirm this by extracting the archive and
running `ls' on the directory.  *Note Writing::, for more information.
(_Please note:_ This is the case unless you employ the `--backup'
option; .)


File: tar.info,  Node: update,  Next: concatenate,  Prev: append,  Up: Advanced tar

Updating an Archive
-------------------

     _(This message will disappear, once this node revised.)_

In the previous section, you learned how to use `--append' (`-r') 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' (`-r')).

   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 ().

* Menu:

* how to update::


File: tar.info,  Node: how to update,  Prev: update,  Up: update

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.  *Note 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 *Note Backups::.


File: tar.info,  Node: concatenate,  Next: delete,  Prev: update,  Up: Advanced tar

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', name the archives to be concatenated on the
command line.  (Nothing happens if you don't list any.)  The members,
and their member names, will be copied verbatim from those archives.  If
this causes multiple members to have the same name, it does not delete
any members; all the members with the same name coexist.  For
information on how this affects reading the archive, .

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

   We can concatenate these two archives with `tar':

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

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

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

   When you use `--concatenate', the source and target archives must
already exist and must have been created using compatable format
parameters ().  The new, concatenated archive will be called by the
same name as the first archive listed on the command line.

   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.
*Note Ignore Zeros::, for further information on dealing with archives
improperly combined using the `cat' shell utility.

   You must specify the source archives using `--file=ARCHIVE-NAME'
(`-f ARCHIVE-NAME') (*note file::).  If you do not specify the target
archive, `tar' uses the value of the environment variable `TAPE', or,
if this has not been set, the default archive name.


File: tar.info,  Node: delete,  Next: compare,  Prev: concatenate,  Up: Advanced tar

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=ARCHIVE-NAME' (`-f
ARCHIVE-NAME') and then specify the names of the members to be deleted;
if you list no member names, nothing will be deleted.  The `--verbose'
(`-v') option will cause `tar' to print the names of the members as
they are deleted.  As with `--extract' (`--get', `-x'), 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.  *Note 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
     practice/blues
     practice/folk
     practice/jazz
     practice/rock
     practice/blues
     $ 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'.


File: tar.info,  Node: compare,  Prev: delete,  Up: Advanced tar

Comparing Archive Members with the File System
----------------------------------------------

     _(This message will disappear, once this node revised.)_

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

Depending on the system where you are running `tar' and the version you
are running, `tar' may have a different error message, such as:

     funk: does not exist

   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, *Note verify::.


File: tar.info,  Node: extract options,  Next: backup,  Prev: Advanced tar,  Up: operations

Options Used by `--extract'
===========================

     _(This message will disappear, once this node revised.)_

The previous chapter showed how to use `--extract' (`--get', `-x') to
extract an archive into the filesystem.  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
*Note extract:: for more basic information about the `--extract'
operation.

* Menu:

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


File: tar.info,  Node: Reading,  Next: Writing,  Prev: extract options,  Up: extract options

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' (`--get', `-x') or `--list'
(`-t') operations.  *Note 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.  *Note Blocking Factor::.

* Menu:

* read full records::
* Ignore Zeros::
* Ignore Failed Read::


File: tar.info,  Node: read full records,  Next: Ignore Zeros,  Prev: Reading,  Up: Reading

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.


File: tar.info,  Node: Ignore Zeros,  Next: Ignore Failed Read,  Prev: read full records,  Up: Reading

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 which was created by `cat'-ing 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
compatablity among archiving utilities.

`--ignore-zeros'
`-i'
     To ignore blocks of zeros (ie. end-of-archive entries) which may be
     encountered while reading an archive.  Use in conjunction with
     `--extract' (`--get', `-x') or `--list' (`-t').


File: tar.info,  Node: Ignore Failed Read,  Prev: Ignore Zeros,  Up: Reading

Ignore Fail Read
................

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


File: tar.info,  Node: Writing,  Next: Scarce,  Prev: Reading,  Up: extract options

Changing How `tar' Writes Files
-------------------------------

     _(This message will disappear, once this node revised.)_

* Menu:

* Prevention Overwriting::
* Keep Old Files::
* Unlink First::
* Recursive Unlink::
* Modification Times::
* Setting Access Permissions::
* Writing to Standard Output::
* remove files::


File: tar.info,  Node: Prevention Overwriting,  Next: Keep Old Files,  Prev: Writing,  Up: Writing

Options to Prevent Overwriting Files
....................................

Normally, `tar' writes 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.
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 found to be on the way of the proper extraction.

   To prevent `tar' from extracting an archive member from an archive
if doing so will overwrite a file in the file system, use
`--keep-old-files' (`-k') in conjunction with `--extract'.  When this
option is specified, `tar' will report an error stating the name of the
files in conflict instead of overwriting the file with the
corresponding extracted archive member.

   The `--unlink-first' (`-U') option removes existing files, symbolic
links, empty directories, devices, etc., _prior_ to extracting over
them.  In particular, using this option will prevent replacing an
already existing symbolic link by the name of an extracted file, since
the link itself is removed prior to the extraction, rather than the
file it points to.  On some systems, the backing store for the
executable _is_ the original program text.  You could use the
`--unlink-first' (`-U') option to prevent segmentation violations or
other woes when extracting arbitrary executables over currently running
copies.  Note that if something goes wrong with the extraction and you
_did_ use this option, you might end up with no file at all.  Without
this option, if something goes wrong with the extraction, the existing
file is not overwritten and preserved.

   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.  For example, someone using this feature
may be very surprised at the results when extracting a directory entry
from the archive.  This option can be dangerous; be very aware of what
you are doing if you choose to use it.

* Menu:

* Keep Old Files::
* Unlink First::
* Recursive Unlink::


File: tar.info,  Node: Keep Old Files,  Next: Unlink First,  Prev: Prevention Overwriting,  Up: Writing

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

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


File: tar.info,  Node: Unlink First,  Next: Recursive Unlink,  Prev: Keep Old Files,  Up: Writing

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

`--unlink-first'
`-U'
     Try removing files before extracting over them, instead of trying
     to overwrite them.


File: tar.info,  Node: Recursive Unlink,  Next: Modification Times,  Prev: Unlink First,  Up: Writing

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

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

   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
filesystem when the archive was created.  It is debatable that this
would always be a proper behaviour.  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 behaviour.  In any case, single files are
silently removed.


File: tar.info,  Node: Modification Times,  Next: Setting Access Permissions,  Prev: Recursive Unlink,  Up: Writing

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

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

   To set the 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 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').


File: tar.info,  Node: Setting Access Permissions,  Next: Writing to Standard Output,  Prev: Modification Times,  Up: Writing

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

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

`--preserve-permission'
`--same-permission'
`--ignore-umask'
`-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').


File: tar.info,  Node: Writing to Standard Output,  Next: remove files,  Prev: Setting Access Permissions,  Up: Writing

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.  Used in conjunction with
     `--extract' (`--get', `-x').  Extract files to standard output.
     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').


File: tar.info,  Node: remove files,  Prev: Writing to Standard Output,  Up: Writing

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

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


File: tar.info,  Node: Scarce,  Prev: Writing,  Up: extract options

Coping with Scarce Resources
----------------------------

     _(This message will disappear, once this node revised.)_

* Menu:

* Starting File::
* Same Order::


File: tar.info,  Node: Starting File,  Next: Same Order,  Prev: Scarce,  Up: Scarce

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=NAME' (`-K NAME')
is not necessary.  *Note Inc Dumps::, *Note interactive::, and
*Note exclude::.)


File: tar.info,  Node: Same Order,  Prev: Starting File,  Up: Scarce

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.


File: tar.info,  Node: backup,  Next: Applications,  Prev: extract options,  Up: operations

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'
     Make backups of files that are about to be overwritten or removed.
     Without this option, the original versions are destroyed.

`--suffix=SUFFIX'
     Append SUFFIX to each backup file made with `-b'.  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.

`--version-control=METHOD'
     Use METHOD to determine the type of backups made with `--backup'.
     If this option is not specified, the value of the `VERSION_CONTROL'
     environment variable is used.  And if `VERSION_CONTROL' is not set,
     the default backup type is `existing'.

     This option corresponds to the Emacs variable `version-control';
     the same values for METHOD are accepted as in Emacs.  This options
     also more descriptive name.  The valid METHODs (unique
     abbreviations are accepted):

    `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.



   Some people express the desire to _always_ use the OP-BACKUP option,
by defining some kind of alias or script.  This is not as easy as one
may thing, due to the fact old style options should appear first and
consume arguments a bit inpredictably for an alias or script.  But, if
you are ready to give up using old style options, you may resort to
using something like (a Bourne shell function here):

     tar () { /usr/local/bin/tar --backup $*; }


File: tar.info,  Node: Applications,  Next: looking ahead,  Prev: backup,  Up: operations

Notable `tar' Usages
====================

     _(This message will disappear, once this node revised.)_

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 -)

The command also works using short option forms:

     $ cd sourcedir; tar --create --file=- . | (cd targetdir; tar --extract --file=-)

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


File: tar.info,  Node: looking ahead,  Prev: Applications,  Up: operations

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 how to

   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.
*Note files::.

   There are various ways of causing `tar' to skip over some files, and
not archive them.  *Note Choosing::.


File: tar.info,  Node: Backups,  Next: Choosing,  Prev: operations,  Up: Top

Performing Backups and Restoring Files
**************************************

     _(This message will disappear, once this node revised.)_

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

   Here is a possible plan for a future documentation about the
backuping scripts which are provided within the GNU `tar' distribution.

     .* dumps
     . + what are dumps
     
     . + different levels of dumps
     .  - full dump = dump everything
     .  - level 1, level 2 dumps etc, -
     	A level n dump dumps everything changed since the last level
     	n-1 dump (?)
     
     . + how to use scripts for dumps  (ie, the concept)
     .  - scripts to run after editing backup specs (details)
     
     . + Backup Specs, what is it.
     .  - how to customize
     .  - actual text of script  [/sp/dump/backup-specs]
     
     . + Problems
     .  - rsh doesn't work
     .  - rtape isn't installed
     .  - (others?)
     
     . + the --incremental option of tar
     
     . + tapes
     .  - write protection
     .  - types of media
     .   : different sizes and types, useful for different things
     .  - files and tape marks
          one tape mark between files, two at end.
     .  - positioning the tape
          MT writes two at end of write,
            backspaces over one when writing again.

   This chapter documents both the provided FSF 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 accidently deleted).  File system "backups" are also called
"dumps".

* Menu:

* Full Dumps::                  Using `tar' to Perform Full Dumps
* Inc Dumps::                   Using `tar' to Perform Incremental Dumps
* incremental and listed-incremental::  The Incremental Options
* Backup Levels::               Levels of Backups
* Backup Parameters::           Setting Parameters for Backups and Restoration
* Scripted Backups::            Using the Backup Scripts
* Scripted Restoration::        Using the Restore Script


File: tar.info,  Node: Full Dumps,  Next: Inc Dumps,  Prev: Backups,  Up: Backups

Using `tar' to Perform Full Dumps
=================================

     _(This message will disappear, once this node revised.)_

Full dumps should only be made when no other people or programs are
modifying files in the filesystem.  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 filesystem 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 filesystem separately you will need to use
the `--one-file-system' (`-l') option to prevent `tar' from crossing
filesystem boundaries when storing (sub)directories.

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

   Unless you are in a hurry, and trust the `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.

   `--listed-incremental=SNAPSHOT-FILE' (`-g SNAPSHOT-FILE') take a
file name argument always.  If the file doesn't exist, run a level zero
dump, creating the file.  If the file exists, uses that file to see
what has changed.

   `--incremental' (`-G')

   `--incremental' (`-G') handle old GNU-format incremental backup.

   This option should only be used when creating an incremental backup
of a filesystem.  When the `--incremental' (`-G') option is used, `tar'
writes, at the beginning of the archive, an entry for each of the
directories that will be operated on.  The entry for a directory
includes a list of all the files in the directory at the time the dump
was done, and a flag for each file indicating whether the file is going
to be put in the archive.  This information is used when doing a
complete incremental restore.

   Note that this option causes `tar' to create a non-standard archive
that may not be readable by non-GNU versions of the `tar' program.

   The `--incremental' (`-G') option means the archive is an incremental
backup.  Its meaning depends on the command that it modifies.

   If the `--incremental' (`-G') option is used with `--list' (`-t'),
`tar' will list, for each directory in the archive, the list of files in
that directory at the time the archive was created.  This information
is put out in a format that is not easy for humans to read, but which
is unambiguous for a program: each file name is preceded by either a
`Y' if the file is present in the archive, an `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).  Each file name is terminated by a null
character.  The last file is followed by an additional null and a
newline to indicate the end of the data.

   If the `--incremental' (`-G') option is used with `--extract'
(`--get', `-x'), then when the entry for a directory is found, all
files that currently exist in that directory but are not listed in the
archive _are deleted from the directory_.

   This behavior is convenient when you are restoring a damaged file
system from a succession of incremental backups: it restores the entire
state of the file system to that which obtained when the backup was
made.  If you don't use `--incremental' (`-G'), the file system will
probably fill up with files that shouldn't exist any more.

   `--listed-incremental=SNAPSHOT-FILE' (`-g SNAPSHOT-FILE') handle new
GNU-format incremental backup.  This option handles new GNU-format
incremental backup.  It has much the same effect as `--incremental'
(`-G'), but also the time when the dump is done and the list of
directories dumped is written to the given FILE. When restoring, only
files newer than the saved time are restored, and the direcotyr list is
used to speed up operations.

   `--listed-incremental=SNAPSHOT-FILE' (`-g SNAPSHOT-FILE') acts like
`--incremental' (`-G'), but when used in conjunction with `--create'
(`-c') will also cause `tar' to use the file FILE, which contains
information about the state of the filesystem at the time of the last
backup, to decide which files to include in the archive being created.
That file will then be updated by `tar'.  If the file FILE does not
exist when this option is specified, `tar' will create it, and include
all appropriate files in the archive.

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

   GNU `tar' actually writes the file twice: once before the data and
written, and once after.


File: tar.info,  Node: Inc Dumps,  Next: incremental and listed-incremental,  Prev: Full Dumps,  Up: Backups

Using `tar' to Perform Incremental Dumps
========================================

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Performing incremental dumps is similar to performing full dumps,
although a few more options will usually be needed.

   You will need to use the `-N DATE' option to tell `tar' to only
store files that have been modified since DATE.  DATE should be the
date and time of the last full/incremental dump.

   A standard scheme is to do a _monthly_ (full) dump once a month, a
_weekly_ dump once a week of everything since the last monthly and a
_daily_ every day of everything since the last (weekly or monthly) dump.

   Here is a copy of the script used to dump the filesystems of the
machines here at the Free Software Foundation.  This script is run via
`cron' late at night when people are least likely to be using the
machines.  This script dumps several filesystems from several machines
at once (via NFS).  The operator is responsible for ensuring that all
the machines will be up at the time the dump happens.  If a machine is
not running, its files will not be dumped, and the next day's
incremental dump will _not_ store files that would have gone onto that
dump.

     #!/bin/csh
     # Dump thingie
     set now = `date`
     set then = `cat date.nfs.dump`
     /u/hack/bin/tar -c -G -v\
      -f /dev/rtu20\
      -b 126\
      -N "$then"\
      -V "Dump from $then to $now"\
      /alpha-bits/gp\
      /gnu/hack\
      /hobbes/u\
      /spiff/u\
      /sugar-bombs/u
     echo $now > date.nfs.dump
     mt -f /dev/rtu20 rew

   Output from this script is stored in a file, for the operator to
read later.

   This script uses the file `date.nfs.dump' to store the date/time of
the last dump.

   Since this is a streaming tape drive, no attempt to verify the
archive is done.  This is also why the high blocking factor (126) is
used.  The tape drive must also be rewound by the `mt' command after
the dump is made.


File: tar.info,  Node: incremental and listed-incremental,  Next: Backup Levels,  Prev: Inc Dumps,  Up: Backups

The Incremental Options
=======================

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`--incremental' (`-G') is used in conjunction with `--create' (`-c'),
`--extract' (`--get', `-x') or `--list' (`-t') when backing up and
restoring file systems.  An archive cannot be extracted or listed with
the `--incremental' (`-G') option specified unless it was created with
the option specified.  This option should only be used by a script, not
by the user, and is usually disregarded in favor of
`--listed-incremental=SNAPSHOT-FILE' (`-g SNAPSHOT-FILE'), which is
described below.

   `--incremental' (`-G') in conjunction with `--create' (`-c') causes
`tar' to write, at the beginning of the archive, an entry for each of
the directories that will be archived.  The entry for a directory
includes a list of all the files in the directory at the time the
archive was created and a flag for each file indicating whether or not
the file is going to be put in the archive.

   Note that this option causes `tar' to create a non-standard archive
that may not be readable by non-GNU versions of the `tar' program.

   `--incremental' (`-G') in conjunction with `--extract' (`--get',
`-x') causes `tar' to read the lists of directory contents previously
stored in the archive, _delete_ files in the file system that did not
exist in their directories when the archive was created, and then
extract the files in the archive.

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

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

   `--listed-incremental=SNAPSHOT-FILE' (`-g SNAPSHOT-FILE') acts like
`--incremental' (`-G'), but when used in conjunction with `--create'
(`-c') will also cause `tar' to use the file SNAPSHOT-FILE, which
contains information about the state of the file system at the time of
the last backup, to decide which files to include in the archive being
created.  That file will then be updated by `tar'.  If the file FILE
does not exist when this option is specified, `tar' will create it, and
include all appropriate files in the archive.

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

   Despite it should be obvious that a device has a non-volatile value,
NFS devices have non-dependable values when an automounter gets in the
picture.  This led to a great deal of spurious redumping in incremental
dumps, so it is somewhat useless to compare two NFS devices numbers
over time.  So `tar' now considers all NFS devices as being equal when
it comes to comparing directories; this is fairly gross, but there does
not seem to be a better way to go.


File: tar.info,  Node: Backup Levels,  Next: Backup Parameters,  Prev: incremental and listed-incremental,  Up: Backups

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

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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 a incremental dump.  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
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.  .  .  Once the backup parameters are set,
you can perform backups or restoration by running the appropriate
script.

   The name of the restore script is `restore'.  .  The names of the
level one and full backup scripts are, respectively, `level-1' and
`level-0'.  The `level-0' script also exists under the name `weekly',
and the `level-1' under the name `daily'--these additional names can be
changed according to your backup schedule.  , for more information on
running the restoration script.  , for more information on running the
backup scripts.

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


File: tar.info,  Node: Backup Parameters,  Next: Scripted Backups,  Prev: Backup Levels,  Up: Backups

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

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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.

   , for an explanation of this syntax.

`ADMINISTRATOR'
     The user name of the backup administrator.

`BACKUP_HOUR'
     The hour at which the backups are done.  This can be a number from
     0 to 23, or the string `now'.

`TAPE_FILE'
     The device `tar' writes the archive to.  This device should be
     attached to the host on which the dump scripts are run.

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

`BLOCKING'
     The blocking factor `tar' will use when writing the dump archive.
     *Note Blocking Factor::.

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

     The host name specifies which host to run `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 (ie. 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.

`BACKUP_FILES'
     A list of individual files to be dumped.  These should be
     accessible from the machine on which the backup script is run.


* Menu:

* backup-specs example::        An Example Text of `Backup-specs'
* Script Syntax::               Syntax for `Backup-specs'


File: tar.info,  Node: backup-specs example,  Next: Script Syntax,  Prev: Backup Parameters,  Up: Backup Parameters

An Example Text of `Backup-specs'
---------------------------------

     _(This message will disappear, once this node revised.)_

The following is the text of `backup-specs' as it appears at FSF:

     # site-specific parameters for file system backup.
     
     ADMINISTRATOR=friedman
     BACKUP_HOUR=1
     TAPE_FILE=/dev/nrsmt0
     TAPE_STATUS="mts -t $TAPE_FILE"
     BLOCKING=124
     BACKUP_DIRS="
     	albert:/fs/fsf
     	apple-gunkies:/gd
     	albert:/fs/gd2
     	albert:/fs/gp
     	geech:/usr/jla
     	churchy:/usr/roland
     	albert:/
     	albert:/usr
     	apple-gunkies:/
     	apple-gunkies:/usr
     	gnu:/hack
     	gnu:/u
     	apple-gunkies:/com/mailer/gnu
     	apple-gunkies:/com/archive/gnu"
     
     BACKUP_FILES="/com/mailer/aliases /com/mailer/league*[a-z]"


File: tar.info,  Node: Script Syntax,  Prev: backup-specs example,  Up: Backup Parameters

Syntax for `Backup-specs'
-------------------------

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`backup-specs' is in shell script syntax.  The following conventions
should be considered when editing the script:

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

   A quoted string typically cannot contain wildcards.  In
`backup-specs', however, the parameters BACKUP_DIRS and BACKUP_FILES
can contain wildcards.


File: tar.info,  Node: Scripted Backups,  Next: Scripted Restoration,  Prev: Backup Parameters,  Up: Backups

Using the Backup Scripts
========================

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The syntax for running a backup script is:

     `script-name' [TIME-TO-BE-RUN]

   where TIME-TO-BE-RUN can be a specific system time, or can be `now'.
If you do not specify a time, the script runs at the time specified in
`backup-specs' ().

   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.  .  .

   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.  , 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-MMM-DDD-YYYY-level-1' or `log-MMM-DDD-YYYY-full'.

   The script also prints the name of each system being dumped to the
standard output.


File: tar.info,  Node: Scripted Restoration,  Prev: Scripted Backups,  Up: Backups

Using the Restore Script
========================

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     *Warning:* The GNU `tar' distribution does _not_ provide any such
     `restore' script yet.  This section is only listed here for
     documentation maintenance purposes.  In any case, all contents is
     subject to change as things develop.

   To restore files that were archived using a scripted backup, use the
`restore' script.  The syntax for the script is:

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

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

   If you specify `--all' as the FILES argument, the `restore' script
extracts all the files in the archived file system into the active file
system.

     *Warning:* The script will delete files from the active file
     system if they were not in the file system when the archive was
     made.

   *Note Inc Dumps::, and *Note Inc Dumps::, for an explanation of how
the script makes that determination.


File: tar.info,  Node: Choosing,  Next: Date input formats,  Prev: Backups,  Up: Top

Choosing Files and Names for `tar'
**********************************

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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.

* Menu:

* file::                        Choosing the Archive's Name
* Selecting Archive Members::
* files::                       Reading Names from a File
* exclude::                     Excluding Some Files
* Wildcards::
* after::                       Operating Only on New Files
* recurse::                     Descending into Directories
* one::                         Crossing Filesystem Boundaries


File: tar.info,  Node: file,  Next: Selecting Archive Members,  Prev: Choosing,  Up: Choosing

Choosing and Naming Archive Files
=================================

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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 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 (ie. `/dev/tu00').  `tar' always needs
an archive name.

   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.

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

   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 `/usr/ucb/rmt' program installed.  If you need to use a file whose
name includes a colon, then the remote tape drive behavior can be
inhibited by using the `--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.


File: tar.info,  Node: Selecting Archive Members,  Next: files,  Prev: file,  Up: Choosing

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.
*Note 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 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 when `tar' is invoked with `--create'
(`-c'), `tar' operates on all the non-directory files in the working
directory.  If you specify either `--list' (`-t') or `--extract'
(`--get', `-x'), `tar' operates on all the archive members in the
archive.  If you specify any operation other than one of these three,
`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.


File: tar.info,  Node: files,  Next: exclude,  Prev: Selecting Archive Members,  Up: Choosing

Reading Names from a File
=========================

     _(This message will disappear, once this node revised.)_

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.

   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'; *note gzip:: for more information.)

     $ find . -size -400 -print > small-files
     $ tar -c -v -z -T small-files -f little.tgz

* Menu:

* nul::


File: tar.info,  Node: nul,  Prev: files,  Up: files

`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 causes `--directory=DIRECTORY' (`-C DIRECTORY') options
to be treated as file names to archive, in case there are any files out
there called `-C'.

   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' just 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


File: tar.info,  Node: exclude,  Next: Wildcards,  Prev: files,  Up: Choosing

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=PATTERN' or `--exclude-from=FILE-OF-PATTERNS' (`-X
FILE-OF-PATTERNS') options.

`--exclude=PATTERN'
     Causes `tar' to ignore files that match the PATTERN.

   The `--exclude=PATTERN' option will prevent any file or member which
matches the shell wildcards (PATTERN) from being operated on (PATTERN
can be a single file name or a more complex expression).  For example,
if you want to create an archive with all the contents of `/tmp' except
the file `/tmp/foo', you can use the command `tar --create
--file=arch.tar --exclude=foo'.  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=FILE-OF-PATTERNS' option to read a list of
shell wildcards, one per line, from FILE; `tar' will ignore files
matching those regular expressions.  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.

* Menu:

* problems with exclude::


File: tar.info,  Node: problems with exclude,  Prev: exclude,  Up: exclude

Problems with Using the `exclude' Options
-----------------------------------------

Some users find `exclude' options confusing.  Here are some common
pitfalls:

   * The main operating mode of `tar' will always act on file names
     listed on the command line, no matter whether or not there is an
     exclusion which would otherwise affect them.  In the example
     above, if you create an archive and exclude files that end with
     `*.o', but explicitly name the file `catc.o' after all the options
     have been listed, `catc.o' _will_ be included in the archive.

   * You can sometimes confuse the meanings of `--exclude=PATTERN' and
     `--exclude-from=FILE-OF-PATTERNS' (`-X FILE-OF-PATTERNS').  Be
     careful: use `--exclude=PATTERN' when files to be excluded are
     given as a pattern on the command line.  Use
     `--exclude-from=FILE-OF-PATTERNS' to introduce the name of a file
     which contains a list of patterns, one per line; each of these
     patterns can exclude zero, one, or many files.

   * 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 -X '*/tmp/*' DIRECTORY

     rather than:

          $ tar -c -f ARCHIVE.TAR -X */tmp/* 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=FILE-OF-PATTERNS' option was called
     `--exclude-PATTERN' instead.  Now, `--exclude=PATTERN' applies to
     patterns listed on the command line and
     `--exclude-from=FILE-OF-PATTERNS' applies to patterns listed in a
     file.



File: tar.info,  Node: Wildcards,  Next: after,  Prev: exclude,  Up: Choosing

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.  However, `tar' often uses wildcard patterns for
matching (or globbing) archive members instead of actual files in the
filesystem.  Wildcard patterns are also used for verifying volume
labels of `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: excluding a directory also excludes all the files beneath it.

   There are some discussions floating in the air and asking for
modifications in the way GNU `tar' accomplishes wildcard matches.  We
perceive any change of semantics in this area as a delicate thing to
impose on GNU `tar' users.  On the other hand, the GNU project should be
progressive enough to correct any ill design: compatibility at all price
is not always a good attitude.  In conclusion, it is _possible_ that
slight amendments be later brought to the previous description.  Your
opinions on the matter are welcome.


File: tar.info,  Node: after,  Next: recurse,  Prev: Wildcards,  Up: Choosing

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 modification or inode-changed times are
newer than the DATE given.  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 actual contents of the file (rather than inode
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 modification or inode-changed times are
     later than DATE.  Use in conjunction with any operation.

`--newer-mtime=DATE'
     Acts like `--after-date=DATE' (`--newer=DATE', `-N DATE'), but
     only looks at modification times.

   These options limit `tar' to only operating on files which have been
modified after the date specified.  A file is considered to have
changed if the contents have been modified, or if the owner,
permissions, and so forth, have been changed.  (For more information on
how to specify a date, see *Note Date input formats::; remember that the
entire date argument must be quoted if it contains any spaces.)

   Gurus would say that `--after-date=DATE' (`--newer=DATE', `-N DATE')
tests both the `mtime' (time the contents of the file were last
modified) and `ctime' (time the file's status was last changed: owner,
permissions, etc) fields, while `--newer-mtime=DATE' tests only `mtime'
field.

   To be precise, `--after-date=DATE' (`--newer=DATE', `-N DATE')
checks _both_ `mtime' and `ctime' and processes the file if either one
is more recent than DATE, while `--newer-mtime=DATE' only checks
`mtime' and disregards `ctime'.  Neither uses `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.

     *Please Note:* `--after-date=DATE' (`--newer=DATE', `-N DATE') and
     `--newer-mtime=DATE' should not be used for incremental backups.
     Some files (such as those in renamed directories) are not selected
     properly by these options.  *Note incremental and
     listed-incremental::.

   To select files newer than the modification time of a file that
already exists, you can use the `--reference' (`-r') option of GNU
`date', available in GNU shell utilities 1.13 or later.  It returns the
timestamp of that already existing file; this timestamp expands to
become the referent date which `--newer' uses to determine which files
to archive.  For example, you could say,

     $ tar -cf ARCHIVE.TAR --newer="`date -r FILE`" /home

which tells .


File: tar.info,  Node: recurse,  Next: one,  Prev: after,  Up: Choosing

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=FILE-OF-NAMES'
(`-T FILE-OF-NAMES') 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 *Note files:: for more information on using `find' with
`tar', or look.

`--no-recursion'
     Prevents `tar' from recursively descending directories.

   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
`! -d' option to `find'  as they usually do not want all the files in a
directory.  They then use the {No value for `op-file-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.


File: tar.info,  Node: one,  Prev: recurse,  Up: Choosing

Crossing Filesystem 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'
(`-l').  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=FILE-OF-NAMES' (`-T FILE-OF-NAMES'), regardless of where
they reside.

`--one-file-system'
`-l'
     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 filesystem 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.

   It is reported that using this option, the mount point is is
archived, but nothing under it.

   This option is useful for making full or incremental archival
backups of a file system.  If this option is used in conjunction with
`--verbose' (`-v'), files that are excluded are mentioned by name on the
standard error.

* Menu:

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


File: tar.info,  Node: directory,  Next: absolute,  Prev: one,  Up: one

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=FILE-OF-NAMES' (`-T FILE-OF-NAMES'), use
`--directory=DIRECTORY' (`-C DIRECTORY').  This will change the working
directory to the directory 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' (*note files::), you can put `-C' options
in the file list.  Unfortunately, you cannot put `--directory' options
in the file list.  (This interpretation can be disabled by using the
`--null' option.)


File: tar.info,  Node: absolute,  Prev: directory,  Up: one

Absolute File Names
-------------------

     _(This message will disappear, once this node revised.)_

`-P'
`--absolute-names'
     Do not strip leading slashes from file names.

   By default, GNU `tar' drops a leading `/' on input or output.  This
option turns off this behavior; it is equivalent to changing to the
root directory before running `tar' (except it also turns off the usual
warning message).

   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'.

   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'.

   If you use the `--absolute-names' (`-P') option, `tar' will do
neither 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' (`-P') option, but using this option may be more
convenient than switching to root.

`--absolute-names'
     Preserves full file names (inclusing superior dirctory 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)
     $ tar -c -f archive.tar -C  / home


File: tar.info,  Node: Date input formats,  Next: Formats,  Prev: Choosing,  Up: Top

Date input formats
******************

     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 `getdate'
function) is not described here.

   Although the date syntax here can represent any possible time since
zero A.D., computer integers are not big enough for such a
(comparatively) long time.  The earliest date semantically allowed on
Unix systems is midnight, 1 January 1970 UCT.

* Menu:

* General date syntax::         Common rules.
* Calendar date item::          19 Dec 1994.
* Time of day item::            9:20pm.
* Timezone item::               EST, DST, BST, UCT, AHST, ...
* Day of week item::            Monday and others.
* Relative item in date strings::  next tuesday, 2 years ago.
* Pure numbers in date strings::  19931219, 1440.
* Authors of getdate::          Bellovin, Salz, Berets, et al.


File: tar.info,  Node: General date syntax,  Next: Calendar date item,  Prev: Date input formats,  Up: Date input formats

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 the 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 numbers may be written out in words in most contexts.  This is
most useful for specifying day of the week items or relative items (see
below).  Here is the list: `first' for 1, `next' for 2, `third' 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.  Also, `last' means exactly -1.

   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.

   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.


File: tar.info,  Node: Calendar date item,  Next: Time of day item,  Prev: General date syntax,  Up: Date input formats

Calendar date item
==================

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:

     1970-09-17           # ISO 8601.
     70-9-17              # This century assumed by default.
     70-09-17             # Leading zeros are ignored.
     9/17/72              # Common U.S. writing.
     24 September 1972
     24 Sept 72           # September has a special abbreviation.
     24 Sep 72            # Three-letter abbreviations always allowed.
     Sep 24, 1972
     24-sep-72
     24sep72

   The year can also be omitted.  In this case, the last specified year
is used, or the current year if none.  For example:

     9/17
     sep 17

   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 less than 100, then 1900 is
added to it to force a date in this century.  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


File: tar.info,  Node: Time of day item,  Next: Timezone item,  Prev: Calendar date item,  Up: Date input formats

Time of day item
================

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:

     20:02:0
     20:02
     8:02pm
     20:02-0500      # In EST (Eastern U.S. Standard Time).

   More generally, the time of the 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.
Alternatively, `:SECOND' can be omitted, in which case it is taken to
be zero.

   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 timezone correction,
expressed as `SHHMM', where S is `+' or `-', HH is a number of zone
hours and MM is a number of zone minutes.  When a timezone correction
is given this way, it forces interpretation of the time in UTC,
overriding any previous specification for the timezone or the local
timezone.  The MINUTE part of the time of the day may not be elided
when a timezone correction is used.  This is the only way to specify a
timezone correction by fractional parts of an hour.

   Either `am'/`pm' or a timezone correction may be specified, but not
both.


File: tar.info,  Node: Timezone item,  Next: Day of week item,  Prev: Time of day item,  Up: Date input formats

Timezone item
=============

A "timezone item" specifies an international timezone, indicated by a
small set of letters.  Any included period is ignored.  Military
timezone designations use a single letter.  Currently, only integral
zone hours may be represented in a timezone item.  See the previous
section for a finer control over the timezone correction.

   Here are many non-daylight-savings-time timezones, indexed by the
zone hour value.

+000
     `GMT' for Greenwich Mean, `UT' or `UTC' for Universal
     (Coordinated), `WET' for Western European and `Z' for militaries.

+100
     `WAT' for West Africa and `A' for militaries.

+200
     `AT' for Azores and `B' for militaries.

+300
     `C' for militaries.

+400
     `AST' for Atlantic Standard and `D' for militaries.

+500
     `E' for militaries and `EST' for Eastern Standard.

+600
     `CST' for Central Standard and `F' for militaries.

+700
     `G' for militaries and `MST' for Mountain Standard.

+800
     `H' for militaries and `PST' for Pacific Standard.

+900
     `I' for militaries and `YST' for Yukon Standard.

+1000
     `AHST' for Alaska-Hawaii Standard, `CAT' for Central Alaska, `HST'
     for Hawaii Standard and `K' for militaries.

+1100
     `L' for militaries and `NT' for Nome.

+1200
     `IDLW' for International Date Line West and `M' for militaries.

-100
     `CET' for Central European, `FWT' for French Winter, `MET' for
     Middle European, `MEWT' for Middle European Winter, `N' for
     militaries and `SWT' for Swedish Winter.

-200
     `EET' for Eastern European, USSR Zone 1 and `O' for militaries.

-300
     `BT' for Baghdad, USSR Zone 2 and `P' for militaries.

-400
     `Q' for militaries and `ZP4' for USSR Zone 3.

-500
     `R' for militaries and `ZP5' for USSR Zone 4.

-600
     `S' for militaries and `ZP6' for USSR Zone 5.

-700
     `T' for militaries and `WAST' for West Australian Standard.

-800
     `CCT' for China Coast, USSR Zone 7 and `U' for militaries.

-900
     `JST' for Japan Standard, USSR Zone 8 and `V' for militaries.

-1000
     `EAST' for East Australian Standard, `GST' for Guam Standard, USSR
     Zone 9 and `W' for militaries.

-1100
     `X' for militaries.

-1200
     `IDLE' for International Date Line East, `NZST' for New Zealand
     Standard, `NZT' for New Zealand and `Y' for militaries.

   Here are many DST timezones, indexed by the zone hour value.  Also,
by following a non-DST timezone by the string `DST' in a separate word
(that is, separated by some whitespace), the corresponding DST timezone
may be specified.

0
     `BST' for British Summer.

+400
     `ADT' for Atlantic Daylight.

+500
     `EDT' for Eastern Daylight.

+600
     `CDT' for Central Daylight.

+700
     `MDT' for Mountain Daylight.

+800
     `PDT' for Pacific Daylight.

+900
     `YDT' for Yukon Daylight.

+1000
     `HDT' for Hawaii Daylight.

-100
     `MEST' for Middle European Summer, `MESZ' for Middle European
     Summer, `SST' for Swedish Summer and `FST' for French Summer.

-700
     `WADT' for West Australian Daylight.

-1000
     `EADT' for Eastern Australian Daylight.

-1200
     `NZDT' for New Zealand Daylight.


File: tar.info,  Node: Day of week item,  Next: Relative item in date strings,  Prev: Timezone item,  Up: Date input formats

Day of week item
================

The explicit mention of a day of the week will forward the date (only
if necessary) to reach that day of the week in the future.

   Days of the week may be spelled out in full: `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.


File: tar.info,  Node: Relative item in date strings,  Next: Pure numbers in date strings,  Prev: Day of week item,  Up: Date input formats

Relative item 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 multiplicator 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 change 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 makes the resulting date to cross the boundary
between DST and non-DST (or vice-versa), the hour is adjusted according
to the local time.


File: tar.info,  Node: Pure numbers in date strings,  Next: Authors of getdate,  Prev: Relative item in date strings,  Up: Date input formats

Pure numbers in date strings
============================

The precise intepretation of a pure decimal number is dependent of the
context in the date string.

   If the decimal number is of the form YYYYMMDD and no other calendar
date item (*note Calendar date item::) 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
the 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.


File: tar.info,  Node: Authors of getdate,  Prev: Pure numbers in date strings,  Up: Date input formats

Authors of `getdate'
====================

`getdate' was originally implemented by Steven M. Bellovin
(`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 (`rsalz@bbn.com') and
Jim Berets (`jberets@bbn.com') in August, 1990.  Various revisions for
the GNU system were made by David MacKenzie, Jim Meyering, and others.

   This chapter was originally produced by Franc,ois Pinard
(`pinard@iro.umontreal.ca') from the `getdate.y' source code, and then
edited by K. Berry (`kb@cs.umb.edu').


File: tar.info,  Node: Formats,  Next: Media,  Prev: Date input formats,  Up: Top

Controlling the Archive Format
******************************

* Menu:

* Portability::                 Making `tar' Archives More Portable
* Compression::                 Using Less Space through Compression
* Attributes::                  Handling File Attributes
* Standard::                    The Standard Format
* Extensions::                  GNU Extensions to the Archive Format
* cpio::                        Comparison of `tar' and `cpio'


File: tar.info,  Node: Portability,  Next: Compression,  Prev: Formats,  Up: Formats

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 comptible 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.

* Menu:

* Portable Names::              Portable Names
* dereference::                 Symbolic Links
* old::                         Old V7 Archives
* posix::                       POSIX archives
* Checksumming::                Checksumming Problems


File: tar.info,  Node: Portable Names,  Next: dereference,  Prev: Portability,  Up: Portability

Portable Names
--------------

Use _straight_ file and directory names, made up of printable ASCII
characters, avoiding colons, slashes, backslashes, spaces, and other
_dangerous_ characters.  Avoid deep directory nesting.  Accounting for
oldish System V machines, limit your file and directory names to 14
characters or less.

   If you intend to have your `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.


File: tar.info,  Node: dereference,  Next: old,  Prev: Portable Names,  Up: Portability

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 filesystem 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.


File: tar.info,  Node: old,  Next: posix,  Prev: dereference,  Up: Portability

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
`--old-archive' (`-o') option in conjunction with the `--create'
(`-c').  `tar' also accepts `--portability' 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 `--old-archive' (`-o') unless
the archive was created with 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 `tar's are able to read old
format archives, so it might be safer for you to always use
`--old-archive' (`-o') for your distributions.


File: tar.info,  Node: posix,  Next: Checksumming,  Prev: old,  Up: Portability

GNU `tar' and POSIX `tar'
-------------------------

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' is incompatible with the current
POSIX spec, and with `tar' programs that follow it.

   We plan to reimplement these GNU extensions in a new way which is
upward compatible with the latest POSIX `tar' format, but we don't know
when this will be done.

   In the mean time, there is simply no telling what might happen if you
read a GNU `tar' archive, which uses the GNU extensions, using some
other `tar' program.  So if you want to read the archive with another
`tar' program, be sure to write it using the `--old-archive' option
(`-o').

   Traditionally, old `tar's have a limit of 100 characters.  GNU `tar'
attempted two different approaches to overcome this limit, using and
extending a format specified by a draft of some P1003.1.  The first way
was not that successful, and involved `@MaNgLeD@' file names, or such;
while a second approach used `././@LongLink' and other tricks, yielding
better success.  In theory, GNU `tar' should be able to handle file
names of practically unlimited length.  So, if GNU `tar' fails to dump
and retrieve files having more than 100 characters, then there is a bug
in GNU `tar', indeed.

   But, being strictly POSIX, the limit was still 100 characters.  For
various other purposes, GNU `tar' used areas left unassigned in the
POSIX draft.  POSIX later revised P1003.1 `ustar' format by assigning
previously unused header fields, in such a way that the upper limit for
file name length was raised to 256 characters.  However, the actual
POSIX limit oscillates between 100 and 256, depending on the precise
location of slashes in full file name (this is rather ugly).  Since GNU
`tar' use the same fields for quite other purposes, it became
incompatible with the latest POSIX standards.

   For longer or non-fitting file names, we plan to use yet another set
of GNU extensions, but this time, complying with the provisions POSIX
offers for extending the format, rather than conflicting with it.
Whenever an archive uses old GNU `tar' extension format or POSIX
extensions, would it be for very long file names or other specialities,
this archive becomes non-portable to other `tar' implementations.  In
fact, anything can happen.  The most forgiving `tar's will merely
unpack the file using a wrong name, and maybe create another file named
something like `@LongName', with the true file name in it.  `tar's not
protecting themselves may segment violate!

   Compatibility concerns make all this thing more difficult, as we
will have to support _all_ these things together, for a while.  GNU
`tar' should be able to produce and read true POSIX format files, while
being able to detect old GNU `tar' formats, besides old V7 format, and
process them conveniently.  It would take years before this whole area
stabilizes...

   There are plans to raise this 100 limit to 256, and yet produce POSIX
conformant archives.  Past 256, I do not know yet if GNU `tar' will go
non-POSIX again, or merely refuse to archive the file.

   There are plans so GNU `tar' support more fully the latest POSIX
format, while being able to read old V7 format, GNU (semi-POSIX plus
extension), as well as full POSIX.  One may ask if there is part of the
POSIX format that we still cannot support.  This simple question has a
complex answer.  Maybe that, on intimate look, some strong limitations
will pop up, but until now, nothing sounds too difficult (but see
below).  I only have these few pages of POSIX telling about `Extended
tar Format' (P1003.1-1990 - section 10.1.1), and there are references
to other parts of the standard I do not have, which should normally
enforce limitations on stored file names (I suspect things like fixing
what `/' and `<NUL>' means).  There are also some points which the
standard does not make clear, Existing practice will then drive what I
should do.

   POSIX mandates that, when a file name cannot fit within 100 to 256
characters (the variance comes from the fact a `/' is ideally needed as
the 156'th character), or a link name cannot fit within 100 characters,
a warning should be issued and the file _not_ be stored.  Unless some
`--posix' option is given (or `POSIXLY_CORRECT' is set), I suspect that
GNU `tar' should disobey this specification, and automatically switch
to using GNU extensions to overcome file name or link name length
limitations.

   There is a problem, however, which I did not intimately studied yet.
Given a truly POSIX archive with names having more than 100 characters,
I guess that GNU `tar' up to 1.11.8 will process it as if it were an
old V7 archive, and be fooled by some fields which are coded
differently.  So, the question is to decide if the next generation of
GNU `tar' should produce POSIX format by default, whenever possible,
producing archives older versions of GNU `tar' might not be able to read
correctly.  I fear that we will have to suffer such a choice one of
these days, if we want GNU `tar' to go closer to POSIX.  We can rush it.
Another possibility is to produce the current GNU `tar' format by
default for a few years, but have GNU `tar' versions from some 1.POSIX
and up able to recognize all three formats, and let older GNU `tar'
fade out slowly.  Then, we could switch to producing POSIX format by
default, with not much harm to those still having (very old at that
time) GNU `tar' versions prior to 1.POSIX.

   POSIX format cannot represent very long names, volume headers,
splitting of files in multi-volumes, sparse files, and incremental
dumps; these would be all disallowed if `--posix' or `POSIXLY_CORRECT'.
Otherwise, if `tar' is given long names, or `-[VMSgG]', then it should
automatically go non-POSIX.  I think this is easily granted without
much discussion.

   Another point is that only `mtime' is stored in POSIX archives,
while GNU `tar' currently also store `atime' and `ctime'.  If we want
GNU `tar' to go closer to POSIX, my choice would be to drop `atime' and
`ctime' support on average.  On the other hand, I perceive that full
dumps or incremental dumps need `atime' and `ctime' support, so for
those special applications, POSIX has to be avoided altogether.

   A few users requested that `--sparse' (`-S') be always active by
default, I think that before replying to them, we have to decide if we
want GNU `tar' to go closer to POSIX on average, while producing files.
My choice would be to go closer to POSIX in the long run.  Besides
possible double reading, I do not see any point of not trying to save
files as sparse when creating archives which are neither POSIX nor
old-V7, so the actual `--sparse' (`-S') would become selected by
default when producing such archives, whatever the reason is.  So,
`--sparse' (`-S') alone might be redefined to force GNU-format
archives, and recover its previous meaning from this fact.

   GNU-format as it exists now can easily fool other POSIX `tar', as it
uses fields which POSIX considers to be part of the file name prefix.
I wonder if it would not be a good idea, in the long run, to try
changing GNU-format so any added field (like `ctime', `atime', file
offset in subsequent volumes, or sparse file descriptions) be wholly
and always pushed into an extension block, instead of using space in
the POSIX header block.  I could manage to do that portably between
future GNU `tar's.  So other POSIX `tar's might be at least able to
provide kind of correct listings for the archives produced by GNU
`tar', if not able to process them otherwise.

   Using these projected extensions might induce older `tar's to fail.
We would use the same approach as for POSIX.  I'll put out a `tar'
capable of reading POSIXier, yet extended archives, but will not produce
this format by default, in GNU mode.  In a few years, when newer GNU
`tar's will have flooded out `tar' 1.11.X and previous, we could switch
to producing POSIXier extended archives, with no real harm to users, as
almost all existing GNU `tar's will be ready to read POSIXier format.
In fact, I'll do both changes at the same time, in a few years, and
just prepare `tar' for both changes, without effecting them, from
1.POSIX.  (Both changes: 1--using POSIX convention for getting over 100
characters; 2--avoiding mangling POSIX headers for GNU extensions,
using only POSIX mandated extension techniques).

   So, a future `tar' will have a `--posix' flag forcing the usage of
truly POSIX headers, and so, producing archives previous GNU `tar' will
not be able to read.  So, _once_ pretest will announce that feature, it
would be particularly useful that users test how exchangeable will be
archives between GNU `tar' with `--posix' and other POSIX `tar'.

   In a few years, when GNU `tar' will produce POSIX headers by
default, `--posix' will have a strong meaning and will disallow GNU
extensions.  But in the meantime, for a long while, `--posix' in GNU
tar will not disallow GNU extensions like `--label=ARCHIVE-LABEL' (`-V
ARCHIVE-LABEL'), `--multi-volume' (`-M'), `--sparse' (`-S'), or very
long file or link names.  However, `--posix' with GNU extensions will
use POSIX headers with reserved-for-users extensions to headers, and I
will be curious to know how well or bad POSIX `tar's will react to
these.

   GNU `tar' prior to 1.POSIX, and after 1.POSIX without `--posix',
generates and checks `ustar  ', with two suffixed spaces.  This is
sufficient for older GNU `tar' not to recognize POSIX archives, and
consequently, wrongly decide those archives are in old V7 format.  It
is a useful bug for me, because GNU `tar' has other POSIX
incompatibilities, and I need to segregate GNU `tar' semi-POSIX
archives from truly POSIX archives, for GNU `tar' should be somewhat
compatible with itself, while migrating closer to latest POSIX
standards.  So, I'll be very careful about how and when I will do the
correction.


File: tar.info,  Node: Checksumming,  Prev: posix,  Up: Portability

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.


File: tar.info,  Node: Compression,  Next: Attributes,  Prev: Portability,  Up: Formats

Using Less Space through Compression
====================================

* Menu:

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


File: tar.info,  Node: gzip,  Next: sparse,  Prev: Compression,  Up: Compression

Creating and Reading Compressed Archives
----------------------------------------

     _(This message will disappear, once this node revised.)_

`-z'
`--gzip'
`--ungzip'
     Filter the archive through `gzip'.

   Some format parameters must be taken into consideration when
modifying an archive: .  Compressed archives cannot be modified.

   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,
avoid the `--gzip' (`--gunzip', `--ungzip', `-z') option and run `gzip'
explicitly.  (Or set the `GZIP' environment variable.)

   The `--gzip' (`--gunzip', `--ungzip', `-z') option does not work
with the `--multi-volume' (`-M') option, or with the `--update' (`-u'),
`--append' (`-r'), `--concatenate' (`--catenate', `-A'), or `--delete'
operations.

   It is not exact to say that GNU `tar' is to work in concert with
`gzip' in a way similar to `zip', say.  Surely, it is possible that
`tar' and `gzip' be done with a single call, like in:

     $ tar cfz archive.tar.gz subdir

to save all of `subdir' into a `gzip''ed archive.  Later you can do:

     $ tar xfz archive.tar.gz

to explode and unpack.

   The difference is that the whole archive is compressed.  With `zip',
archive members are archived individually.  `tar''s method yields
better compression.  On the other hand, one can view the contents of a
`zip' archive without having to decompress it.  As for the `tar' and
`gzip' tandem, you need to decompress the archive to see its contents.
However, this may be done without needing disk space, by using pipes
internally:

     $ tar tfz 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 unsychronized, and there
is little chance that you could recover later in the archive.

   There are pending suggestions for having a per-volume or per-file
compression in GNU `tar'.  This would allow for viewing the contents
without decompression, and for resynchronizing decompression at every
volume or file, in case of corrupted archives.  Doing so, we might
loose some compressibility.  But this would have make recovering easier.
So, there are pros and cons.  We'll see!

`-Z'
`--compress'
`--uncompress'
     Filter the archive through `compress'.  Otherwise like `--gzip'
     (`--gunzip', `--ungzip', `-z').

`--use-compress-program=PROG'
     Filter through PROG (must accept `-d').

   `--compress' (`--uncompress', `-Z') stores an archive in compressed
format.  This option is useful in saving time over networks and space
in pipes, and when storage space is at a premium.  `--compress'
(`--uncompress', `-Z') causes `tar' to compress when writing the
archive, or to uncompress when reading the archive.

   To perform compression and uncompression on the archive, `tar' runs
the `compress' utility.  `tar' uses the default compression parameters;
if you need to override them, avoid the `--compress' (`--uncompress',
`-Z') option and run the `compress' utility explicitly.  It is useful
to be able to call the `compress' utility from within `tar' because the
`compress' utility by itself cannot access remote tape drives.

   The `--compress' (`--uncompress', `-Z') option will not work in
conjunction with the `--multi-volume' (`-M') option or the `--append'
(`-r'), `--update' (`-u'), `--append' (`-r') and `--delete' operations.
*Note Operations::, for more information on these operations.

   If there is no compress utility available, `tar' will report an
error.  *Please note* that the `compress' program may be covered by a
patent, and therefore we recommend you stop using it.

`--compress'
`--uncompress'
`-z'
`-Z'
     When this option is specified, `tar' will compress (when writing
     an archive), or uncompress (when reading an archive).  Used in
     conjunction with the `--create' (`-c'), `--extract' (`--get',
     `-x'), `--list' (`-t') and `--compare' (`--diff', `-d') operations.

   You can have archives be compressed by using the `--gzip'
(`--gunzip', `--ungzip', `-z') option.  This will arrange for `tar' to
use the `gzip' program to be used to compress or uncompress the archive
wren writing or reading it.

   To use the older, obsolete, `compress' program, use the `--compress'
(`--uncompress', `-Z') option.  The GNU Project recommends you not use
`compress', because there is a patent covering the algorithm it uses.
You could be sued for patent infringment merely by running `compress'.

   I have one question, or maybe it's a suggestion if there isn't a way
to do it now. I would like to use `--gzip' (`--gunzip', `--ungzip',
`-z'), but I'd also like the output to be fed through a program like
GNU `ecc' (actually, right now that's `exactly' what I'd like to use
:-)), basically adding ECC protection on top of compression.  It seems
as if this should be quite easy to do, but I can't work out exactly how
to go about it.  Of course, I can pipe the standard output of `tar'
through `ecc', but then I lose (though I haven't started using it yet,
I confess) the ability to have `tar' use `rmt' for it's I/O (I think).

   I think the most straightforward thing would be to let me specify a
general set of filters outboard of compression (preferably ordered, so
the order can be automatically reversed on input operations, and with
the options they require specifiable), but beggars shouldn't be
choosers and anything you decide on would be fine with me.

   By the way, I like `ecc' but if (as the comments say) it can't deal
with loss of block sync, I'm tempted to throw some time at adding that
capability.  Supposing I were to actually do such a thing and get it
(apparantly) working, do you accept contributed changes to utilities
like that?  (Leigh Clayton `loc@soliton.com', May 1995).

   Isn't that exactly the role of the `--use-compress-prog=PROGRAM'
option?  I never tried it myself, but I suspect you may want to write a
PROG script or program able to filter stdin to stdout to way you want.
It should recognize the `-d' option, for when extraction is needed
rather than creation.

   It has been reported that if one writes compressed data (through the
`--gzip' (`--gunzip', `--ungzip', `-z') or `--compress'
(`--uncompress', `-Z') options) to a DLT and tries to use the DLT
compression mode, the data will actually get bigger and one will end up
with less space on the tape.


File: tar.info,  Node: sparse,  Prev: gzip,  Up: Compression

Archiving Sparse Files
----------------------

     _(This message will disappear, once this node revised.)_

`-S'
`--sparse'
     Handle sparse files efficiently.

   This option causes all files to be put in the archive to be tested
for sparseness, and handled specially if they are.  The `--sparse'
(`-S') option is useful when many `dbm' files, for example, are being
backed up.  Using this option dramatically decreases the amount of
space needed to store such a file.

   In later versions, this option may be removed, and the testing and
treatment of sparse files may be done automatically with any special
GNU options.  For now, it is an option needing to be specified on the
command line with the creation or updating of an archive.

   Files in the filesystem occasionally have "holes."  A hole in a file
is a section of the file's contents which was never written.  The
contents of a hole read as all zeros.  On many operating systems,
actual disk storage is not allocated for holes, but they are counted in
the length of the file.  If you archive such a file, `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 the
`--sparse' (`-S') 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'
(`-S') is not needed on extraction) any such files have hols created
wherever the continuous stretches of zeros were found.  Thus, if you
use `--sparse' (`-S'), `tar' archives won't take more space than the
original.

   A file is sparse if it contains blocks of zeros whose existence is
recorded, but that have no space allocated on disk.  When you specify
the `--sparse' (`-S') option in conjunction with the `--create' (`-c')
operation, `tar' tests all files for sparseness while archiving.  If
`tar' finds a file to be sparse, it uses a sparse representation of the
file in the archive.  *Note create::, for more information about
creating archives.

   `--sparse' (`-S') is useful when archiving files, such as dbm files,
likely to contain many nulls.  This option dramatically decreases the
amount of space needed to store such an archive.

     *Please Note:* Always use `--sparse' (`-S') 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' (`-S') 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).

   `tar' ignores the `--sparse' (`-S') option when reading an archive.

`--sparse'
`-S'
     Files stored sparsely in the file system are represented sparsely
     in the archive.  Use in conjunction with write operations.

   However, users should be well aware that at archive creation time,
GNU `tar' still has to read whole disk file to locate the "holes", and
so, even if sparse files use little space on disk and in the archive,
they may sometimes require inordinate amount of time for reading and
examining all-zero blocks of a file.  Although it works, it's painfully
slow for a large (sparse) file, even though the resulting tar archive
may be small.  (One user reports that dumping a `core' file of over 400
megabytes, but with only about 3 megabytes of actual data, took about 9
minutes on a Sun Sparstation ELC, with full CPU utilisation.)

   This reading is required in all cases and is not related to the fact
the `--sparse' (`-S') option is used or not, so by merely _not_ using
the option, you are not saving time(1).

   Programs like `dump' do not have to read the entire file; by
examining the file system directly, they can determine in advance
exactly where the holes are and thus avoid reading through them.  The
only data it need read are the actual allocated data blocks.  GNU `tar'
uses a more portable and straightforward archiving approach, it would
be fairly difficult that it does otherwise.  Elizabeth Zwicky writes to
`comp.unix.internals', on 1990-12-10:

     What I did say is that you cannot tell the difference between a
     hole and an equivalent number of nulls without reading raw blocks.
     `st_blocks' at best tells you how many holes there are; it
     doesn't tell you _where_.  Just as programs may, conceivably, care
     what `st_blocks' is (care to name one that does?), they may also
     care where the holes are (I have no examples of this one either,
     but it's equally imaginable).

     I conclude from this that good archivers are not portable.  One can
     arguably conclude that if you want a portable program, you can in
     good conscience restore files with as many holes as possible,
     since you can't get it right.

   ---------- Footnotes ----------

   (1) Well!  We should say the whole truth, here.  When `--sparse'
(`-S') is selected while creating an archive, the current `tar'
algorithm requires sparse files to be read twice, not once.  We hope to
develop a new archive format for saving sparse files in which one pass
will be sufficient.


File: tar.info,  Node: Attributes,  Next: Standard,  Prev: Compression,  Up: Formats

Handling File Attributes
========================

     _(This message will disappear, once this node revised.)_

When `tar' reads files, this causes them to have the access times
updated.  To have `tar' attempt to set the access times back to what
they were before they were read, use the `--atime-preserve' option.
This doesn't work for files that you don't own, unless you're root, and
it doesn't interact with incremental dumps nicely (*note Backups::),
but it is good enough for some purposes.

   Handling of file attributes

`--atime-preserve'
     Do not change access times on dumped files.

`-m'
`--touch'
     Do not extract file modified time.

     When this option is used, `tar' leaves the modification times of
     the files it extracts as the time when the files were extracted,
     instead of setting it to the time recorded in the archive.

     This option is meaningless with `--list' (`-t').

`--same-owner'
     Create extracted files with the same ownership they have in the
     archive.

     When using super-user at extraction time, ownership is always
     restored.  So, this option is meaningful only for non-root users,
     when `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,
     and doing a `chmod' like when you use `--same-permissions'
     (`--preserve-permissions', `-p') (), 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.

`--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 filesystem(s) being
     extracted.  This occurs, for example, if you are restoring your
     files after a major crash and had booted from an emergency floppy
     with no password file or put your disk into another machine to do
     the restore.

     The numeric ids are _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 meaningless with `--list' (`-t').

`--preserve'
     Same as both `--same-permissions' (`--preserve-permissions', `-p')
     and `--same-order' (`--preserve-order', `-s').

     The `--preserve' option has no equivalent short option name.  It
     is equivalent to `--same-permissions' (`--preserve-permissions',
     `-p') plus `--same-order' (`--preserve-order', `-s').



File: tar.info,  Node: Standard,  Next: Extensions,  Prev: Attributes,  Up: Formats

The Standard 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 512 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 *Note update::,
and to learn more about having more than one archive member with the
same name, see .

   In addition to entries describing archive members, an archive may
contain entries which `tar' itself uses to store information.
*Note 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 may be a block filled
with binary zeros as an end-of-file marker.  A reasonable system should
write a block of zeros at the end, but must not assume that such a
block exists when reading an archive.

   The blocks may be "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':

     /* GNU tar Archive Format description.  */
     
     /* If OLDGNU_COMPATIBILITY is not zero, tar produces archives which, by
        default, are readable by older versions of GNU tar.  This can be
        overriden by using --posix; in this case, POSIXLY_CORRECT in environment
        may be set for enforcing stricter conformance.  If OLDGNU_COMPATIBILITY
        is zero or undefined, tar will eventually produces archives which, by
        default, POSIX compatible; then either using --posix or defining
        POSIXLY_CORRECT enforces stricter conformance.
     
        This #define will disappear in a few years.  FP, June 1995.  */
     #define OLDGNU_COMPATIBILITY 1
     
     /*---------------------------------------------.
     | `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 */
     
     /* 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
     
     /* The GNU extra header contains some information GNU tar needs, but not
        foreseen in POSIX header format.  It is only used after a POSIX header
        (and never with old GNU headers), and immediately follows this POSIX
        header, when typeflag is a letter rather than a digit, so signaling a GNU
        extension.  */
     
     struct extra_header
     {                               /* byte offset */
       char atime[12];               /*   0 */
       char ctime[12];               /*  12 */
       char offset[12];              /*  24 */
       char realsize[12];            /*  36 */
       char longnames[4];            /*  48 */
       char unused_pad1[68];         /*  52 */
       struct sparse sp[SPARSES_IN_EXTRA_HEADER];
                                     /* 120 */
       char isextended;              /* 504 */
                                     /* 505 */
     };
     
     /* 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 */
       char ctime[12];               /* 357 */
       char offset[12];              /* 369 */
       char longnames[4];            /* 381 */
       char unused_pad2;             /* 385 */
       struct sparse sp[SPARSES_IN_OLDGNU_HEADER];
                                     /* 386 */
       char isextended;              /* 482 */
       char realsize[12];            /* 483 */
                                     /* 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.  */
     
     /* 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'
     
     /*--------------------------------------.
     | tar Header Block, overall structure.  |
     `--------------------------------------*/
     
     /* tar files are made in basic blocks of this size.  */
     #define BLOCKSIZE 512
     
     enum archive_format
     {
       DEFAULT_FORMAT,               /* format to be decided later */
       V7_FORMAT,                    /* old V7 tar format */
       OLDGNU_FORMAT,                /* GNU format as per before tar 1.12 */
       POSIX_FORMAT,                 /* restricted, pure POSIX format */
       GNU_FORMAT                    /* POSIX format with GNU extensions */
     };
     
     union block
     {
       char buffer[BLOCKSIZE];
       struct posix_header header;
       struct extra_header extra_header;
       struct oldgnu_header oldgnu_header;
       struct sparse_header sparse_header;
     };
     
     /* End of Format description.  */

   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 fileds are zero-filled
octal numbers in ASCII.  Each numeric field of width W contains W minus
2 digits, a space, and a null, except `size', and `mtime', which do not
contain the trailing 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.  , in particular the
`--incremental' (`-G') option.

   The `mtime' field is the modification time of the file at the time
it was archived.  It is the ASCII representation of the octal value of
the last time the file was modified, represented as an integer number of
seconds since January 1, 1970, 00:00 Coordinated Universal Time.

   The `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 time
and last inode-change time.

   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 alloted 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_header's 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'.


File: tar.info,  Node: Extensions,  Next: cpio,  Prev: Standard,  Up: Formats

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.


File: tar.info,  Node: cpio,  Prev: Extensions,  Up: Formats

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);
`cpio's way requires you to play some games (in its "binary" format,
i-numbers are only 16 bits, and in its "portable ASCII" format, they're
18 bits--it would have to play games with the "file system ID" field of
the header to make sure that the file system ID/i-number pairs of
different files were always different), and I don't know which `cpio's,
if any, play those games.  Those that don't might get confused and
think two files are the same file when they're not, and make hard links
between them.

   `tar's way of handling multiple hard links to a file places only one
copy of the link on the tape, but the name attached to that copy is the
_only_ one you can use to retrieve the file; `cpio's 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 re-syncing.
However, lots of tape driver software won't allow you to continue past
a media error which should be the only reason for getting out of sync
unless a file changed sizes while you were writing the archive.

     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 `tar'red as sparse files (which you can easily
test, because the resulting archive gets smaller, and GNU `cpio' can no
longer read it).


File: tar.info,  Node: Media,  Next: Index,  Prev: Formats,  Up: Top

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 formated at 1600 bits per inch.  The
physically smaller EXABYTE tape cartridge holds 2.3 gigabytes.

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

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

* Menu:

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


File: tar.info,  Node: Device,  Next: Remote Tape Server,  Prev: Media,  Up: Media

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 `/etc/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
`/etc/rmt'.  This program is free software from the University of
California, and a copy of the source code can be found with the sources
for `tar'; it's compiled and installed by default.

   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 preferrable to him/her.
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
     overriden 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.
     *Note 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').


File: tar.info,  Node: Remote Tape Server,  Next: Common Problems and Solutions,  Prev: Device,  Up: Media

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 `/etc/rmt' on any machine
whose tape drive you want to use.  `tar' calls `/etc/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.  Instructions for compiling and installing
it are included in the `Makefile'.

   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' (`-P') option, or use the
command `tar -C / ...'.

   Some versions of Unix (Ultrix 3.1 is know to have this problem), can
claim that a short write near the end of a tape succeeded, when it
actually failed.  This will result in the -M option not working
correctly.  The best workaround at the moment is to use a significantly
larger blocking factor than the default 20.

   In order to update an archive, `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' (`-r'), `--update' (`-u'),
`--concatenate' (`--catenate', `-A'), 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' (`-M'),
`--label=ARCHIVE-LABEL' (`-V ARCHIVE-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.


File: tar.info,  Node: Common Problems and Solutions,  Next: Blocking,  Prev: Remote Tape Server,  Up: Media

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


File: tar.info,  Node: Blocking,  Next: Many,  Prev: Common Problems and Solutions,  Up: Media

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.  *Note 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=512-SIZE' (`-b 512-SIZE')) larger than the actual
blocking factor, and then use the `--read-full-records' (`-B') option.
(If you specify a blocking factor with `--blocking-factor=512-SIZE'
(`-b 512-SIZE') and don't use the `--read-full-records' (`-B') 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=512-SIZE' (`-b 512-SIZE') 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=512-SIZE' (`-b 512-SIZE') 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.

* Menu:

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


File: tar.info,  Node: Format Variations,  Next: Blocking Factor,  Prev: Blocking,  Up: Blocking

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=512-SIZE' (`-b 512-SIZE') option specified
(*note Blocking Factor::), you must specify that blocking-factor when
operating on the archive.  *Note Formats::, for other examples of
format parameter considerations.

