This is tar.info, produced by makeinfo version 4.13 from tar.texi.

This manual is for GNU `tar' (version 1.26, 12 March 2011), which
creates and extracts files from archives.

   Copyright (C) 1992, 1994, 1995, 1996, 1997, 1999, 2000, 2001, 2003,
2004, 2005, 2006, 2007, 2008, 2009, 2010 Free Software Foundation, Inc.

     Permission is granted to copy, distribute and/or modify this
     document under the terms of the GNU Free Documentation License,
     Version 1.3 or any later version published by the Free Software
     Foundation; with no Invariant Sections, with the Front-Cover Texts
     being "A GNU Manual", and with the Back-Cover Texts as in (a)
     below.  A copy of the license is included in the section entitled
     "GNU Free Documentation License".

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

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

INFO-DIR-SECTION Individual utilities
START-INFO-DIR-ENTRY
* tar: (tar)tar invocation.                     Invoking GNU `tar'.
END-INFO-DIR-ENTRY


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

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

This manual is for GNU `tar' (version 1.26, 12 March 2011), which
creates and extracts files from archives.

   Copyright (C) 1992, 1994, 1995, 1996, 1997, 1999, 2000, 2001, 2003,
2004, 2005, 2006, 2007, 2008, 2009, 2010 Free Software Foundation, Inc.

     Permission is granted to copy, distribute and/or modify this
     document under the terms of the GNU Free Documentation License,
     Version 1.3 or any later version published by the Free Software
     Foundation; with no Invariant Sections, with the Front-Cover Texts
     being "A GNU Manual", and with the Back-Cover Texts as in (a)
     below.  A copy of the license is included in the section entitled
     "GNU Free Documentation License".

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

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

* Menu:

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

Appendices

* Changes::
* Configuring Help Summary::
* Fixing Snapshot Files::
* Tar Internals::
* Genfile::
* Free Software Needs Free Documentation::
* GNU Free Documentation License::
* Index of Command Line Options::
* Index::

 --- The Detailed Node Listing ---

Introduction

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

Tutorial Introduction to `tar'

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

Two Frequently Used Options

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

How to Create Archives

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

How to List Archives

* list dir::

How to Extract Members from an Archive

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

Invoking GNU `tar'

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

The Three Option Styles

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

All `tar' Options

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

GNU `tar' Operations

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

Advanced GNU `tar' Operations

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

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

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

Updating an Archive

* how to update::

Options Used by `--create'

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

Options Used by `--extract'

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

Options to Help Read Archives

* read full records::
* Ignore Zeros::

Changing How `tar' Writes Files

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

Coping with Scarce Resources

* Starting File::
* Same Order::

Performing Backups and Restoring Files

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

Setting Parameters for Backups and Restoration

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

Choosing Files and Names for `tar'

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

Reading Names from a File

* nul::

Excluding Some Files

* problems with exclude::

Wildcards Patterns and Matching

* controlling pattern-matching::

Crossing File System Boundaries

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

Date input formats

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

Controlling the Archive Format

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

Using Less Space through Compression

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

Creating and Reading Compressed Archives

* lbzip2::  Using lbzip2 with GNU `tar'.

Making `tar' Archives More Portable

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

GNU `tar' and POSIX `tar'

* PAX keywords:: Controlling Extended Header Keywords.

How to Extract GNU-Specific Data Using Other `tar' Implementations

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

Tapes and Other Archive Media

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

Blocking

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

Many Archives on One Tape

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

Using Multiple Tapes

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


Tar Internals

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

Storing Sparse Files

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

Genfile

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

Copying This Manual

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


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

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

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

* Menu:

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


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

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

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

   The second chapter is a tutorial (*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 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

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

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

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

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


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

1.3 What `tar' Does
===================

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

   Initially, `tar' archives were used to store files conveniently on
magnetic tape.  The name `tar' comes from this use; it stands for
`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 file system.

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


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

1.4 How `tar' Archives are Named
================================

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

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


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

1.5 GNU `tar' Authors
=====================

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

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

   Jay Fenlason put together a draft of a GNU `tar' manual, borrowing
notes from the original man page from John Gilmore.  This was withdrawn
in version 1.11.  Thomas Bushnell, n/BSG and Amy Gorin worked on a
tutorial and manual for GNU `tar'.  Franc,ois Pinard put version 1.11.8
of the manual together by taking information from all these sources and
merging them.  Melissa Weisshaus finally edited and redesigned the book
to create version 1.12.  The book for versions from 1.14 up to 1.26
were edited by the current maintainer, Sergey Poznyakoff.

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

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

   Support for POSIX archives was added by Sergey Poznyakoff.


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

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

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

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


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

2 Tutorial Introduction to `tar'
********************************

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

* 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,  Up: Tutorial

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

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

   * Before you start to work through this tutorial, you should
     understand what the terms "archive" and "archive member" mean
     (*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 file system.  You should have
     some basic understanding of directory structure and how files are
     named according to which directory they are in.  You should
     understand concepts such as standard output and standard input,
     what various definitions of the term `argument' mean, and the
     differences between relative and absolute file names.

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

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

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


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

2.3 Basic `tar' Operations and Options
======================================

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

   Some arguments fall into a class called "operations"; exactly one of
these is both allowed and required for any instance of using `tar'; you
may _not_ specify more than one.  People sometimes speak of "operating
modes".  You are in a particular operating mode when you have specified
the operation which specifies it; there are eight operations in total,
and thus there are eight operating modes.

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

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

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

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

`--create'
`-c'
     Create a new `tar' archive.

`--list'
`-t'
     List the contents of an archive.

`--extract'
`-x'
     Extract one or more members from an archive.


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

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

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

* Menu:

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


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

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

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

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

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

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

   For example, to create an archive in verbose mode:

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

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

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

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

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

Note that you must double the hyphens properly each time.

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

   The full output consists of six fields:

   * File type and permissions in symbolic form.  These are displayed
     in the same format as the first column of `ls -l' output (*note
     format=verbose: (fileutils)What information is listed.).

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

   * Size of the file, in bytes.

   * File modification date in ISO 8601 format.

   * File modification time.

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

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

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

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

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

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

    `--Volume Header--'
          The archive member is a GNU "volume header" (*note Tape
          Files::).

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

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


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

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



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

2.6 How to Create Archives
==========================

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

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

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

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

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

* Menu:

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


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

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

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

   Now `cd' to the directory named `practice'; `practice' is now your
"working directory".  (_Please note_: Although the full file name of
this directory is `/HOMEDIR/practice', in our examples we will refer to
this directory as `practice'; the HOMEDIR is presumed.)

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

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

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

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

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

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

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

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

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

   The order of the options becomes more important when you begin to use
short forms.  With short forms, if you type commands in the wrong order
(even if you type them correctly in all other ways), you may end up with
results you don't expect.  For this reason, it is a good idea to get
into the habit of typing options in the order that makes inherent sense.
*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.  *Note Choosing::, for the detailed discussion on these.)
Now that they are in the archive, they are called _archive members_,
not files.  (*note members: Definitions.).

   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

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

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

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

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

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


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

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

As we said before, the `--create' (`-c') operation is one of the most
basic uses of `tar', and you will use it countless times.  Eventually,
you will probably want to use abbreviated (or "short") forms of
options.  A full discussion of the three different forms that options
can take appears in *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.


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

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

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

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

     $ cd ..
     $

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

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

`tar' should output:

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

   Note that the archive thus created is not in the subdirectory
`practice', but rather in the current working directory--the directory
from which `tar' was invoked.  Before trying to archive a directory
from its superior directory, you should make sure you have write access
to the superior directory itself, not only the directory you are trying
archive with `tar'.  For example, you will probably not be able to
store your home directory in an archive by invoking `tar' from the root
directory; *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 `foo.tar', and
skips it.  (It makes no sense to put an archive into itself.)  GNU `tar'
will continue in this case, and create the archive normally, except for
the exclusion of that one file.  (_Please note:_ Other implementations
of `tar' may not be so clever; they will enter an infinite loop when
this happens, so you should not depend on this behavior unless you are
certain you are running GNU `tar'.  In general, it is wise to always
place the archive outside of the directory being dumped.)


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

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

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

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

The output of `tar' would then be:

     blues
     folk
     jazz

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

     ./birds
     baboon
     ./box

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

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

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

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

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

     $ tar --create --verbose --file archive /etc/mail
     tar: Removing leading `/' from member names
     /etc/mail/
     /etc/mail/sendmail.cf
     /etc/mail/aliases
     $ tar --test --file archive
     etc/mail/
     etc/mail/sendmail.cf
     etc/mail/aliases

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

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

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

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

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

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

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

* Menu:

* list dir::


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

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

To get information about the contents of an archived directory, use the
directory name as a file name argument in conjunction with `--list'
(`-t').  To find out file attributes, include the `--verbose' (`-v')
option.

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

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

   `tar' responds:

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

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


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

2.8 How to Extract Members from an Archive
==========================================

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

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

* Menu:

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


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

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

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

     $ tar -xvf collection.tar

produces this:

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


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

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

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

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

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

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

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

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

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

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

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

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

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

   (1) This is only accidentally true, but not in general.  Whereas
modification times are always restored, in most cases, one has to be
root for restoring the owner, and use a special option for restoring
permissions.  Here, it just happens that the restoring user is also the
owner of the archived members, and that the current `umask' is
compatible with original permissions.


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

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

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

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

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

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

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

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

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


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

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

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

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

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


File: tar.info,  Node: failing commands,  Prev: extracting untrusted archives,  Up: extract

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

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

   If you try to use this command,

     $ tar -xvf music.tar folk jazz

you will get the following response:

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

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

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

Likewise, if you try to use this command,

     $ tar -tvf music.tar folk jazz

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

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


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

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

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


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

3 Invoking GNU `tar'
********************

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' (*note verbose::) and
`--interactive' options (*note interactive::).

* Menu:

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


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

3.1 General Synopsis of `tar'
=============================

The GNU `tar' program is invoked as either one of:

     tar OPTION... [NAME]...
     tar LETTER... [ARGUMENT]... [OPTION]... [NAME]...

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

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

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

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

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

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

   The distinction between file names and archive member names is
especially important when shell globbing is used, and sometimes a
source of confusion for newcomers.  *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', `--extract',
`--compare', and `--update') will act on the entire contents of the
archive.

   Besides successful exits, GNU `tar' may fail for many reasons.  Some
reasons correspond to bad usage, that is, when the `tar' command line
is improperly written.  Errors may be encountered later, while
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 be 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.

   Possible exit codes of GNU `tar' are summarized in the following
table:

0
     `Successful termination'.

1
     `Some files differ'.  If tar was invoked with `--compare'
     (`--diff', `-d') command line option, this means that some files
     in the archive differ from their disk counterparts (*note
     compare::).  If tar was given `--create', `--append' or `--update'
     option, this exit code means that some files were changed while
     being archived and so the resulting archive does not contain the
     exact copy of the file set.

2
     `Fatal error'.  This means that some fatal, unrecoverable error
     occurred.

   If `tar' has invoked a subprocess and that subprocess exited with a
nonzero exit code, `tar' exits with that code as well.  This can
happen, for example, if `tar' was given some compression option (*note
gzip::) and the external compressor program failed.  Another example is
`rmt' failure during backup to the remote device (*note Remote Tape
Server::).


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

3.2 Using `tar' Options
=======================

GNU `tar' has a total of eight operating modes which allow you to
perform a variety of tasks.  You are required to choose one operating
mode each time you employ the `tar' program by specifying one, and only
one operation as an argument to the `tar' command (the corresponding
options 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::.)

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

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

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


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

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

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

   Some options must take an argument(1).  Where you _place_ the
arguments generally depends on which style of options you choose.  We
will detail specific information relevant to each option style in the
sections on the different option styles, below.  The differences are
subtle, yet can often be very important; incorrect option placement can
cause you to overwrite a number of important files.  We urge you to
note these differences, and only use the option style(s) which makes
the most sense to you until you feel comfortable with the others.

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

* Menu:

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

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

   (1) For example, `--file' (`-f') takes the name of an archive file
as an argument.  If you do not supply an archive file name, `tar' will
use a default, but this can be confusing; thus, we recommend that you
always supply a specific archive file name.


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

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

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

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

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

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

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

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


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

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

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

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

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

   Short options which take optional arguments take their arguments
immediately following the option letter, _without any intervening white
space characters_.

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

3.3.3 Old Option Style
----------------------

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 white space; old options cannot appear
anywhere else.  The letter of an old option is exactly the same letter
as the corresponding short option.  For example, the old option `t' is
the same as the short option `-t', and consequently, the same as the
long option `--list'.  So for example, the command `tar cv' specifies
the option `-v' in addition to the operation `-c'.

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

     $ tar cvbf 20 /dev/rmt0

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

   On the other hand, this old style syntax makes it difficult to match
option letters with their corresponding arguments, and is often
confusing.  In the command `tar cvbf 20 /dev/rmt0', for example, `20'
is the argument for `-b', `/dev/rmt0' is the argument for `-f', and
`-v' does not have a corresponding argument.  Even using short options
like in `tar -c -v -b 20 -f /dev/rmt0' is clearer, putting all
arguments next to the option they pertain to.

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

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

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

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

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

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

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

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

   ---------- 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

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

All three styles may be intermixed in a single `tar' command, so long
as the rules for each style are fully respected(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 white space).
Modern options may be given only after all arguments to the old options
have been collected.  If this rule is not respected, a modern option
might be falsely interpreted as the value of the argument to one of the
old style options.

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

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

   On the other hand, the following commands are _not_ equivalent to
the previous set:

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

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

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

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


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

3.4 All `tar' Options
=====================

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

* Menu:

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


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

3.4.1 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 an 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

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

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

`--after-date'
     (See `--newer', *note after::)

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

`--atime-preserve'
`--atime-preserve=replace'
`--atime-preserve=system'
     Attempt to preserve the access time of files when reading them.
     This option currently is effective only on files that you own,
     unless you have superuser privileges.

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

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

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

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

`--auto-compress'
`-a'
     During a `--create' operation, enables automatic compressed format
     recognition based on the archive suffix.  The effect of this
     option is cancelled by `--no-auto-compress'.  *Note gzip::.

`--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.  *Note backup::.

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

`--blocking-factor=BLOCKING'
`-b BLOCKING'
     Sets the blocking factor `tar' uses to BLOCKING x 512 bytes per
     record.  *Note Blocking Factor::.

`--bzip2'
`-j'
     This option tells `tar' to read or write archives through `bzip2'.
     *Note gzip::.

`--check-device'
     Check device numbers when creating a list of modified files for
     incremental archiving.  This is the default.  *Note device
     numbers::, for a detailed description.

`--checkpoint[=NUMBER]'
     This option directs `tar' to print periodic checkpoint messages as
     it reads through the archive.  It is intended for when you want a
     visual indication that `tar' is still running, but don't want to
     see `--verbose' output.  You can also instruct `tar' to execute a
     list of actions on each checkpoint, see `--checkpoint-action'
     below.  For a detailed description, see *note checkpoints::.

`--checkpoint-action=ACTION'
     Instruct `tar' to execute an action upon hitting a breakpoint.
     Here we give only a brief outline.  *Note checkpoints::, for a
     complete description.

     The ACTION argument can be one of the following:

    bell
          Produce an audible bell on the console.

    dot
    .
          Print a single dot on the standard listing stream.

    echo
          Display a textual message on the standard error, with the
          status and number of the checkpoint.  This is the default.

    echo=STRING
          Display STRING on the standard error.  Before output, the
          string is subject to meta-character expansion.

    exec=COMMAND
          Execute the given COMMAND.

    sleep=TIME
          Wait for TIME seconds.

    ttyout=STRING
          Output STRING on the current console (`/dev/tty').

     Several `--checkpoint-action' options can be specified.  The
     supplied actions will be executed in order of their appearance in
     the command line.

     Using `--checkpoint-action' without `--checkpoint' assumes default
     checkpoint frequency of one checkpoint per 10 records.

`--check-links'
`-l'
     If this option was given, `tar' will check the number of links
     dumped for each processed file.  If this number does not match the
     total number of hard links for the file, a warning message will be
     output (1).

     *Note hard links::.

`--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.  *Note gzip::.

`--confirmation'
     (See `--interactive'.)  *Note interactive::.

`--delay-directory-restore'
     Delay setting modification times and permissions of extracted
     directories until the end of extraction. *Note Directory
     Modification Times and Permissions::.

`--dereference'
`-h'
     When reading or writing a file to be archived, `tar' accesses the
     file that a symbolic link points to, rather than the symlink
     itself.  *Note dereference::.

`--directory=DIR'
`-C DIR'
     When this option is specified, `tar' will change its current
     directory to DIR before performing any operations.  When this
     option is used during archive creation, it is order sensitive.
     *Note directory::.

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

`--exclude-backups'
     Exclude backup and lock files.  *Note exclude-backups: exclude.

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

`--exclude-caches'
     Exclude from dump any directory containing a valid cache directory
     tag file, but still dump the directory node and the tag file
     itself.

     *Note exclude-caches: exclude.

`--exclude-caches-under'
     Exclude from dump any directory containing a valid cache directory
     tag file, but still dump the directory node itself.

     *Note exclude::.

`--exclude-caches-all'
     Exclude from dump any directory containing a valid cache directory
     tag file.  *Note exclude::.

`--exclude-tag=FILE'
     Exclude from dump any directory containing file named FILE, but
     dump the directory node and FILE itself.  *Note exclude-tag:
     exclude.

`--exclude-tag-under=FILE'
     Exclude from dump the contents of any directory containing file
     named FILE, but dump the directory node itself.  *Note
     exclude-tag-under: exclude.

`--exclude-tag-all=FILE'
     Exclude from dump any directory containing file named FILE.  *Note
     exclude-tag-all: exclude.

`--exclude-vcs'
     Exclude from dump directories and files, that are internal for some
     widely used version control systems.

     *Note exclude-vcs: exclude.

`--file=ARCHIVE'
`-f ARCHIVE'
     `tar' will use the file ARCHIVE as the `tar' archive it performs
     operations on, rather than `tar''s compilation dependent default.
     *Note file tutorial::.

`--files-from=FILE'
`-T FILE'
     `tar' will use the contents of FILE as a list of archive members
     or files to operate on, in addition to those specified on the
     command-line.  *Note files::.

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

`--format=FORMAT'
`-H FORMAT'
     Selects output archive format.  FORMAT may be one of the following:

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

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

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

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

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


     *Note Formats::, for a detailed discussion of these formats.

`--full-time'
     This option instructs `tar' to print file times to their full
     resolution.  Usually this means 1-second resolution, but that
     depends on the underlying file system.  The `--full-time' option
     takes effect only when detailed output (verbosity level 2 or
     higher) has been requested using the `--verbose' option, e.g.,
     when listing or extracting archives:

          $ tar -t -v --full-time -f archive.tar

     or, when creating an archive:

          $ tar -c -vv --full-time -f archive.tar .

     Notice, thar when creating the archive you need to specify
     `--verbose' twice to get a detailed output (*note verbose
     tutorial::).

`--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.  *Note override::.

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

`--gzip'
`--gunzip'
`--ungzip'
`-z'
     This option tells `tar' to read or write archives through `gzip',
     allowing `tar' to directly operate on several kinds of compressed
     archives transparently.  *Note gzip::.

`--hard-dereference'
     When creating an archive, dereference hard links and store the
     files they refer to, instead of creating usual hard link members.

     *Note hard links::.

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

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

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

`--ignore-failed-read'
     Do not exit unsuccessfully merely because an unreadable file was
     encountered.  *Note Reading::.

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

`--incremental'
`-G'
     Informs `tar' that it is working with an old GNU-format
     incremental backup archive.  It is intended primarily for
     backwards compatibility only.  *Note Incremental Dumps::, for a
     detailed discussion of incremental archives.

`--index-file=FILE'
     Send verbose output to FILE instead of to standard output.

`--info-script=SCRIPT-FILE'
`--new-volume-script=SCRIPT-FILE'
`-F SCRIPT-FILE'
     When `tar' is performing multi-tape backups, SCRIPT-FILE is run at
     the end of each tape.  If SCRIPT-FILE exits with nonzero status,
     `tar' fails immediately.  *Note info-script::, for a detailed
     discussion of SCRIPT-FILE.

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

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

`--keep-old-files'
`-k'
     Do not overwrite existing files when extracting files from an
     archive.  *Note Keep Old Files::.

`--label=NAME'
`-V NAME'
     When creating an archive, instructs `tar' to write NAME as a name
     record in the archive.  When extracting or listing archives, `tar'
     will only operate on archives that have a label matching the
     pattern specified in NAME.  *Note Tape Files::.

`--level=N'
     Force incremental backup of level N.  As of GNU `tar' version
     1.26, the option `--level=0' truncates the snapshot file, thereby
     forcing the level 0 dump.  Other values of N are effectively
     ignored.  *Note --level=0::, for details and examples.

     The use of this option is valid only in conjunction with the
     `--listed-incremental' option.  *Note Incremental Dumps::, for a
     detailed description.

`--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.  *Note Incremental Dumps::.

`--lzip'
     This option tells `tar' to read or write archives through `lzip'.
     *Note gzip::.

`--lzma'
     This option tells `tar' to read or write archives through `lzma'.
     *Note gzip::.

`--lzop'
     This option tells `tar' to read or write archives through `lzop'.
     *Note gzip::.

`--mode=PERMISSIONS'
     When adding files to an archive, `tar' will use PERMISSIONS for
     the archive members, rather than the permissions from the files.
     PERMISSIONS can be specified either as an octal number or as
     symbolic permissions, like with `chmod'. *Note override::.

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

`--multi-volume'
`-M'
     Informs `tar' that it should create or otherwise operate on a
     multi-volume `tar' archive.  *Note Using Multiple Tapes::.

`--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.  If DATE begins with `/' or `.', it is taken
     to be the name of a file whose data modification time specifies
     the date.  *Note after::.

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

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

`--no-auto-compress'
     Disables automatic compressed format recognition based on the
     archive suffix.  *Note --auto-compress::.  *Note gzip::.

`--no-check-device'
     Do not check device numbers when creating a list of modified files
     for incremental archiving.  *Note device numbers::, for a detailed
     description.

`--no-delay-directory-restore'
     Modification times and permissions of extracted directories are
     set when all files from this directory have been extracted.  This
     is the default.  *Note Directory Modification Times and
     Permissions::.

`--no-ignore-case'
     Use case-sensitive matching.  *Note controlling pattern-matching::.

`--no-ignore-command-error'
     Print warnings about subprocesses that terminated with a nonzero
     exit code. *Note Writing to an External Program::.

`--no-null'
     If the `--null' option was given previously, this option cancels
     its effect, so that any following `--files-from' options will
     expect their file lists to be newline-terminated.

`--no-overwrite-dir'
     Preserve metadata of existing directories when extracting files
     from an archive.  *Note Overwrite Old Files::.

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

`--no-recursion'
     With this option, `tar' will not recurse into directories.  *Note
     recurse::.

`--no-same-owner'
`-o'
     When extracting an archive, do not attempt to preserve the owner
     specified in the `tar' archive.  This the default behavior for
     ordinary users.

`--no-same-permissions'
     When extracting an archive, subtract the user's umask from files
     from the permissions specified in the archive.  This is the
     default behavior for ordinary users.

`--no-seek'
     The archive media does not support seeks to arbitrary locations.
     Usually `tar' determines automatically whether the archive can be
     seeked or not.  Use this option to disable this mechanism.

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

`--no-wildcards'
     Do not use wildcards.  *Note controlling pattern-matching::.

`--no-wildcards-match-slash'
     Wildcards do not match `/'.  *Note controlling pattern-matching::.

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

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

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

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

     *Note Changes::, for more information.

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

     This option instructs `tar' to process only the NUMBERth
     occurrence of each named file.  NUMBER defaults to 1, so

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

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

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

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

`--overwrite'
     Overwrite existing files and directory metadata when extracting
     files from an archive.  *Note Overwrite Old Files::.

`--overwrite-dir'
     Overwrite the metadata of existing directories when extracting
     files from an archive.  *Note Overwrite Old Files::.

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

     This option does not affect extraction from archives.

`--pax-option=KEYWORD-LIST'
     This option enables creation of the archive in POSIX.1-2001 format
     (*note posix::) and modifies the way `tar' handles the extended
     header keywords.  KEYWORD-LIST is a comma-separated list of
     keyword options.  *Note PAX keywords::, for a detailed discussion.

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

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

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

`--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 Setting Access
     Permissions::.

`--quote-chars=STRING'
     Always quote characters from STRING, even if the selected quoting
     style would not quote them (*note quoting styles::).

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

`--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[SUF]'
     Instructs `tar' to use SIZE bytes per record when accessing the
     archive.  The argument can be suffixed with a "size suffix", e.g.
     `--record-size=10K' for 10 Kilobytes.  *Note size-suffixes::, for
     a list of valid suffixes.   *Note Blocking Factor::, for a detailed
     description of this option.

`--recursion'
     With this option, `tar' recurses into directories (default).
     *Note recurse::.

`--recursive-unlink'
     Remove existing directory hierarchies before extracting
     directories of the same name from the archive.  *Note Recursive
     Unlink::.

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

`--restrict'
     Disable use of some potentially harmful `tar' options.  Currently
     this option disables shell invocation from multi-volume menu
     (*note Using Multiple Tapes::).

`--rmt-command=CMD'
     Notifies `tar' that it should use CMD instead of the default
     `/usr/libexec/rmt' (*note Remote Tape Server::).

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

`--same-order'
`--preserve-order'
`-s'
     This option is an optimization for `tar' when running on machines
     with small amounts of memory.  It informs `tar' that the list of
     file arguments has already been sorted to match the order of files
     in the archive.  *Note Reading::.

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

`--same-permissions'
     (See `--preserve-permissions'; *note Setting Access Permissions::.)

`--seek'
`-n'
     Assume that the archive media supports seeks to arbitrary
     locations.  Usually `tar' determines automatically whether the
     archive can be seeked or not.  This option is intended for use in
     cases when such recognition fails.  It takes effect only if the
     archive is open for reading (e.g. with `--list' or `--extract'
     options).

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

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

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

`--show-omitted-dirs'
     Instructs `tar' to mention the directories it is skipping when
     operating on a `tar' archive.  *Note show-omitted-dirs::.

`--show-transformed-names'
`--show-stored-names'
     Display file or member names after applying any transformations
     (*note transform::).  In particular, when used in conjunction with
     one of the archive creation operations it instructs `tar' to list
     the member names stored in the archive, as opposed to the actual
     file names.  *Note listing member and file names::.

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

`--sparse-version=VERSION'
     Specifies the "format version" to use when archiving sparse files.
     Implies `--sparse'.  *Note sparse::. For the description of the
     supported sparse formats, *Note Sparse Formats::.

`--starting-file=NAME'
`-K NAME'
     This option affects extraction only; `tar' will skip extracting
     files in the archive until it finds one that matches NAME.  *Note
     Scarce::.

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

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

     would extract this file to file `name'.

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

`--tape-length=NUM[SUF]'
`-L NUM[SUF]'
     Specifies the length of tapes that `tar' is writing as being
     NUM x 1024 bytes long.  If optional SUF is given, it specifies a
     multiplicative factor to be used instead of 1024.  For example,
     `-L2M' means 2 megabytes.  *Note size-suffixes::, for a list of
     allowed suffixes.  *Note Using Multiple Tapes::, for a detailed
     discussion of this option.

`--test-label'
     Reads the volume label.  If an argument is specified, test whether
     it matches the volume label.  *Note --test-label option::.

`--to-command=COMMAND'
     During extraction `tar' will pipe extracted files to the standard
     input of COMMAND.  *Note Writing to an External Program::.

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

`--totals[=SIGNO]'
     Displays the total number of bytes transferred when processing an
     archive.  If an argument is given, these data are displayed on
     request, when signal SIGNO is delivered to `tar'.  *Note totals::.

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

`--transform=SED-EXPR'
`--xform=SED-EXPR'
     Transform file or member names using `sed' replacement expression
     SED-EXPR.  For example,

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

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

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

`--uncompress'
     (See `--compress', *note gzip::)

`--ungzip'
     (See `--gzip', *note gzip::)

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

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

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

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

`--verbose'
`-v'
     Specifies that `tar' should be more verbose about the operations
     it is performing.  This option can be specified multiple times for
     some operations to increase the amount of information displayed.
     *Note verbose::.

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

`--version'
     Print information about the program's name, version, origin and
     legal status, all on standard output, and then exit successfully.
     *Note help::.

`--volno-file=FILE'
     Used in conjunction with `--multi-volume'.  `tar' will keep track
     of which volume of a multi-volume archive it is working in FILE.
     *Note volno-file::.

`--warning=KEYWORD'
     Enable or disable warning messages identified by KEYWORD.  The
     messages are suppressed if KEYWORD is prefixed with `no-'.  *Note
     warnings::.

`--wildcards'
     Use wildcards when matching member names with patterns.  *Note
     controlling pattern-matching::.

`--wildcards-match-slash'
     Wildcards match `/'.  *Note controlling pattern-matching::.

`--xz'
`-J'
     Use `xz' for compressing or decompressing the archives.  *Note
     gzip::.


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

   (1) Earlier versions of GNU `tar' understood `-l' as a synonym for
`--one-file-system'.  The current semantics, which complies to UNIX98,
was introduced with version 1.15.91. *Note Changes::, for more
information.


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

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

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

Short Option   Reference
-------------------------------------------------------------------------- 
-A             *note --concatenate::.
-B             *note --read-full-records::.
-C             *note --directory::.
-F             *note --info-script::.
-G             *note --incremental::.
-J             *note --xz::.
-K             *note --starting-file::.
-L             *note --tape-length::.
-M             *note --multi-volume::.
-N             *note --newer::.
-O             *note --to-stdout::.
-P             *note --absolute-names::.
-R             *note --block-number::.
-S             *note --sparse::.
-T             *note --files-from::.
-U             *note --unlink-first::.
-V             *note --label::.
-W             *note --verify::.
-X             *note --exclude-from::.
-Z             *note --compress::.
-b             *note --blocking-factor::.
-c             *note --create::.
-d             *note --compare::.
-f             *note --file::.
-g             *note --listed-incremental::.
-h             *note --dereference::.
-i             *note --ignore-zeros::.
-j             *note --bzip2::.
-k             *note --keep-old-files::.
-l             *note --check-links::.
-m             *note --touch::.
-o             When creating, *note --no-same-owner::, when extracting --
               *note --portability::.
               
                  The latter usage is deprecated.  It is retained for
               compatibility with the earlier versions of GNU `tar'.  In
               future releases `-o' will be equivalent to
               `--no-same-owner' only.
-p             *note --preserve-permissions::.
-r             *note --append::.
-s             *note --same-order::.
-t             *note --list::.
-u             *note --update::.
-v             *note --verbose::.
-w             *note --interactive::.
-x             *note --extract::.
-z             *note --gzip::.


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

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

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

     tar (GNU tar) 1.26
     Copyright (C) 2010 Free Software Foundation, Inc.
     Copyright (C) 2010 Free Software Foundation, Inc.
     License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.
     This is free software: you are free to change and redistribute it.
     There is NO WARRANTY, to the extent permitted by law.

     Written by John Gilmore and Jay Fenlason.

The first occurrence of `tar' in the result above is the program name
in the package (for example, `rmt' is another program), while the
second occurrence of `tar' is the name of the package itself,
containing possibly many programs.  The package is currently named
`tar', after the name of the main program it contains(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.  Notice, however, that some `tar'
options have long description lines and the above command will list
only the first of them.

   The exact look of the option summary displayed by `tar --help' is
configurable. *Note Configuring Help Summary::, for a detailed
description.

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

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

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

   ---------- 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 output `tar (GNU paxutils) 3.2'.


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

3.6 Obtaining GNU `tar' default values
======================================

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

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

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

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


File: tar.info,  Node: verbose,  Next: checkpoints,  Prev: defaults,  Up: tar invocation

3.7 Checking `tar' progress
===========================

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

   This example shows the default checkpoint message used by `tar'.  If
you place a dot immediately after the equal sign, it will print a `.'
at each checkpoint(1).  For example:

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

   The `--checkpoint' option provides a flexible mechanism for
executing arbitrary actions upon hitting checkpoints, see the next
section (*note checkpoints::), for more information on it.

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

   If `--block-number' (`-R') is used, `tar' prints, along with every
message it would normally produce, the block number within the archive
where the message was triggered.  Also, supplementary messages are
triggered when reading blocks full of NULs, or when hitting end of file
on the archive.  As of now, if the archive is 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).  *Note backup::.

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

   (1) This is actually a shortcut for `--checkpoint=N
--checkpoint-action=dot'.  *Note dot: checkpoints.


File: tar.info,  Node: checkpoints,  Next: warnings,  Prev: verbose,  Up: tar invocation

3.8 Checkpoints
===============

A "checkpoint" is a moment of time before writing Nth record to the
archive (a "write checkpoint"), or before reading Nth record from the
archive (a "read checkpoint").  Checkpoints allow to periodically
execute arbitrary actions.

   The checkpoint facility is enabled using the following option:

`--checkpoint[=N]'
     Schedule checkpoints before writing or reading each Nth record.
     The default value for N is 10.

   A list of arbitrary "actions" can be executed at each checkpoint.
These actions include: pausing, displaying textual messages, and
executing arbitrary external programs.  Actions are defined using the
`--checkpoint-action' option.

`--checkpoint-action=ACTION'
     Execute an ACTION at each checkpoint.

   The simplest value of ACTION is `echo'.  It instructs `tar' to
display the default message on the standard error stream upon arriving
at each checkpoint.  The default message is (in POSIX locale) `Write
checkpoint N', for write checkpoints, and `Read checkpoint N', for read
checkpoints.  Here, N represents ordinal number of the checkpoint.

   In another locales, translated versions of this message are used.

   This is the default action, so running:

     $ tar -c --checkpoint=1000 --checkpoint-action=echo /var

is equivalent to:

     $ tar -c --checkpoint=1000 /var

   The `echo' action also allows to supply a customized message.  You
do so by placing an equals sign and the message right after it, e.g.:

     --checkpoint-action="echo=Hit %s checkpoint #%u"

   The `%s' and `%u' in the above example are "meta-characters".  The
`%s' meta-character is replaced with the "type" of the checkpoint:
`write' or `read' (or a corresponding translated version in locales
other than POSIX).  The `%u' meta-character is replaced with the
ordinal number of the checkpoint.  Thus, the above example could
produce the following output when used with the `--create' option:

     tar: Hit write checkpoint #10
     tar: Hit write checkpoint #20
     tar: Hit write checkpoint #30

   Aside from meta-character expansion, the message string is subject to
"unquoting", during which the backslash "escape sequences" are replaced
with their corresponding ASCII characters (*note escape sequences::).
E.g. the following action will produce an audible bell and the message
described above at each checkpoint:

     --checkpoint-action='echo=\aHit %s checkpoint #%u'

   There is also a special action which produces an audible signal:
`bell'.  It is not equivalent to `echo='\a'', because `bell' sends the
bell directly to the console (`/dev/tty'), whereas `echo='\a'' sends it
to the standard error.

   The `ttyout=STRING' action outputs STRING to `/dev/tty', so it can
be used even if the standard output is redirected elsewhere.  The
STRING is subject to the same modifications as with `echo' action.  In
contrast to the latter, `ttyout' does not prepend `tar' executable name
to the string, nor does it output a newline after it.  For example, the
following action will print the checkpoint message at the same screen
line, overwriting any previous message:

     --checkpoint-action="ttyout=\rHit %s checkpoint #%u"

   Another available checkpoint action is `dot' (or `.').  It instructs
`tar' to print a single dot on the standard listing stream, e.g.:

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

   For compatibility with previous GNU `tar' versions, this action can
be abbreviated by placing a dot in front of the checkpoint frequency,
as shown in the previous section.

   Yet another action, `sleep', pauses `tar' for a specified amount of
seconds.  The following example will stop for 30 seconds at each
checkpoint:

     $ tar -c --checkpoint=1000 --checkpoint-action=sleep=30

   Finally, the `exec' action executes a given external program.  For
example:

     $ tar -c --checkpoint=1000 --checkpoint-action=exec=/sbin/cpoint

   This program is executed using `/bin/sh -c', with no additional
arguments.  Its exit code is ignored.  It gets a copy of `tar''s
environment plus the following variables:

`TAR_VERSION'
     GNU `tar' version number.

`TAR_ARCHIVE'
     The name of the archive `tar' is processing.

`TAR_BLOCKING_FACTOR'
     Current blocking factor (*note Blocking::).

`TAR_CHECKPOINT'
     Number of the checkpoint.

`TAR_SUBCOMMAND'
     A short option describing the operation `tar' is executing.  *Note
     Operations::, for a complete list of subcommand options.

`TAR_FORMAT'
     Format of the archive being processed. *Note Formats::, for a
     complete list of archive format names.

   Any number of actions can be defined, by supplying several
`--checkpoint-action' options in the command line.  For example, the
command below displays two messages, pauses execution for 30 seconds
and executes the `/sbin/cpoint' script:

     $ tar -c -f arc.tar \
            --checkpoint-action='\aecho=Hit %s checkpoint #%u' \
            --checkpoint-action='echo=Sleeping for 30 seconds' \
            --checkpoint-action='sleep=30' \
            --checkpoint-action='exec=/sbin/cpoint'

   This example also illustrates the fact that `--checkpoint-action'
can be used without `--checkpoint'.  In this case, the default
checkpoint frequency (at each 10th record) is assumed.


File: tar.info,  Node: warnings,  Next: interactive,  Prev: checkpoints,  Up: tar invocation

3.9 Controlling Warning Messages
================================

Sometimes, while performing the requested task, GNU `tar' notices some
conditions that are not exactly errors, but which the user should be
aware of.  When this happens, `tar' issues a "warning message"
describing the condition.  Warning messages are output to the standard
error and they do not affect the exit code of `tar' command.

   GNU `tar' allows the user to suppress some or all of its warning
messages:

`--warning=KEYWORD'
     Control display of the warning messages identified by KEYWORD.  If
     KEYWORD starts with the prefix `no-', such messages are
     suppressed.  Otherwise, they are enabled.

     Multiple `--warning' messages accumulate.

     The tables below list allowed values for KEYWORD along with the
     warning messages they control.

Keywords controlling `tar' operation
------------------------------------

all
     Enable all warning messages.  This is the default.  

none
     Disable all warning messages.  

filename-with-nuls
     `%s: file name read contains nul character' 

alone-zero-block
     `A lone zero block at %s'

Keywords applicable for `tar --create'
--------------------------------------

cachedir
     `%s: contains a cache directory tag %s; %s' 

file-shrank
     `%s: File shrank by %s bytes; padding with zeros' 

xdev
     `%s: file is on a different filesystem; not dumped' 

file-ignored
     `%s: Unknown file type; file ignored'
     `%s: socket ignored'
     `%s: door ignored' 

file-unchanged
     `%s: file is unchanged; not dumped' 

ignore-archive
     `%s: file is the archive; not dumped' 

file-removed
     `%s: File removed before we read it' 

file-changed
     `%s: file changed as we read it'

Keywords applicable for `tar --extract'
---------------------------------------

timestamp
     `%s: implausibly old time stamp %s'
     `%s: time stamp %s is %s s in the future' 

contiguous-cast
     `Extracting contiguous files as regular files' 

symlink-cast
     `Attempting extraction of symbolic links as hard links' 

unknown-cast
     `%s: Unknown file type `%c', extracted as normal file' 

ignore-newer
     `Current %s is newer or same age' 

unknown-keyword
     `Ignoring unknown extended header keyword `%s'' 

decompress-program
     Controls verbose description of failures occurring when trying to
     run alternative decompressor programs (*note alternative
     decompression programs::).  This warning is disabled by default
     (unless `--verbose' is used).  A common example of what you can get
     when using this warning is:

          $ tar --warning=decompress-program -x -f archive.Z
          tar (child): cannot run compress: No such file or directory
          tar (child): trying gzip

     This means that `tar' first tried to decompress `archive.Z' using
     `compress', and, when that failed, switched to `gzip'.

Keywords controlling incremental extraction:
--------------------------------------------

rename-directory
     `%s: Directory has been renamed from %s'
     `%s: Directory has been renamed' 

new-directory
     `%s: Directory is new' 

xdev
     `%s: directory is on a different device: not purging' 

bad-dumpdir
     `Malformed dumpdir: 'X' never used'


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

3.10 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

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

* Menu:

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


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

4.1 Basic GNU `tar' Operations
==============================

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

`--create'
`-c'
     Creating an empty archive would have some kind of elegance.  One
     can initialize an empty archive and later use `--append' (`-r')
     for adding all members.  Some applications would not welcome
     making an exception in the way of adding the first archive member.
     On the other hand, many people reported that it is dangerously too
     easy for `tar' to destroy a magnetic tape with an empty
     archive(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 to each
          other on the QWERTY keyboard.  Instead of being unpacked, the
          archive then gets wholly destroyed.  When users speak about
          "exploding" an archive, they usually mean something else :-).

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

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

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

`--extract'
`--get'
`-x'
     A socket is stored, within a GNU `tar' archive, as a pipe.

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

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


   ---------- 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: create options,  Prev: Basic tar,  Up: operations

4.2 Advanced GNU `tar' Operations
=================================

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

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

* Menu:

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


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

4.2.1 The Five Advanced `tar' Operations
----------------------------------------

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', and the two archive
files you created are `collection.tar' and `music.tar'.

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

   Unless we state otherwise, all practicing you do and examples you
follow in this chapter will take place in the `practice' directory that
you created in the previous chapter; see *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'
`-u'
     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: append,  Next: update,  Prev: Operations,  Up: Advanced tar

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

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

   Other operations don't deal with these members as perfectly as you
might prefer; if you were to use `--extract' to extract the archive,
only the most recently added copy of a member with the same name as
other members would end up in the working directory.  This is because
`--extract' extracts an archive in the order the members appeared in
the archive; the most recently archived members will be extracted last.
Additionally, an extracted member will _replace_ a file of the same
name which existed in the directory already, and `tar' will not prompt
you about this(1).  Thus, only the most recently archived member will
end up being extracted, as it will replace the one extracted before it,
and so on.

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

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

would extract only the second copy.  *Note --occurrence: Option
Summary, for the description of `--occurrence' option.

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

* Menu:

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

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

   (1) Unless you give it `--keep-old-files' option, or the disk copy
is newer than the one in the archive and you invoke `tar' with
`--keep-newer-files' option.


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

4.2.2.1 Appending Files to an Archive
.....................................

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


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

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

You can use `--append' (`-r') to add copies of files which have been
updated since the archive was created.  (However, we do not recommend
doing this since there is another `tar' option called `--update'; *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
replace a file of the same name which was archived earlier, even though
the older version of the file will remain in the archive unless you
delete all versions of the file.

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

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

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

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

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

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

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

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

   *Note Writing::, for more information on `--extract' and see *note
-occurrence: Option Summary, for a description of `--occurrence' option.


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

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

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

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

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

* Menu:

* how to update::


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

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

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

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

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

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

   The reason `tar' does not overwrite the older file when updating it
is because writing to the middle of a section of tape is a difficult
process.  Tapes are not designed to go backward.  *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

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

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

   To use `--concatenate', give the first archive with `--file' option
and name the rest of archives to be concatenated on the command line.
The members, and their member names, will be copied verbatim from those
archives to the first one(1).  The new, concatenated archive will be
called by the same name as the one given with the `--file' option.  As
usual, if you omit `--file', `tar' will use the value of the environment
variable `TAPE', or, if this has not been set, the default archive name.

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

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

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

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

   We can concatenate these two archives with `tar':

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

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

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

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

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

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

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

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

   (1) This can cause multiple members to have the same name.  For
information on how this affects reading the archive, see *note
multiple::.


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

4.2.5 Removing Archive Members Using `--delete'
-----------------------------------------------

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

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

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

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

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

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

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

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

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

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


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

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

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

* Menu:

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


File: tar.info,  Node: override,  Next: Ignore Failed Read,  Up: create options

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

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

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

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

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

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

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

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

     For example:

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

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

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

          $ tar -c -f archive.tar --owner=0 .

     or:

          $ tar -c -f archive.tar --owner=root .

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


File: tar.info,  Node: Ignore Failed Read,  Prev: override,  Up: create options

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

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


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

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

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

4.4.1 Options to Help Read Archives
-----------------------------------

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


File: tar.info,  Node: read full records,  Next: Ignore Zeros,  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,  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 that was created by concatenating several archives
together).

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

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


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

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

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

* Menu:

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


File: tar.info,  Node: Dealing with Old Files,  Next: Overwrite Old Files,  Up: Writing

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

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

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

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

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

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


File: tar.info,  Node: Overwrite Old Files,  Next: Keep Old Files,  Prev: Dealing with Old Files,  Up: Writing

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

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

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

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

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


File: tar.info,  Node: Keep Old Files,  Next: Keep Newer Files,  Prev: Overwrite Old Files,  Up: Writing

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

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


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

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

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


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

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

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


File: tar.info,  Node: Recursive Unlink,  Next: Data 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!_

   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.


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

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

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

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

`--touch'
`-m'
     Sets the data modification time of extracted archive members to
     the time they were extracted, not the time recorded for them in
     the archive.  Use in conjunction with `--extract' (`--get', `-x').


File: tar.info,  Node: Setting Access Permissions,  Next: Directory Modification Times and Permissions,  Prev: Data 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-permissions' in
conjunction with the `--extract' (`--get', `-x') operation.

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


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

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

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

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

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

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

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

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

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

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

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


File: tar.info,  Node: Writing to Standard Output,  Next: Writing to an External Program,  Prev: Directory Modification Times and 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.  Use only in conjunction with
     `--extract' (`--get', `-x').  When this option is used, instead of
     creating the files specified, `tar' writes the contents of the
     files extracted to its standard output.  This may be useful if you
     are only extracting the files in order to send them through a
     pipe.  This option is meaningless with `--list' (`-t').

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

     tar -xOzf foo.tgz bigfile | process

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

     tar -xOzf foo.tgz bigfile1 bigfile2 | process

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


File: tar.info,  Node: Writing to an External Program,  Next: remove files,  Prev: Writing to Standard Output,  Up: Writing

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

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

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

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

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

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

     Currently only regular files are supported.

`TAR_MODE'
     File mode, an octal number.

`TAR_FILENAME'
     The name of the file.

`TAR_REALNAME'
     Name of the file as stored in the archive.

`TAR_UNAME'
     Name of the file owner.

`TAR_GNAME'
     Name of the file owner group.

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

`TAR_MTIME'
     Time of last modification.

`TAR_CTIME'
     Time of last status change.

`TAR_SIZE'
     Size of the file.

`TAR_UID'
     UID of the file owner.

`TAR_GID'
     GID of the file owner.

   Additionally, the following variables contain information about tar
mode and the archive being processed:

`TAR_VERSION'
     GNU `tar' version number.

`TAR_ARCHIVE'
     The name of the archive `tar' is processing.

`TAR_BLOCKING_FACTOR'
     Current blocking factor (*note Blocking::).

`TAR_VOLUME'
     Ordinal number of the volume `tar' is processing.

`TAR_FORMAT'
     Format of the archive being processed. *Note Formats::, for a
     complete list of archive format names.

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

     tar: 2345: Child returned status 1

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

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

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

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


File: tar.info,  Node: remove files,  Prev: Writing to an External Program,  Up: Writing

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

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


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

4.4.3 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,  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 resume the same
`tar' operation.  In this case, `--starting-file' is not necessary.)
See also *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

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

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

   Backup options may prove unexpectedly useful when extracting archives
containing many members having identical name, or when extracting
archives on systems having file name limitations, making different
members appear as having similar names through the side-effect of name
truncation.

   When any existing file is backed up before being overwritten by
extraction, then clashing files are automatically be renamed to be
unique, and the true name is kept for only the last file of a series of
clashing files.  By using verbose mode, users may track exactly what
happens.

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

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

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

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

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

    `t'
    `numbered'
          Always make numbered backups.

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

    `never'
    `simple'
          Always make simple backups.


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



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

4.6 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":

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

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

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

The command also works using long option forms:

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

or

     $ tar --directory sourcedir --create --file=- . \
            | tar --directory targetdir --extract --file=-

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


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

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

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

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

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

GNU `tar' is distributed along with the scripts for performing backups
and restores.  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 from `http://www.amanda.org'.

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

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

* Menu:

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


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

5.1 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 file system.  If files are modified while `tar'
is making the backup, they may not be stored properly in the archive,
in which case you won't be able to restore them if you have to.  (Files
not being modified are written with no trouble, and do not corrupt the
entire archive.)

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

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

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

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

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


File: tar.info,  Node: Incremental Dumps,  Next: Backup Levels,  Prev: Full Dumps,  Up: Backups

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

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

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

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

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

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

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

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

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

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

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

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

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

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

   You can force `level 0' backups either by removing the snapshot file
before running `tar', or by supplying the `--level=0' option, e.g.:

     $ tar --create \
                --file=archive.2.tar \
                --listed-incremental=/var/log/usr.snar-0 \
                --level=0 \
                /usr

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

   Metadata stored in snapshot files include device numbers, which,
obviously are supposed to be non-volatile values.  However, it turns
out that NFS devices have undependable values when an automounter gets
in the picture.  This can lead to a great deal of spurious redumping in
incremental dumps, so it is somewhat useless to compare two NFS devices
numbers over time.  The solution implemented currently is to consider
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.

   Apart from using NFS, there are a number of cases where relying on
device numbers can cause spurious redumping of unmodified files.  For
example, this occurs when archiving LVM snapshot volumes.  To avoid
this, use `--no-check-device' option:

`--no-check-device'
     Do not rely on device numbers when preparing a list of changed
     files for an incremental dump.

`--check-device'
     Use device numbers when preparing a list of changed files for an
     incremental dump.  This is the default behavior.  The purpose of
     this option is to undo the effect of the `--no-check-device' if it
     was given in `TAR_OPTIONS' environment variable (*note
     TAR_OPTIONS::).

   There is also another way to cope with changing device numbers.  It
is described in detail in *note Fixing Snapshot Files::.

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

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

   When extracting from the incremental backup GNU `tar' attempts to
restore the exact state the file system had when the archive was
created.  In particular, it will _delete_ those files in the file
system that did not exist in their directories when the archive was
created.  If you have created several levels of incremental files, then
in order to restore the exact contents the file system  had when the
last level was created, you will need to restore from all backups in
turn.  Continuing our example, to restore the state of `/usr' file
system, one would do(1):

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

   To list the contents of an incremental archive, use `--list' (*note
list::), as usual.  To obtain more information about the archive, use
`--listed-incremental' or `--incremental' combined with two `--verbose'
options(2):

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

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

     X FILE

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

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

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

   (1) Notice, that since both archives were created without `-P'
option (*note absolute::), these commands should be run from the root
file system.

   (2) Two `--verbose' options were selected to avoid breaking usual
verbose listing output (`--list --verbose') when using in scripts.

   Versions of GNU `tar' up to 1.15.1 used to dump verbatim binary
contents of the DUMPDIR header (with terminating nulls) when
`--incremental' or `--listed-incremental' option was given, no matter
what the verbosity level.  This behavior, and, especially, the binary
output it produced were considered inconvenient and were changed in
version 1.16.


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

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

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

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

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

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

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

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

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


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

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

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

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

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

* Menu:

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


File: tar.info,  Node: General-Purpose Variables,  Next: Magnetic Tape Control,  Up: Backup Parameters

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

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

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

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

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

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

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

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

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

 -- Backup variable: DIRLIST
     The name of the file that contains a list of file systems to backup
     or restore.  By default it is `/etc/backup/dirs'.

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

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

 -- Backup variable: FILELIST
     The name of the file that contains a list of individual files to
     backup or restore.  By default it is `/etc/backup/files'.

 -- Backup variable: MT
     Full file name of `mt' binary.

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

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

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

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

     This variable affects only `backup'.

 -- Backup variable: SLEEP_TIME
     Time to sleep between dumps of any two successive file systems

     This variable affects only `backup'.

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


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

 -- Backup variable: TAR
     Full file name of the GNU `tar' executable.  If this is not set,
     backup scripts will search `tar' in the current shell path.


File: tar.info,  Node: Magnetic Tape Control,  Next: User Hooks,  Prev: General-Purpose Variables,  Up: Backup Parameters

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

Backup scripts access tape device using special "hook functions".
These functions take a single argument -- the name of the tape device.
Their names are kept in the following variables:

 -- Backup variable: MT_BEGIN
     The name of "begin" function.  This function is called before
     accessing the drive.  By default it retensions the tape:

          MT_BEGIN=mt_begin

          mt_begin() {
              mt -f "$1" retension
          }

 -- Backup variable: MT_REWIND
     The name of "rewind" function.  The default definition is as
     follows:

          MT_REWIND=mt_rewind

          mt_rewind() {
              mt -f "$1" rewind
          }


 -- Backup variable: MT_OFFLINE
     The name of the function switching the tape off line.  By default
     it is defined as follows:

          MT_OFFLINE=mt_offline

          mt_offline() {
              mt -f "$1" offl
          }

 -- Backup variable: MT_STATUS
     The name of the function used to obtain the status of the archive
     device, including error count.  Default definition:

          MT_STATUS=mt_status

          mt_status() {
              mt -f "$1" status
          }


File: tar.info,  Node: User Hooks,  Next: backup-specs example,  Prev: Magnetic Tape Control,  Up: Backup Parameters

5.4.3 User Hooks
----------------

"User hooks" are shell functions executed before and after each `tar'
invocation.  Thus, there are "backup hooks", which are executed before
and after dumping each file system, and "restore hooks", executed
before and after restoring a file system.  Each user hook is a shell
function taking four arguments:

 -- User Hook Function: hook LEVEL HOST FS FSNAME
     Its arguments are:

    LEVEL
          Current backup or restore level.

    HOST
          Name or IP address of the host machine being dumped or
          restored.

    FS
          Full file name of the file system being dumped or restored.

    FSNAME
          File system name with directory separators replaced with
          colons.  This is useful, e.g., for creating unique files.

   Following variables keep the names of user hook functions:

 -- Backup variable: DUMP_BEGIN
     Dump begin function.  It is executed before dumping the file
     system.

 -- Backup variable: DUMP_END
     Executed after dumping the file system.

 -- Backup variable: RESTORE_BEGIN
     Executed before restoring the file system.

 -- Backup variable: RESTORE_END
     Executed after restoring the file system.


File: tar.info,  Node: backup-specs example,  Prev: User Hooks,  Up: Backup Parameters

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

The following is an example of `backup-specs':

     # site-specific parameters for file system backup.

     ADMINISTRATOR=friedman
     BACKUP_HOUR=1
     TAPE_FILE=/dev/nrsmt0

     # Use `ssh' instead of the less secure `rsh'
     RSH=/usr/bin/ssh
     RSH_COMMAND=/usr/bin/ssh

     # Override MT_STATUS function:
     my_status() {
           mts -t $TAPE_FILE
     }
     MT_STATUS=my_status

     # Disable MT_OFFLINE function
     MT_OFFLINE=:

     BLOCKING=124
     BACKUP_DIRS="
             albert:/fs/fsf
             apple-gunkies:/gd
             albert:/fs/gd2
             albert:/fs/gp
             geech:/usr/jla
             churchy:/usr/roland
             albert:/
             albert:/usr
             apple-gunkies:/
             apple-gunkies:/usr
             gnu:/hack
             gnu:/u
             apple-gunkies:/com/mailer/gnu
             apple-gunkies:/com/archive/gnu"

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


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

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

The syntax for running a backup script is:

     backup --level=LEVEL --time=TIME

   The `--level' option requests the dump level.  Thus, to produce a
full dump, specify `--level=0' (this is the default, so `--level' may
be omitted if its value is `0')(1).

   The `--time' option determines when should the backup be run.  TIME
may take three forms:

HH:MM
     The dump must be run at HH hours MM minutes.

HH
     The dump must be run at HH hours.

now
     The dump must be run immediately.

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

   The backup scripts write two files on the file system.  The first is
a record file in `/etc/tar-backup/', which is used by the scripts to
store and retrieve information about which files were dumped.  This
file is not meant to be read by humans, and should not be deleted by
them.  *Note Snapshot Files::, for a more detailed explanation of this
file.

   The second file is a log file containing the names of the file
systems and files dumped, what time the backup was made, and any error
messages that were generated, as well as how much space was left in the
media volume after the last volume of the archive was written.  You
should check this log file after every backup.  The file name is
`log-MM-DD-YYYY-level-N', where MM-DD-YYYY represents current date, and
N represents current dump level number.

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

   Following is the full list of options accepted by `backup' script:

`-l LEVEL'
`--level=LEVEL'
     Do backup level LEVEL (default 0).

`-f'
`--force'
     Force backup even if today's log file already exists.

`-v[LEVEL]'
`--verbose[=LEVEL]'
     Set verbosity level.  The higher the level is, the more debugging
     information will be output during execution.  Default LEVEL is
     100, which means the highest debugging level.

`-t START-TIME'
`--time=START-TIME'
     Wait till TIME, then do backup.

`-h'
`--help'
     Display short help message and exit.

`-V'
`--version'
     Display information about the program's name, version, origin and
     legal status, all on standard output, and then exit successfully.

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

   (1) For backward compatibility, the `backup' will also try to deduce
the requested dump level from the name of the script itself.  If the
name consists of a string `level-' followed by a single decimal digit,
that digit is taken as the dump level number.  Thus, you may create a
link from `backup' to `level-1' and then run `level-1' whenever you
need to create a level one dump.


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

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

To restore files that were archived using a scripted backup, use the
`restore' script.  Its usage is quite straightforward.  In the simplest
form, invoke `restore --all', it will then restore all the file systems
and files specified in `backup-specs' (*note BACKUP_DIRS:
General-Purpose Variables.).

   You may select the file systems (and/or files) to restore by giving
`restore' a list of "patterns" in its command line.  For example,
running

     restore 'albert:*'

will restore all file systems on the machine `albert'.  A more
complicated example:

     restore 'albert:*' '*:/var'

This command will restore all file systems on the machine `albert' as
well as `/var' file system on all machines.

   By default `restore' will start restoring files from the lowest
available dump level (usually zero) and will continue through all
available dump levels.  There may be situations where such a thorough
restore is not necessary.  For example, you may wish to restore only
files from the recent level one backup.  To do so, use `--level'
option, as shown in the example below:

     restore --level=1

   The full list of options accepted by `restore' follows:

`-a'
`--all'
     Restore all file systems and files specified in `backup-specs'.

`-l LEVEL'
`--level=LEVEL'
     Start restoring from the given backup level, instead of the
     default 0.

`-v[LEVEL]'
`--verbose[=LEVEL]'
     Set verbosity level.  The higher the level is, the more debugging
     information will be output during execution.  Default LEVEL is
     100, which means the highest debugging level.

`-h'
`--help'
     Display short help message and exit.

`-V'
`--version'
     Display information about the program's name, version, origin and
     legal status, all on standard output, and then exit successfully.

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

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

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


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

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

Certain options to `tar' enable you to specify a name for your archive.
Other options let you decide which files to include or exclude from the
archive, based on when or whether files were modified, whether the file
names do or don't match specified patterns, or whether files are in
specified directories.

   This chapter discusses these options in detail.

* Menu:

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


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

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

By default, `tar' uses an archive file name that was compiled when it
was built on the system; usually this name refers to some physical tape
drive on the machine.  However, the person who installed `tar' on the
system may not have set the default to a meaningful value as far as
most users are concerned.  As a result, you will usually want to tell
`tar' where to find (or create) the archive.  The `--file=ARCHIVE-NAME'
(`-f ARCHIVE-NAME') option allows you to either specify or name a file
to use as the archive instead of the default archive file location.

`--file=ARCHIVE-NAME'
`-f ARCHIVE-NAME'
     Name the archive to create or operate on.  Use in conjunction with
     any operation.

   For example, in this `tar' command,

     $ tar -cvf collection.tar blues folk jazz

`collection.tar' is the name of the archive.  It must directly follow
the `-f' option, since whatever directly follows `-f' _will_ end up
naming the archive.  If you neglect to specify an archive name, you may
end up overwriting a file in the working directory with the archive you
create since `tar' will use this file's name for the archive name.

   An archive can be saved as a file in the file system, sent through a
pipe or over a network, or written to an I/O device such as a tape,
floppy disk, or CD write drive.

   If you do not name the archive, `tar' uses the value of the
environment variable `TAPE' as the file name for the archive.  If that
is not available, `tar' uses a default, compiled-in archive name,
usually that for tape unit zero (i.e., `/dev/tu00').

   If you use `-' as an ARCHIVE-NAME, `tar' reads the archive from
standard input (when listing or extracting files), or writes it to
standard output (when creating an archive).  If you use `-' as an
ARCHIVE-NAME when modifying an archive, `tar' reads the original
archive from its standard input and writes the entire new archive to
its standard output.

   The following example is a convenient way of copying directory
hierarchy from `sourcedir' to `targetdir'.

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

   The `-C' option allows to avoid using subshells:

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

   In both examples above, the leftmost `tar' invocation archives the
contents of `sourcedir' to the standard output, while the rightmost one
reads this archive from its standard input and extracts it.  The `-p'
option tells it to restore permissions of the extracted files.

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

     --file=HOSTNAME:/DEV/FILE-NAME

`tar' will set up the remote connection, if possible, and prompt you
for a username and password.  If you use
`--file=@HOSTNAME:/DEV/FILE-NAME', `tar' will attempt to set up the
remote connection using your username as the username on the remote
machine.

   If the archive file name includes a colon (`:'), then it is assumed
to be a file on another machine.  If the archive file is
`USER@HOST:FILE', then FILE is used on the host HOST.  The remote host
is accessed using the `rsh' program, with a username of USER.  If the
username is omitted (along with the `@' sign), then your user name will
be used.  (This is the normal `rsh' behavior.)  It is necessary for the
remote machine, in addition to permitting your `rsh' access, to have
the `rmt' program installed (this command is included in the GNU `tar'
distribution and by default is installed under `PREFIX/libexec/rmt',
where PREFIX means your installation prefix).  If you need to use a
file whose name includes a colon, then the remote tape drive behavior
can be inhibited by using the `--force-local' option.

   When the archive is being created to `/dev/null', GNU `tar' tries to
minimize input and output operations.  The Amanda backup system, when
used with GNU `tar', has an initial sizing pass which uses this feature.


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

6.2 Selecting Archive Members
=============================

"File Name arguments" specify which files in the file system `tar'
operates on, when creating or adding to an archive, or which archive
members `tar' operates on, when reading or deleting from an archive.
*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 a file name begins with dash (`-'), precede it with `--add-file'
option to prevent it from being treated as an option.

   By default GNU `tar' attempts to "unquote" each file or member name,
replacing "escape sequences" according to the following table:

Escape         Replaced with
----------------------------------------------------------- 
\a             Audible bell (ASCII 7)
\b             Backspace (ASCII 8)
\f             Form feed (ASCII 12)
\n             New line (ASCII 10)
\r             Carriage return (ASCII 13)
\t             Horizontal tabulation (ASCII 9)
\v             Vertical tabulation (ASCII 11)
\?             ASCII 127
\N             ASCII N (N should be an octal number
                         of up to 3 digits)

   A backslash followed by any other symbol is retained.

   This default behavior is controlled by the following command line
option:

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

`--no-unquote'
     Disable unquoting input file or member names.

   If you specify a directory name as a file name argument, all the
files in that directory are operated on by `tar'.

   If you do not specify files, `tar' behavior differs depending on the
operation mode as described below:

   When `tar' is invoked with `--create' (`-c'), `tar' will stop
immediately, reporting the following:

     $ tar cf a.tar
     tar: Cowardly refusing to create an empty archive
     Try `tar --help' or `tar --usage' for more information.

   If you specify either `--list' (`-t') or `--extract' (`--get',
`-x'), `tar' operates on all the archive members in the archive.

   If run with `--diff' option, tar will compare the archive with the
contents of the current working directory.

   If you specify any other operation, `tar' does nothing.

   By default, `tar' takes file names from the command line.  However,
there are other ways to specify file or member names, or to modify the
manner in which `tar' selects the files or members upon which to
operate.  In general, these methods work both for specifying the names
of files and archive members.


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

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

Instead of giving the names of files or archive members on the command
line, you can put the names into a file, and then use the
`--files-from=FILE-OF-NAMES' (`-T FILE-OF-NAMES') option to `tar'.
Give the name of the file which contains the list of files to include
as the argument to `--files-from'.  In the list, the file names should
be separated by newlines.  You will frequently use this option when you
have generated the list of files to archive with the `find' utility.

`--files-from=FILE-NAME'
`-T FILE-NAME'
     Get names to extract or create from file FILE-NAME.

   If you give a single dash as a file name for `--files-from', (i.e.,
you specify either `--files-from=-' or `-T -'), then the file names are
read from standard input.

   Unless you are running `tar' with `--create', you can not use both
`--files-from=-' and `--file=-' (`-f -') in the same command.

   Any number of `-T' options can be given in the command line.

   The following example shows how to use `find' to generate a list of
files smaller than 400K in length and put that list into a file called
`small-files'.  You can then use the `-T' option to `tar' to specify
the files from that file, `small-files', to create the archive
`little.tgz'.  (The `-z' option to `tar' compresses the archive with
`gzip'; *note gzip:: for more information.)

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

In the file list given by `-T' option, any file name beginning with `-'
character is considered a `tar' option and is processed accordingly(1).
For example, the common use of this feature is to change to another
directory by specifying `-C' option:

     $ cat list
     -C/etc
     passwd
     hosts
     -C/lib
     libc.a
     $ tar -c -f foo.tar --files-from list

In this example, `tar' will first switch to `/etc' directory and add
files `passwd' and `hosts' to the archive.  Then it will change to
`/lib' directory and will archive the file `libc.a'.  Thus, the
resulting archive `foo.tar' will contain:

     $ tar tf foo.tar
     passwd
     hosts
     libc.a


   Notice that the option parsing algorithm used with `-T' is stricter
than the one used by shell.  Namely, when specifying option arguments,
you should observe the following rules:

   * When using short (single-letter) option form, its argument must
     immediately follow the option letter, without any intervening
     whitespace.  For example: `-Cdir'.

   * When using long option form, the option argument must be separated
     from the option by a single equal sign.  No whitespace is allowed
     on any side of the equal sign.  For example: `--directory=dir'.

   * For both short and long option forms, the option argument can be
     given on the next line after the option name, e.g.:

          --directory
          dir

     and

          -C
          dir

   If you happen to have a file whose name starts with `-', precede it
with `--add-file' option to prevent it from being recognized as an
option.  For example: `--add-file=--my-file'.

* Menu:

* nul::

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

   (1) Versions of GNU `tar' up to 1.15.1 recognized only `-C' option
in file lists, and only if the option and its argument occupied two
consecutive lines.


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

6.3.1 `NUL'-Terminated File Names
---------------------------------

The `--null' option causes `--files-from=FILE-OF-NAMES' (`-T
FILE-OF-NAMES') to read file names terminated by a `NUL' instead of a
newline, so files whose names contain newlines can be archived using
`--files-from'.

`--null'
     Only consider `NUL'-terminated file names, instead of files that
     terminate in a newline.

`--no-null'
     Undo the effect of any previous `--null' option.

   The `--null' option is just like the one in GNU `xargs' and `cpio',
and is useful with the `-print0' predicate of GNU `find'.  In `tar',
`--null' also disables special handling for file names that begin with
dash.

   This example shows how to use `find' to generate a list of files
larger than 800K in length and put that list into a file called
`long-files'.  The `-print0' option to `find' is just like `-print',
except that it separates files with a `NUL' rather than with a newline.
You can then run `tar' with both the `--null' and `-T' options to
specify that `tar' gets 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

   The `--no-null' option can be used if you need to read both
`NUL'-terminated and newline-terminated files on the same command line.
For example, if `flist' is a newline-terminated file, then the
following command can be used to combine it with the above command:

     $ find . -size +800 -print0 |
       tar -c -f big.tar --null -T - --no-null -T flist

   This example uses short options for typographic reasons, to avoid
very long lines.

   GNU `tar' is able to automatically detect `NUL'-terminated file
lists, so it is safe to use them even without the `--null' option.  In
this case `tar' will print a warning and continue reading such a file
as if `--null' were actually given:

     $ find . -size +800 -print0 | tar -c -f big.tar -T -
     tar: -: file name read contains nul character

   The null terminator, however, remains in effect only for this
particular file, any following `-T' options will assume newline
termination.  Of course, the null autodetection applies to these
eventual surplus `-T' options as well.


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

6.4 Excluding Some Files
========================

To avoid operating on files whose names match a particular pattern, use
the `--exclude' or `--exclude-from' options.

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

   The `--exclude=PATTERN' option prevents any file or member whose
name matches the shell wildcard (PATTERN) from being operated on.  For
example, to create an archive with all the contents of the directory
`src' except for files whose names end in `.o', use the command `tar
-cf src.tar --exclude='*.o' src'.

   You may give multiple `--exclude' options.

`--exclude-from=FILE'
`-X FILE'
     Causes `tar' to ignore files that match the patterns listed in
     FILE.

   Use the `--exclude-from' option to read a list of patterns, one per
line, from FILE; `tar' will ignore files matching those patterns.  Thus
if `tar' is called as `tar -c -X foo .' and the file `foo' contains a
single line `*.o', no files whose names end in `.o' will be added to
the archive.

   Notice, that lines from FILE are read verbatim. One of the frequent
errors is leaving some extra whitespace after a file name, which is
difficult to catch using text editors.

   However, empty lines are OK.

`--exclude-vcs'
     Exclude files and directories used by following version control
     systems: `CVS', `RCS', `SCCS', `SVN', `Arch', `Bazaar',
     `Mercurial', and `Darcs'.

     As of version 1.26, the following files are excluded:

        * `CVS/', and everything under it

        * `RCS/', and everything under it

        * `SCCS/', and everything under it

        * `.git/', and everything under it

        * `.gitignore'

        * `.cvsignore'

        * `.svn/', and everything under it

        * `.arch-ids/', and everything under it

        * `{arch}/', and everything under it

        * `=RELEASE-ID'

        * `=meta-update'

        * `=update'

        * `.bzr'

        * `.bzrignore'

        * `.bzrtags'

        * `.hg'

        * `.hgignore'

        * `.hgrags'

        * `_darcs'

`--exclude-backups'
     Exclude backup and lock files.  This option causes exclusion of
     files that match the following shell globbing patterns:

    .#*

    *~

    #*#


   When creating an archive, the `--exclude-caches' option family
causes `tar' to exclude all directories that contain a "cache directory
tag". A cache directory tag is a short file with the well-known name
`CACHEDIR.TAG' and having a standard header specified in
`http://www.brynosaurus.com/cachedir/spec.html'.  Various applications
write cache directory tags into directories they use to hold
regenerable, non-precious data, so that such data can be more easily
excluded from backups.

   There are three `exclude-caches' options, each providing a different
exclusion semantics:

`--exclude-caches'
     Do not archive the contents of the directory, but archive the
     directory itself and the `CACHEDIR.TAG' file.

`--exclude-caches-under'
     Do not archive the contents of the directory, nor the
     `CACHEDIR.TAG' file, archive only the directory itself.

`--exclude-caches-all'
     Omit directories containing `CACHEDIR.TAG' file entirely.

   Another option family, `--exclude-tag', provides a generalization of
this concept.  It takes a single argument, a file name to look for.
Any directory that contains this file will be excluded from the dump.
Similarly to `exclude-caches', there are three options in this option
family:

`--exclude-tag=FILE'
     Do not dump the contents of the directory, but dump the directory
     itself and the FILE.

`--exclude-tag-under=FILE'
     Do not dump the contents of the directory, nor the FILE, archive
     only the directory itself.

`--exclude-tag-all=FILE'
     Omit directories containing FILE file entirely.

   Multiple `--exclude-tag*' options can be given.

   For example, given this directory:

     $ find dir
     dir
     dir/blues
     dir/jazz
     dir/folk
     dir/folk/tagfile
     dir/folk/sanjuan
     dir/folk/trote

   The `--exclude-tag' will produce the following:

     $ tar -cf archive.tar --exclude-tag=tagfile -v dir
     dir/
     dir/blues
     dir/jazz
     dir/folk/
     tar: dir/folk/: contains a cache directory tag tagfile;
       contents not dumped
     dir/folk/tagfile

   Both the `dir/folk' directory and its tagfile are preserved in the
archive, however the rest of files in this directory are not.

   Now, using the `--exclude-tag-under' option will exclude `tagfile'
from the dump, while still preserving the directory itself, as shown in
this example:

     $ tar -cf archive.tar --exclude-tag-under=tagfile -v dir
     dir/
     dir/blues
     dir/jazz
     dir/folk/
     ./tar: dir/folk/: contains a cache directory tag tagfile;
       contents not dumped

   Finally, using `--exclude-tag-all' omits the `dir/folk' directory
entirely:

     $ tar -cf archive.tar --exclude-tag-all=tagfile -v dir
     dir/
     dir/blues
     dir/jazz
     ./tar: dir/folk/: contains a cache directory tag tagfile;
       directory not dumped

* Menu:

* problems with exclude::


File: tar.info,  Node: problems with 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' does not act on a file name
     explicitly listed on the command line, if one of its file name
     components is excluded.  In the example above, if you create an
     archive and exclude files that end with `*.o', but explicitly name
     the file `dir.o/foo' after all the options have been listed,
     `dir.o/foo' will be excluded from the archive.

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

   * When you use `--exclude=PATTERN', be sure to quote the PATTERN
     parameter, so GNU `tar' sees wildcard characters like `*'.  If you
     do not do this, the shell might expand the `*' itself using files
     at hand, so `tar' might receive a list of files instead of one
     pattern, or none at all, making the command somewhat illegal.
     This might not correspond to what you want.

     For example, write:

          $ tar -c -f ARCHIVE.TAR --exclude '*.o' DIRECTORY

     rather than:

          # _Wrong!_
          $ tar -c -f ARCHIVE.TAR --exclude *.o DIRECTORY

   * You must use use shell syntax, or globbing, rather than `regexp'
     syntax, when using exclude options in `tar'.  If you try to use
     `regexp' syntax to describe files to be excluded, your command
     might fail.

   *  In earlier versions of `tar', what is now the `--exclude-from'
     option was called `--exclude' instead.  Now, `--exclude' applies
     to patterns listed on the command line and `--exclude-from'
     applies to patterns listed in a file.



File: tar.info,  Node: wildcards,  Next: quoting styles,  Prev: exclude,  Up: Choosing

6.5 Wildcards Patterns and Matching
===================================

"Globbing" is the operation by which "wildcard" characters, `*' or `?'
for example, are replaced and expanded into all existing files matching
the given pattern.  GNU `tar' can use wildcard patterns for matching
(or globbing) archive members when extracting from or listing an
archive.  Wildcard patterns are also used for verifying volume labels
of `tar' archives.  This section has the purpose of explaining wildcard
syntax for `tar'.

   A PATTERN should be written according to shell syntax, using wildcard
characters to effect globbing.  Most characters in the pattern stand
for themselves in the matched string, and case is significant: `a' will
match only `a', and not `A'.  The character `?' in the pattern matches
any single character in the matched string.  The character `*' in the
pattern matches zero, one, or more single characters in the matched
string.  The character `\' says to take the following character of the
pattern _literally_; it is useful when one needs to match the `?', `*',
`[' or `\' characters, themselves.

   The character `[', up to the matching `]', introduces a character
class.  A "character class" is a list of acceptable characters for the
next single character of the matched string.  For example, `[abcde]'
would match any of the first five letters of the alphabet.  Note that
within a character class, all of the "special characters" listed above
other than `\' lose their special meaning; for example, `[-\\[*?]]'
would match any of the characters, `-', `\', `[', `*', `?', or `]'.
(Due to parsing constraints, the characters `-' and `]' must either
come _first_ or _last_ in a character class.)

   If the first character of the class after the opening `[' is `!' or
`^', then the meaning of the class is reversed.  Rather than listing
character to match, it lists those characters which are _forbidden_ as
the next single character of the matched string.

   Other characters of the class stand for themselves.  The special
construction `[A-E]', using an hyphen between two letters, is meant to
represent all characters between A and E, inclusive.

   Periods (`.') or forward slashes (`/') are not considered special
for wildcard matches.  However, if a pattern completely matches a
directory prefix of a matched string, then it matches the full matched
string: thus, excluding a directory also excludes all the files beneath
it.

* Menu:

* controlling pattern-matching::


File: tar.info,  Node: controlling pattern-matching,  Up: wildcards

Controlling Pattern-Matching
----------------------------

For the purposes of this section, we call "exclusion members" all
member names obtained while processing `--exclude' and `--exclude-from'
options, and "inclusion members" those member names that were given in
the command line or read from the file specified with `--files-from'
option.

   These two pairs of member lists are used in the following operations:
`--diff', `--extract', `--list', `--update'.

   There are no inclusion members in create mode (`--create' and
`--append'), since in this mode the names obtained from the command
line refer to _files_, not archive members.

   By default, inclusion members are compared with archive members
literally (1) and exclusion members are treated as globbing patterns.
For example:

     $ tar tf foo.tar
     a.c
     b.c
     a.txt
     [remarks]
     # Member names are used verbatim:
     $ tar -xf foo.tar -v '[remarks]'
     [remarks]
     # Exclude member names are globbed:
     $ tar -xf foo.tar -v --exclude '*.c'
     a.txt
     [remarks]

   This behavior can be altered by using the following options:

`--wildcards'
     Treat all member names as wildcards.

`--no-wildcards'
     Treat all member names as literal strings.

   Thus, to extract files whose names end in `.c', you can use:

     $ tar -xf foo.tar -v --wildcards '*.c'
     a.c
     b.c

Notice quoting of the pattern to prevent the shell from interpreting it.

   The effect of `--wildcards' option is canceled by `--no-wildcards'.
This can be used to pass part of the command line arguments verbatim
and other part as globbing patterns.  For example, the following
invocation:

     $ tar -xf foo.tar --wildcards '*.txt' --no-wildcards '[remarks]'

instructs `tar' to extract from `foo.tar' all files whose names end in
`.txt' and the file named `[remarks]'.

   Normally, a pattern matches a name if an initial subsequence of the
name's components matches the pattern, where `*', `?', and `[...]' are
the usual shell wildcards, `\' escapes wildcards, and wildcards can
match `/'.

   Other than optionally stripping leading `/' from names (*note
absolute::), patterns and names are used as-is.  For example, trailing
`/' is not trimmed from a user-specified name before deciding whether
to exclude it.

   However, this matching procedure can be altered by the options listed
below.  These options accumulate.  For example:

     --ignore-case --exclude='makefile' --no-ignore-case ---exclude='readme'

ignores case when excluding `makefile', but not when excluding `readme'.

`--anchored'
`--no-anchored'
     If anchored, a pattern must match an initial subsequence of the
     name's components.  Otherwise, the pattern can match any
     subsequence.  Default is `--no-anchored' for exclusion members and
     `--anchored' inclusion members.

`--ignore-case'
`--no-ignore-case'
     When ignoring case, upper-case patterns match lower-case names and
     vice versa.  When not ignoring case (the default), matching is
     case-sensitive.

`--wildcards-match-slash'
`--no-wildcards-match-slash'
     When wildcards match slash (the default for exclusion members), a
     wildcard like `*' in the pattern can match a `/' in the name.
     Otherwise, `/' is matched only by `/'.


   The `--recursion' and `--no-recursion' options (*note recurse::)
also affect how member patterns are interpreted.  If recursion is in
effect, a pattern matches a name if it matches any of the name's parent
directories.

   The following table summarizes pattern-matching default values:

Members                Default settings
-------------------------------------------------------------------------- 
Inclusion              `--no-wildcards --anchored
                       --no-wildcards-match-slash'
Exclusion              `--wildcards --no-anchored
                       --wildcards-match-slash'

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

   (1) Notice that earlier GNU `tar' versions used globbing for
inclusion members, which contradicted to UNIX98 specification and was
not documented. *Note Changes::, for more information on this and other
changes.


File: tar.info,  Node: quoting styles,  Next: transform,  Prev: wildcards,  Up: Choosing

6.6 Quoting Member Names
========================

When displaying member names, `tar' takes care to avoid ambiguities
caused by certain characters.  This is called "name quoting".  The
characters in question are:

   * Non-printable control characters:
     Character     ASCII   Character name
     --------------------------------------------------------------- 
     \a            7       Audible bell
     \b            8       Backspace
     \f            12      Form feed
     \n            10      New line
     \r            13      Carriage return
     \t            9       Horizontal tabulation
     \v            11      Vertical tabulation

   * Space (ASCII 32)

   * Single and double quotes (`'' and `"')

   * Backslash (`\')

   The exact way `tar' uses to quote these characters depends on the
"quoting style".  The default quoting style, called "escape" (see
below), uses backslash notation to represent control characters, space
and backslash.  Using this quoting style, control characters are
represented as listed in column `Character' in the above table, a space
is printed as `\ ' and a backslash as `\\'.

   GNU `tar' offers seven distinct quoting styles, which can be selected
using `--quoting-style' option:

`--quoting-style=STYLE'
     Sets quoting style.  Valid values for STYLE argument are: literal,
     shell, shell-always, c, escape, locale, clocale.

   These styles are described in detail below.  To illustrate their
effect, we will use an imaginary tar archive `arch.tar' containing the
following members:

     # 1. Contains horizontal tabulation character.
     a       tab
     # 2. Contains newline character
     a
     newline
     # 3. Contains a space
     a space
     # 4. Contains double quotes
     a"double"quote
     # 5. Contains single quotes
     a'single'quote
     # 6. Contains a backslash character:
     a\backslash

   Here is how usual `ls' command would have listed them, if they had
existed in the current working directory:

     $ ls
     a\ttab
     a\nnewline
     a\ space
     a"double"quote
     a'single'quote
     a\\backslash

   Quoting styles:

`literal'
     No quoting, display each character as is:

          $ tar tf arch.tar --quoting-style=literal
          ./
          ./a space
          ./a'single'quote
          ./a"double"quote
          ./a\backslash
          ./a     tab
          ./a
          newline

`shell'
     Display characters the same way Bourne shell does: control
     characters, except `\t' and `\n', are printed using backslash
     escapes, `\t' and `\n' are printed as is, and a single quote is
     printed as `\''.  If a name contains any quoted characters, it is
     enclosed in single quotes.  In particular, if a name contains
     single quotes, it is printed as several single-quoted strings:

          $ tar tf arch.tar --quoting-style=shell
          ./
          './a space'
          './a'\''single'\''quote'
          './a"double"quote'
          './a\backslash'
          './a    tab'
          './a
          newline'

`shell-always'
     Same as `shell', but the names are always enclosed in single
     quotes:

          $ tar tf arch.tar --quoting-style=shell-always
          './'
          './a space'
          './a'\''single'\''quote'
          './a"double"quote'
          './a\backslash'
          './a    tab'
          './a
          newline'

`c'
     Use the notation of the C programming language.  All names are
     enclosed in double quotes.  Control characters are quoted using
     backslash notations, double quotes are represented as `\"',
     backslash characters are represented as `\\'.  Single quotes and
     spaces are not quoted:

          $ tar tf arch.tar --quoting-style=c
          "./"
          "./a space"
          "./a'single'quote"
          "./a\"double\"quote"
          "./a\\backslash"
          "./a\ttab"
          "./a\nnewline"

`escape'
     Control characters are printed using backslash notation, a space is
     printed as `\ ' and a backslash as `\\'.  This is the default
     quoting style, unless it was changed when configured the package.

          $ tar tf arch.tar --quoting-style=escape
          ./
          ./a space
          ./a'single'quote
          ./a"double"quote
          ./a\\backslash
          ./a\ttab
          ./a\nnewline

`locale'
     Control characters, single quote and backslash are printed using
     backslash notation.  All names are quoted using left and right
     quotation marks, appropriate to the current locale.  If it does not
     define quotation marks, use ``' as left and `'' as right quotation
     marks.  Any occurrences of the right quotation mark in a name are
     escaped with `\', for example:

     For example:

          $ tar tf arch.tar --quoting-style=locale
          `./'
          `./a space'
          `./a\'single\'quote'
          `./a"double"quote'
          `./a\\backslash'
          `./a\ttab'
          `./a\nnewline'

`clocale'
     Same as `locale', but `"' is used for both left and right
     quotation marks, if not provided by the currently selected locale:

          $ tar tf arch.tar --quoting-style=clocale
          "./"
          "./a space"
          "./a'single'quote"
          "./a\"double\"quote"
          "./a\\backslash"
          "./a\ttab"
          "./a\nnewline"

   You can specify which characters should be quoted in addition to
those implied by the current quoting style:

`--quote-chars=STRING'
     Always quote characters from STRING, even if the selected quoting
     style would not quote them.

   For example, using `escape' quoting (compare with the usual escape
listing above):

     $ tar tf arch.tar --quoting-style=escape --quote-chars=' "'
     ./
     ./a\ space
     ./a'single'quote
     ./a\"double\"quote
     ./a\\backslash
     ./a\ttab
     ./a\nnewline

   To disable quoting of such additional characters, use the following
option:

`--no-quote-chars=STRING'
     Remove characters listed in STRING from the list of quoted
     characters set by the previous `--quote-chars' option.

   This option is particularly useful if you have added `--quote-chars'
to your `TAR_OPTIONS' (*note TAR_OPTIONS::) and wish to disable it for
the current invocation.

   Note, that `--no-quote-chars' does _not_ disable those characters
that are quoted by default in the selected quoting style.


File: tar.info,  Node: transform,  Next: after,  Prev: quoting styles,  Up: Choosing

6.7 Modifying File and Member Names
===================================

`Tar' archives contain detailed information about files stored in them
and full file names are part of that information.  When storing a file
to an archive, its file name is recorded in it, along with the actual
file contents.  When restoring from an archive, a file is created on
disk with exactly the same name as that stored in the archive.  In the
majority of cases this is the desired behavior of a file archiver.
However, there are some cases when it is not.

   First of all, it is often unsafe to extract archive members with
absolute file names or those that begin with a `../'.  GNU `tar' takes
special precautions when extracting such names and provides a special
option for handling them, which is described in *note absolute::.

   Secondly, you may wish to extract file names without some leading
directory components, or with otherwise modified names.  In other cases
it is desirable to store files under differing names in the archive.

   GNU `tar' provides several options for these needs.

`--strip-components=NUMBER'
     Strip given NUMBER of leading components from file names before
     extraction.

   For example, suppose you have archived whole `/usr' hierarchy to a
tar archive named `usr.tar'.  Among other files, this archive contains
`usr/include/stdlib.h', which you wish to extract to the current
working directory.  To do so, you type:

     $ tar -xf usr.tar --strip=2 usr/include/stdlib.h

   The option `--strip=2' instructs `tar' to strip the two leading
components (`usr/' and `include/') off the file name.

   If you add the `--verbose' (`-v') option to the invocation above,
you will note that the verbose listing still contains the full file
name, with the two removed components still in place.  This can be
inconvenient, so `tar' provides a special option for altering this
behavior:

`--show-transformed-names'
     Display file or member names with all requested transformations
     applied.

For example:

     $ tar -xf usr.tar -v --strip=2 usr/include/stdlib.h
     usr/include/stdlib.h
     $ tar -xf usr.tar -v --strip=2 --show-transformed usr/include/stdlib.h
     stdlib.h

   Notice that in both cases the file `stdlib.h' is extracted to the
current working directory, `--show-transformed-names' affects only the
way its name is displayed.

   This option is especially useful for verifying whether the invocation
will have the desired effect.  Thus, before running

     $ tar -x --strip=N

it is often advisable to run

     $ tar -t -v --show-transformed --strip=N

to make sure the command will produce the intended results.

   In case you need to apply more complex modifications to the file
name, GNU `tar' provides a general-purpose transformation option:

`--transform=EXPRESSION'
`--xform=EXPRESSION'
     Modify file names using supplied EXPRESSION.

The EXPRESSION is a `sed'-like replace expression of the form:

     s/REGEXP/REPLACE/[FLAGS]

where REGEXP is a "regular expression", REPLACE is a replacement for
each file name part that matches REGEXP.  Both REGEXP and REPLACE are
described in detail in *note The "s" Command: (sed)The "s" Command.

   Any delimiter can be used in lieu of `/', the only requirement being
that it be used consistently throughout the expression. For example,
the following two expressions are equivalent:

     s/one/two/
     s,one,two,

   Changing delimiters is often useful when the REGEX contains slashes.
For example, it is more convenient to write `s,/,-,' than `s/\//-/'.

   As in `sed', you can give several replace expressions, separated by
a semicolon.

   Supported FLAGS are:

`g'
     Apply the replacement to _all_ matches to the REGEXP, not just the
     first.

`i'
     Use case-insensitive matching.

`x'
     REGEXP is an "extended regular expression" (*note Extended regular
     expressions: (sed)Extended regexps.).

`NUMBER'
     Only replace the NUMBERth match of the REGEXP.

     Note: the POSIX standard does not specify what should happen when
     you mix the `g' and NUMBER modifiers.  GNU `tar' follows the GNU
     `sed' implementation in this regard, so the interaction is defined
     to be: ignore matches before the NUMBERth, and then match and
     replace all matches from the NUMBERth on.


   In addition, several "transformation scope" flags are supported,
that control to what files transformations apply.  These are:

`r'
     Apply transformation to regular archive members.

`R'
     Do not apply transformation to regular archive members.

`s'
     Apply transformation to symbolic link targets.

`S'
     Do not apply transformation to symbolic link targets.

`h'
     Apply transformation to hard link targets.

`H'
     Do not apply transformation to hard link targets.

   Default is `rsh', which means to apply tranformations to both archive
members and targets of symbolic and hard links.

   Default scope flags can also be changed using `flags=' statement in
the transform expression.  The flags set this way remain in force until
next `flags=' statement or end of expression, whichever occurs first.
For example:

       --transform 'flags=S;s|^|/usr/local/|'

   Here are several examples of `--transform' usage:

  1. Extract `usr/' hierarchy into `usr/local/':

          $ tar --transform='s,usr/,usr/local/,' -x -f arch.tar

  2. Strip two leading directory components (equivalent to
     `--strip-components=2'):

          $ tar --transform='s,/*[^/]*/[^/]*/,,' -x -f arch.tar

  3. Convert each file name to lower case:

          $ tar --transform 's/.*/\L&/' -x -f arch.tar

  4. Prepend `/prefix/'  to each file name:

          $ tar --transform 's,^,/prefix/,' -x -f arch.tar

  5. Archive the `/lib' directory, prepending `/usr/local' to each
     archive member:

          $ tar --transform 's,^,/usr/local/,S' -c -f arch.tar /lib

   Notice the use of flags in the last example.  The `/lib' directory
often contains many symbolic links to files within it.  It may look,
for example, like this:

     $ ls -l
     drwxr-xr-x root/root       0 2008-07-08 16:20 /lib/
     -rwxr-xr-x root/root 1250840 2008-05-25 07:44 /lib/libc-2.3.2.so
     lrwxrwxrwx root/root       0 2008-06-24 17:12 /lib/libc.so.6 -> libc-2.3.2.so
     ...

   Using the expression `s,^,/usr/local/,' would mean adding
`/usr/local' to both regular archive members and to link targets. In
this case, `/lib/libc.so.6' would become:

       /usr/local/lib/libc.so.6 -> /usr/local/libc-2.3.2.so

   This is definitely not desired.  To avoid this, the `S' flag is
used, which excludes symbolic link targets from filename
transformations.  The result is:

     $ tar --transform 's,^,/usr/local/,S', -c -v -f arch.tar \
            --show-transformed /lib
     drwxr-xr-x root/root       0 2008-07-08 16:20 /usr/local/lib/
     -rwxr-xr-x root/root 1250840 2008-05-25 07:44 /usr/local/lib/libc-2.3.2.so
     lrwxrwxrwx root/root       0 2008-06-24 17:12 /usr/local/lib/libc.so.6 \
      -> libc-2.3.2.so

   Unlike `--strip-components', `--transform' can be used in any GNU
`tar' operation mode.  For example, the following command adds files to
the archive while replacing the leading `usr/' component with `var/':

     $ tar -cf arch.tar --transform='s,^usr/,var/,' /

   To test `--transform' effect we suggest using
`--show-transformed-names' option:

     $ tar -cf arch.tar --transform='s,^usr/,var/,' \
            --verbose --show-transformed-names /

   If both `--strip-components' and `--transform' are used together,
then `--transform' is applied first, and the required number of
components is then stripped from its result.

   You can use as many `--transform' options in a single command line
as you want.  The specified expressions will then be applied in order
of their appearance.  For example, the following two invocations are
equivalent:

     $ tar -cf arch.tar --transform='s,/usr/var,/var/' \
                             --transform='s,/usr/local,/usr/,'
     $ tar -cf arch.tar \
                    --transform='s,/usr/var,/var/;s,/usr/local,/usr/,'


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

6.8 Operating Only on New Files
===============================

The `--after-date=DATE' (`--newer=DATE', `-N DATE') option causes `tar'
to only work on files whose data modification or status change times
are newer than the DATE given.  If DATE starts with `/' or `.', it is
taken to be a file name; the data modification time of that file is
used as the date. If you use this option when creating or appending to
an archive, the archive will only include new files.  If you use
`--after-date' when extracting an archive, `tar' will only extract
files newer than the DATE you specify.

   If you only want `tar' to make the date comparison based on
modification of the file's data (rather than status changes), then use
the `--newer-mtime=DATE' option.

   You may use these options with any operation.  Note that these
options differ from the `--update' (`-u') operation in that they allow
you to specify a particular date against which `tar' can compare when
deciding whether or not to archive the files.

`--after-date=DATE'
`--newer=DATE'
`-N DATE'
     Only store files newer than DATE.

     Acts on files only if their data modification or status change
     times are later than DATE.  Use in conjunction with any operation.

     If DATE starts with `/' or `.', it is taken to be a file name; the
     data modification time of that file is used as the date.

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

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

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

   To be precise, `--after-date' checks _both_ `mtime' and `ctime' and
processes the file if either one is more recent than DATE, while
`--newer-mtime' only checks `mtime' and disregards `ctime'.  Neither
does it use `atime' (the last time the contents of the file were looked
at).

   Date specifiers can have embedded spaces.  Because of this, you may
need to quote date arguments to keep the shell from parsing them as
separate arguments.  For example, the following command will add to the
archive all the files modified less than two days ago:

     $ tar -cf foo.tar --newer-mtime '2 days ago'

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

     $ tar -c -f archive.tar --after-date='10 days ago' .
     tar: Option --after-date: Treating date `10 days ago' as 2006-06-11
     13:19:37.232434

     *Please Note:* `--after-date' and `--newer-mtime' should not be
     used for incremental backups.  *Note Incremental Dumps::, for
     proper way of creating incremental backups.


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

6.9 Descending into Directories
===============================

Usually, `tar' will recursively explore all directories (either those
given on the command line or through the `--files-from' option) for the
various files they contain.  However, you may not always want `tar' to
act this way.

   The `--no-recursion' option inhibits `tar''s recursive descent into
specified directories.  If you specify `--no-recursion', you can use
the `find' (*note find: (find)Top.)  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'.

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

`--recursion'
     Requires `tar' to recursively descend directories.  This is the
     default.

   When you use `--no-recursion', GNU `tar' grabs directory entries
themselves, but does not descend on them recursively.  Many people use
`find' for locating files they want to back up, and since `tar'
_usually_ recursively descends on directories, they have to use the
`-not -type d' test in their `find' invocation (*note Type:
(find)Type.), as they usually do not want all the files in a directory.
They then use the `--files-from' option to archive the files located
via `find'.

   The problem when restoring files archived in this manner is that the
directories themselves are not in the archive; so the
`--same-permissions' (`--preserve-permissions', `-p') option does not
affect them--while users might really like it to.  Specifying
`--no-recursion' is a way to tell `tar' to grab only the directory
entries given to it, adding no new files on its own.  To summarize, if
you use `find' to create a list of files to be stored in an archive,
use it as follows:

     $ find DIR TESTS | \
       tar -cf ARCHIVE -T - --no-recursion

   The `--no-recursion' option also applies when extracting: it causes
`tar' to extract only the matched directory entries, not the files
under those directories.

   The `--no-recursion' option also affects how globbing patterns are
interpreted (*note controlling pattern-matching::).

   The `--no-recursion' and `--recursion' options apply to later
options and operands, and can be overridden by later occurrences of
`--no-recursion' and `--recursion'.  For example:

     $ tar -cf jams.tar --no-recursion grape --recursion grape/concord

creates an archive with one entry for `grape', and the recursive
contents of `grape/concord', but no entries under `grape' other than
`grape/concord'.


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

6.10 Crossing File System Boundaries
====================================

`tar' will normally automatically cross file system boundaries in order
to archive files which are part of a directory tree.  You can change
this behavior by running `tar' and specifying `--one-file-system'.
This option only affects files that are archived because they are in a
directory that is being archived; `tar' will still archive files
explicitly named on the command line or through `--files-from',
regardless of where they reside.

`--one-file-system'
     Prevents `tar' from crossing file system boundaries when
     archiving.  Use in conjunction with any write operation.

   The `--one-file-system' option causes `tar' to modify its normal
behavior in archiving the contents of directories.  If a file in a
directory is not on the same file system as the directory itself, then
`tar' will not archive that file.  If the file is a directory itself,
`tar' will not archive anything beneath it; in other words, `tar' will
not cross mount points.

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

* Menu:

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


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

6.10.1 Changing the Working Directory
-------------------------------------

To change the working directory in the middle of a list of file names,
either on the command line or in a file specified using `--files-from'
(`-T'), use `--directory' (`-C').  This will change the working
directory to the specified directory after that point in the list.

`--directory=DIRECTORY'
`-C DIRECTORY'
     Changes the working directory in the middle of a command line.

   For example,

     $ tar -c -f jams.tar grape prune -C food cherry

will place the files `grape' and `prune' from the current directory
into the archive `jams.tar', followed by the file `cherry' from the
directory `food'.  This option is especially useful when you have
several widely separated files that you want to store in the same
archive.

   Note that the file `cherry' is recorded in the archive under the
precise name `cherry', _not_ `food/cherry'.  Thus, the archive will
contain three files that all appear to have come from the same
directory; if the archive is extracted with plain `tar --extract', all
three files will be written in the current directory.

   Contrast this with the command,

     $ tar -c -f jams.tar grape prune -C food red/cherry

which records the third file in the archive under the name `red/cherry'
so that, if the archive is extracted using `tar --extract', the third
file will be written in a subdirectory named `red'.

   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 various `tar'
options (including `-C') in the file list.  Notice, however, that in
this case the option and its argument may not be separated by
whitespace.  If you use short option, its argument must either follow
the option letter immediately, without any intervening whitespace, or
occupy the next line.  Otherwise, if you use long option, separate its
argument by an equal sign.

   For instance, the file list for the above example will be:

     -C/etc
     passwd
     hosts
     --directory=/lib
     libc.a

To use it, you would invoke `tar' as follows:

     $ tar -c -f foo.tar --files-from list

   The interpretation of `--directory' is disabled by `--null' option.


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

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

By default, GNU `tar' drops a leading `/' on input or output, and
complains about file names containing a `..' component.  There is an
option that turns off this behavior:

`--absolute-names'
`-P'
     Do not strip leading slashes from file names, and permit file names
     containing a `..' file name component.

   When `tar' extracts archive members from an archive, it strips any
leading slashes (`/') from the member name.  This causes absolute
member names in the archive to be treated as relative file names.  This
allows you to have such members extracted wherever you want, instead of
being restricted to extracting the member in the exact directory named
in the archive.  For example, if the archive member has the name
`/etc/passwd', `tar' will extract it as if the name were really
`etc/passwd'.

   File names containing `..' can cause problems when extracting, so
`tar' normally warns you about such files when creating an archive, and
rejects attempts to extracts such files.

   Other `tar' programs do not do this.  As a result, if you create an
archive whose member names start with a slash, they will be difficult
for other people with a non-GNU `tar' program to use.  Therefore, GNU
`tar' also strips leading slashes from member names when putting
members into the archive.  For example, if you ask `tar' to add the file
`/bin/ls' to an archive, it will do so, but the member name will be
`bin/ls'(1).

   If you use the `--absolute-names' (`-P') option, `tar' will do none
of these transformations.

   To archive or extract files relative to the root directory, specify
the `--absolute-names' (`-P') option.

   Normally, `tar' acts on files relative to the working
directory--ignoring superior directory names when archiving, and
ignoring leading slashes when extracting.

   When you specify `--absolute-names' (`-P'), `tar' stores file names
including all superior directory names, and preserves leading slashes.
If you only invoked `tar' from the root directory you would never need
the `--absolute-names' option, but using this option may be more
convenient than switching to root.

`--absolute-names'
     Preserves full file names (including superior directory names) when
     archiving files.  Preserves leading slash when extracting files.


   `tar' prints out a message about removing the `/' from file names.
This message appears once per GNU `tar' invocation.  It represents
something which ought to be told; ignoring what it means can cause very
serious surprises, later.

   Some people, nevertheless, do not want to see this message.  Wanting
to play really dangerously, one may of course redirect `tar' standard
error to the sink.  For example, under `sh':

     $ tar -c -f archive.tar /home 2> /dev/null

Another solution, both nicer and simpler, would be to change to the `/'
directory first, and then avoid absolute notation.  For example:

     $ tar -c -f archive.tar -C / home

   *Note Integrity::, for some of the security-related implications of
using this option.

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

   (1) A side effect of this is that when `--create' is used with
`--verbose' the resulting output is not, generally speaking, the same
as the one you'd get running `tar --list' command.  This may be
important if you use some scripts for comparing both outputs.  *Note
listing member and file names::, for the information on how to handle
this case.


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

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

First, a quote:

     Our units of temporal measurement, from seconds on up to months,
     are so complicated, asymmetrical and disjunctive so as to make
     coherent mental reckoning in time all but impossible.  Indeed, had
     some tyrannical god contrived to enslave our minds to time, to
     make it all but impossible for us to escape subjection to sodden
     routines and unpleasant surprises, he could hardly have done
     better than handing down our present system.  It is like a set of
     trapezoidal building blocks, with no vertical or horizontal
     surfaces, like a language in which the simplest thought demands
     ornate constructions, useless particles and lengthy
     circumlocutions.  Unlike the more successful patterns of language
     and science, which enable us to face experience boldly or at least
     level-headedly, our system of temporal calculation silently and
     persistently encourages our terror of time.

     ...  It is as though architects had to measure length in feet,
     width in meters and height in ells; as though basic instruction
     manuals demanded a knowledge of five different languages.  It is
     no wonder then that we often look into our own immediate past or
     future, last Tuesday or a week from Sunday, with feelings of
     helpless confusion.  ...

     -- Robert Grudin, `Time and the Art of Living'.

   This section describes the textual date representations that GNU
programs accept.  These are the strings you, as a user, can supply as
arguments to the various programs.  The C interface (via the
`parse_datetime' function) is not described here.

* Menu:

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


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

7.1 General date syntax
=======================

A "date" is a string, possibly empty, containing many items separated
by whitespace.  The whitespace may be omitted when no ambiguity arises.
The empty string means the beginning of today (i.e., midnight).  Order
of the items is immaterial.  A date string may contain many flavors of
items:

   * calendar date items

   * time of day items

   * time zone items

   * day of the week items

   * relative items

   * pure numbers.

We describe each of these item types in turn, below.

   A few ordinal numbers may be written out in words in some contexts.
This is most useful for specifying day of the week items or relative
items (see below).  Among the most commonly used ordinal numbers, the
word `last' stands for -1, `this' stands for 0, and `first' and `next'
both stand for 1.  Because the word `second' stands for the unit of
time there is no way to write the ordinal number 2, but for convenience
`third' stands for 3, `fourth' for 4, `fifth' for 5, `sixth' for 6,
`seventh' for 7, `eighth' for 8, `ninth' for 9, `tenth' for 10,
`eleventh' for 11 and `twelfth' for 12.

   When a month is written this way, it is still considered to be
written numerically, instead of being "spelled in full"; this changes
the allowed strings.

   In the current implementation, only English is supported for words
and abbreviations like `AM', `DST', `EST', `first', `January',
`Sunday', `tomorrow', and `year'.

   The output of the `date' command is not always acceptable as a date
string, not only because of the language problem, but also because
there is no standard meaning for time zone items like `IST'.  When using
`date' to generate a date string intended to be parsed later, specify a
date format that is independent of language and that does not use time
zone items other than `UTC' and `Z'.  Here are some ways to do this:

     $ LC_ALL=C TZ=UTC0 date
     Mon Mar  1 00:21:42 UTC 2004
     $ TZ=UTC0 date +'%Y-%m-%d %H:%M:%SZ'
     2004-03-01 00:21:42Z
     $ date --iso-8601=ns | tr T ' '  # --iso-8601 is a GNU extension.
     2004-02-29 16:21:42,692722128-0800
     $ date --rfc-2822  # a GNU extension
     Sun, 29 Feb 2004 16:21:42 -0800
     $ date +'%Y-%m-%d %H:%M:%S %z'  # %z is a GNU extension.
     2004-02-29 16:21:42 -0800
     $ date +'@%s.%N'  # %s and %N are GNU extensions.
     @1078100502.692722128

   Alphabetic case is completely ignored in dates.  Comments may be
introduced between round parentheses, as long as included parentheses
are properly nested.  Hyphens not followed by a digit are currently
ignored.  Leading zeros on numbers are ignored.

   Invalid dates like `2005-02-29' or times like `24:00' are rejected.
In the typical case of a host that does not support leap seconds, a
time like `23:59:60' is rejected even if it corresponds to a valid leap
second.


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

7.2 Calendar date items
=======================

A "calendar date item" specifies a day of the year.  It is specified
differently, depending on whether the month is specified numerically or
literally.  All these strings specify the same calendar date:

     1972-09-24     # ISO 8601.
     72-9-24        # Assume 19xx for 69 through 99,
                    # 20xx for 00 through 68.
     72-09-24       # Leading zeros are ignored.
     9/24/72        # Common U.S. writing.
     24 September 1972
     24 Sept 72     # September has a special abbreviation.
     24 Sep 72      # Three-letter abbreviations always allowed.
     Sep 24, 1972
     24-sep-72
     24sep72

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

     9/24
     sep 24

   Here are the rules.

   For numeric months, the ISO 8601 format `YEAR-MONTH-DAY' is allowed,
where YEAR is any positive number, MONTH is a number between 01 and 12,
and DAY is a number between 01 and 31.  A leading zero must be present
if a number is less than ten.  If YEAR is 68 or smaller, then 2000 is
added to it; otherwise, if YEAR is less than 100, then 1900 is added to
it.  The construct `MONTH/DAY/YEAR', popular in the United States, is
accepted.  Also `MONTH/DAY', omitting the year.

   Literal months may be spelled out in full: `January', `February',
`March', `April', `May', `June', `July', `August', `September',
`October', `November' or `December'.  Literal months may be abbreviated
to their first three letters, possibly followed by an abbreviating dot.
It is also permitted to write `Sept' instead of `September'.

   When months are written literally, the calendar date may be given as
any of the following:

     DAY MONTH YEAR
     DAY MONTH
     MONTH DAY YEAR
     DAY-MONTH-YEAR

   Or, omitting the year:

     MONTH DAY


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

7.3 Time of day items
=====================

A "time of day item" in date strings specifies the time on a given day.
Here are some examples, all of which represent the same time:

     20:02:00.000000
     20:02
     8:02pm
     20:02-0500      # In EST (U.S. Eastern Standard Time).

   More generally, the time of day may be given as
`HOUR:MINUTE:SECOND', where HOUR is a number between 0 and 23, MINUTE
is a number between 0 and 59, and SECOND is a number between 0 and 59
possibly followed by `.' or `,' and a fraction containing one or more
digits.  Alternatively, `:SECOND' can be omitted, in which case it is
taken to be zero.  On the rare hosts that support leap seconds, SECOND
may be 60.

   If the time is followed by `am' or `pm' (or `a.m.' or `p.m.'), HOUR
is restricted to run from 1 to 12, and `:MINUTE' may be omitted (taken
to be zero).  `am' indicates the first half of the day, `pm' indicates
the second half of the day.  In this notation, 12 is the predecessor of
1: midnight is `12am' while noon is `12pm'.  (This is the zero-oriented
interpretation of `12am' and `12pm', as opposed to the old tradition
derived from Latin which uses `12m' for noon and `12pm' for midnight.)

   The time may alternatively be followed by a time zone correction,
expressed as `SHHMM', where S is `+' or `-', HH is a number of zone
hours and MM is a number of zone minutes.  The zone minutes term, MM,
may be omitted, in which case the one- or two-digit correction is
interpreted as a number of hours.  You can also separate HH from MM
with a colon.  When a time zone correction is given this way, it forces
interpretation of the time relative to Coordinated Universal Time
(UTC), overriding any previous specification for the time zone or the
local time zone.  For example, `+0530' and `+05:30' both stand for the
time zone 5.5 hours ahead of UTC (e.g., India).  This is the best way to
specify a time zone correction by fractional parts of an hour.  The
maximum zone correction is 24 hours.

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


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

7.4 Time zone items
===================

A "time zone item" specifies an international time zone, indicated by a
small set of letters, e.g., `UTC' or `Z' for Coordinated Universal
Time.  Any included periods are ignored.  By following a
non-daylight-saving time zone by the string `DST' in a separate word
(that is, separated by some white space), the corresponding daylight
saving time zone may be specified.  Alternatively, a
non-daylight-saving time zone can be followed by a time zone
correction, to add the two values.  This is normally done only for
`UTC'; for example, `UTC+05:30' is equivalent to `+05:30'.

   Time zone items other than `UTC' and `Z' are obsolescent and are not
recommended, because they are ambiguous; for example, `EST' has a
different meaning in Australia than in the United States.  Instead,
it's better to use unambiguous numeric time zone corrections like
`-0500', as described in the previous section.

   If neither a time zone item nor a time zone correction is supplied,
time stamps are interpreted using the rules of the default time zone
(*note Specifying time zone rules::).


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

7.5 Day of week items
=====================

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

   Days of the week may be spelled out in full: `Sunday', `Monday',
`Tuesday', `Wednesday', `Thursday', `Friday' or `Saturday'.  Days may
be abbreviated to their first three letters, optionally followed by a
period.  The special abbreviations `Tues' for `Tuesday', `Wednes' for
`Wednesday' and `Thur' or `Thurs' for `Thursday' are also allowed.

   A number may precede a day of the week item to move forward
supplementary weeks.  It is best used in expression like `third
monday'.  In this context, `last DAY' or `next DAY' is also acceptable;
they move one week before or after the day that DAY by itself would
represent.

   A comma following a day of the week item is ignored.


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

7.6 Relative items in date strings
==================================

"Relative items" adjust a date (or the current date if none) forward or
backward.  The effects of relative items accumulate.  Here are some
examples:

     1 year
     1 year ago
     3 years
     2 days

   The unit of time displacement may be selected by the string `year'
or `month' for moving by whole years or months.  These are fuzzy units,
as years and months are not all of equal duration.  More precise units
are `fortnight' which is worth 14 days, `week' worth 7 days, `day'
worth 24 hours, `hour' worth 60 minutes, `minute' or `min' worth 60
seconds, and `second' or `sec' worth one second.  An `s' suffix on
these units is accepted and ignored.

   The unit of time may be preceded by a multiplier, given as an
optionally signed number.  Unsigned numbers are taken as positively
signed.  No number at all implies 1 for a multiplier.  Following a
relative item by the string `ago' is equivalent to preceding the unit
by a multiplier with value -1.

   The string `tomorrow' is worth one day in the future (equivalent to
`day'), the string `yesterday' is worth one day in the past (equivalent
to `day ago').

   The strings `now' or `today' are relative items corresponding to
zero-valued time displacement, these strings come from the fact a
zero-valued time displacement represents the current time when not
otherwise changed by previous items.  They may be used to stress other
items, like in `12:00 today'.  The string `this' also has the meaning
of a zero-valued time displacement, but is preferred in date strings
like `this thursday'.

   When a relative item causes the resulting date to cross a boundary
where the clocks were adjusted, typically for daylight saving time, the
resulting date and time are adjusted accordingly.

   The fuzz in units can cause problems with relative items.  For
example, `2003-07-31 -1 month' might evaluate to 2003-07-01, because
2003-06-31 is an invalid date.  To determine the previous month more
reliably, you can ask for the month before the 15th of the current
month.  For example:

     $ date -R
     Thu, 31 Jul 2003 13:02:39 -0700
     $ date --date='-1 month' +'Last month was %B?'
     Last month was July?
     $ date --date="$(date +%Y-%m-15) -1 month" +'Last month was %B!'
     Last month was June!

   Also, take care when manipulating dates around clock changes such as
daylight saving leaps.  In a few cases these have added or subtracted
as much as 24 hours from the clock, so it is often wise to adopt
universal time by setting the `TZ' environment variable to `UTC0'
before embarking on calendrical calculations.


File: tar.info,  Node: Pure numbers in date strings,  Next: Seconds since the Epoch,  Prev: Relative items in date strings,  Up: Date input formats

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

The precise interpretation of a pure decimal number depends on the
context in the date string.

   If the decimal number is of the form YYYYMMDD and no other calendar
date item (*note Calendar date items::) appears before it in the date
string, then YYYY is read as the year, MM as the month number and DD as
the day of the month, for the specified calendar date.

   If the decimal number is of the form HHMM and no other time of day
item appears before it in the date string, then HH is read as the hour
of the day and MM as the minute of the hour, for the specified time of
day.  MM can also be omitted.

   If both a calendar date and a time of day appear to the left of a
number in the date string, but no relative item, then the number
overrides the year.


File: tar.info,  Node: Seconds since the Epoch,  Next: Specifying time zone rules,  Prev: Pure numbers in date strings,  Up: Date input formats

7.8 Seconds since the Epoch
===========================

If you precede a number with `@', it represents an internal time stamp
as a count of seconds.  The number can contain an internal decimal
point (either `.' or `,'); any excess precision not supported by the
internal representation is truncated toward minus infinity.  Such a
number cannot be combined with any other date item, as it specifies a
complete time stamp.

   Internally, computer times are represented as a count of seconds
since an epoch--a well-defined point of time.  On GNU and POSIX
systems, the epoch is 1970-01-01 00:00:00 UTC, so `@0' represents this
time, `@1' represents 1970-01-01 00:00:01 UTC, and so forth.  GNU and
most other POSIX-compliant systems support such times as an extension
to POSIX, using negative counts, so that `@-1' represents 1969-12-31
23:59:59 UTC.

   Traditional Unix systems count seconds with 32-bit two's-complement
integers and can represent times from 1901-12-13 20:45:52 through
2038-01-19 03:14:07 UTC.  More modern systems use 64-bit counts of
seconds with nanosecond subcounts, and can represent all the times in
the known lifetime of the universe to a resolution of 1 nanosecond.

   On most hosts, these counts ignore the presence of leap seconds.
For example, on most hosts `@915148799' represents 1998-12-31 23:59:59
UTC, `@915148800' represents 1999-01-01 00:00:00 UTC, and there is no
way to represent the intervening leap second 1998-12-31 23:59:60 UTC.


File: tar.info,  Node: Specifying time zone rules,  Next: Authors of parse_datetime,  Prev: Seconds since the Epoch,  Up: Date input formats

7.9 Specifying time zone rules
==============================

Normally, dates are interpreted using the rules of the current time
zone, which in turn are specified by the `TZ' environment variable, or
by a system default if `TZ' is not set.  To specify a different set of
default time zone rules that apply just to one date, start the date
with a string of the form `TZ="RULE"'.  The two quote characters (`"')
must be present in the date, and any quotes or backslashes within RULE
must be escaped by a backslash.

   For example, with the GNU `date' command you can answer the question
"What time is it in New York when a Paris clock shows 6:30am on October
31, 2004?" by using a date beginning with `TZ="Europe/Paris"' as shown
in the following shell transcript:

     $ export TZ="America/New_York"
     $ date --date='TZ="Europe/Paris" 2004-10-31 06:30'
     Sun Oct 31 01:30:00 EDT 2004

   In this example, the `--date' operand begins with its own `TZ'
setting, so the rest of that operand is processed according to
`Europe/Paris' rules, treating the string `2004-10-31 06:30' as if it
were in Paris.  However, since the output of the `date' command is
processed according to the overall time zone rules, it uses New York
time.  (Paris was normally six hours ahead of New York in 2004, but
this example refers to a brief Halloween period when the gap was five
hours.)

   A `TZ' value is a rule that typically names a location in the `tz'
database (http://www.twinsun.com/tz/tz-link.htm).  A recent catalog of
location names appears in the TWiki Date and Time Gateway
(http://twiki.org/cgi-bin/xtra/tzdate).  A few non-GNU hosts require a
colon before a location name in a `TZ' setting, e.g.,
`TZ=":America/New_York"'.

   The `tz' database includes a wide variety of locations ranging from
`Arctic/Longyearbyen' to `Antarctica/South_Pole', but if you are at sea
and have your own private time zone, or if you are using a non-GNU host
that does not support the `tz' database, you may need to use a POSIX
rule instead.  Simple POSIX rules like `UTC0' specify a time zone
without daylight saving time; other rules can specify simple daylight
saving regimes.  *Note Specifying the Time Zone with `TZ': (libc)TZ
Variable.


File: tar.info,  Node: Authors of parse_datetime,  Prev: Specifying time zone rules,  Up: Date input formats

7.10 Authors of `parse_datetime'
================================

`parse_datetime' started life as `getdate', as 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, Paul Eggert and others, including renaming it to `get_date' to
avoid a conflict with the alternative Posix function `getdate', and a
later rename to `parse_datetime'.  The Posix function `getdate' can
parse more locale-specific dates using `strptime', but relies on an
environment variable and external file, and lacks the thread-safety of
`parse_datetime'.

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

