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This manual is for GNU tar
(version 1.26, 12 March 2011), which creates and extracts
files from archives.
Copyright © 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.
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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.
1.1 What this Book Contains | ||
1.2 Some Definitions | ||
1.3 What tar Does |
||
1.4 How tar Archives are Named |
||
1.5 GNU tar Authors |
||
1.6 Reporting bugs or suggestions |
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The first part of this chapter introduces you to various terms that will recur throughout the book. It also tells you
who has worked on GNU tar
and its documentation, and where you should send bug reports or
comments.
The second chapter is a tutorial (see section Tutorial Introduction to tar
) which provides
a gentle introduction for people who are new to using tar
. It is meant to be self-contained, not requiring
any reading from subsequent chapters to make sense. It moves from topic to topic in a logical, progressive order, building
on information already explained.
Although the tutorial is paced and structured to allow beginners to learn how to use tar
, it is not intended
solely for beginners. The tutorial explains how to use the three most frequently used operations (create,
list, and extract) as well as two frequently used options (file and verbose).
The other chapters do not refer to the tutorial frequently; however, if a section discusses something which is a complex
variant of a basic concept, there may be a cross-reference to that basic concept. (The entire book, including the tutorial,
assumes that the reader understands some basic concepts of using a Unix-type operating system; see section
Tutorial Introduction to tar
.)
The third chapter presents the remaining five operations, and information about using tar
options and option
syntax.
The other chapters are meant to be used as a reference. Each chapter presents everything that needs to be said about a specific topic.
One of the chapters (see section Date input formats) exists in its entirety in other GNU
manuals, and is mostly self-contained. In addition, one section of this manual (see section Basic Tar
Format) 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.)
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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
.
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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.
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!)
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. [ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
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.
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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, Franηois 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
. Franηois 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 Performing Backups and Restoring Files.
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.
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If you find problems or have suggestions about this program or manual, please report them to [email protected].
When reporting a bug, please be sure to include as much detail as possible, in order to reproduce it.
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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.
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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:
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.
tar
archives with tape drives. [ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
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.
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tar
Operations and Options tar
can take a wide variety of arguments which specify and define the actions it will have on the particular
set of files or the archive. The main types of arguments to tar
fall into one of two classes: operations, and
options.
Some arguments fall into a class called operations; exactly one of these is both allowed and required for any
instance of using tar
; you may not specify more than one. People sometimes speak of operating
modes. You are in a particular operating mode when you have specified the operation which specifies it; there are eight
operations in total, and thus there are eight operating modes.
The other arguments fall into the class known as options. You are not required to specify any options, and you
are allowed to specify more than one at a time (depending on the way you are using tar
at that time). Some
options are used so frequently, and are so useful for helping you type commands more carefully that they are effectively
required. We will discuss them in this chapter.
You can write most of the tar
operations and options in any of three forms: long (mnemonic) form, short
form, and old style. Some of the operations and options have no short or old forms; however, the operations and options
which we will cover in this tutorial have corresponding abbreviations. We will indicate those abbreviations appropriately
to get you used to seeing them. Note, that the old style option forms exist in GNU tar
for compatibility with Unix tar
. In this book we present a full discussion of this way of writing options and
operations (see section Old Option Style), and we discuss the other two styles of writing options (See
section Long Option Style, and see section Short Option Style).
In the examples and in the text of this tutorial, we usually use the long forms of operations and options; but the short
forms produce the same result and can make typing long tar
commands easier. For example, instead of typing
tar --create --verbose --file=afiles.tar apple angst aspic |
you can type
tar -c -v -f afiles.tar apple angst aspic |
or even
tar -cvf afiles.tar apple angst aspic |
For more information on option syntax, see Advanced GNU tar
Operations.
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.
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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.
tar
archive. [ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
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.)
The --file Option | ||
The --verbose Option | ||
Getting Help: Using the --help Option |
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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, see section Obtaining GNU
tar
default values). 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
Choosing and Naming Archive Files.
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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:
ls -l
output (see
format=verbose: (fileutils)What information is listed section `Verbose listing' in GNU file utilities).
Depending on the file type, the name can be followed by some additional information, described in the following table:
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 |
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tar
prints out a very brief list of all operations and option available
for the current version of tar
available on your system. [ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
(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.
2.6.1 Preparing a Practice Directory for Examples | ||
2.6.2 Creating the Archive | ||
2.6.3 Running --create with --verbose | ||
2.6.4 Short Forms with create | ||
2.6.5 Archiving Directories |
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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 (see section Backup options, 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 How to Add Files to Existing Archives: --append for information on how to do this.
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To place the files blues, folk, and jazz into an archive named collection.tar, use the following command:
$ tar --create --file=collection.tar blues folk jazz |
The order of the arguments is not very important, when using long option forms. You could also say:
$ tar blues --create folk --file=collection.tar jazz |
However, you can see that this order is harder to understand; this is why we will list the arguments in the order that
makes the commands easiest to understand (and we encourage you to do the same when you use tar
, to avoid errors).
Note that the sequence --file=collection.tar is considered to be one argument. If you substituted any other string of characters for collection.tar, then that string would become the name of the archive file you create.
The order of the options becomes more important when you begin to use short forms. With short forms, if you type commands in the wrong order (even if you type them correctly in all other ways), you may end up with results you don't expect. For this reason, it is a good idea to get into the habit of typing options in the order that makes inherent sense. See section Short Forms with create, for more information on this.
In this example, you type the command as shown above: --create is the operation which creates the new
archive (collection.tar), and --file is the option which lets you give it the name you chose.
The files, blues, folk, and jazz, are now members of the archive, collection.tar
(they are file name arguments to the --create operation. See section Choosing
Files and Names for tar
, for the detailed discussion on these.) Now that they are in the archive, they
are called archive members, not files. (see section members).
When you create an archive, you must specify which files you want placed in the archive. If you do not specify
any archive members, GNU tar
will complain.
If you now list the contents of the working directory (ls
), you will find the archive file listed as well
as the files you saw previously:
blues folk jazz collection.tar |
Creating the archive collection.tar did not destroy the copies of the files in the directory.
Keep in mind that if you don't indicate an operation, tar
will not run and will prompt you for one. If you
don't name any files, tar
will complain. You must have write access to the working directory, or else you will
not be able to create an archive in that directory.
Caution: Do not attempt to use --create (-c) to add files to an existing archive; it will delete the archive and write a new one. Use --append (-r) instead. See section How to Add Files to Existing Archives: --append.
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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
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.
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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 The Three Option 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.
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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
directorythe directory from which tar
was invoked. Before trying to archive a directory from its superior
directory, you should make sure you have write access to the superior directory itself, not only the directory you are trying
archive with tar
. For example, you will probably not be able to store your home directory in an archive by
invoking tar
from the root directory; See section Absolute File Names. (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.)
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Frequently, you will find yourself wanting to determine exactly what a particular archive contains. You can use the --list (-t) operation to get the member names as they currently appear in the archive, as well as various attributes of the files at the time they were archived. For example, you can examine the archive collection.tar that you created in the last section with the command,
$ tar --list --file=collection.tar |
The output of tar
would then be:
blues folk jazz |
The archive bfiles.tar would list as follows:
./birds baboon ./box |
Be sure to use a --file=archive-name (-f archive-name) option just as with --create (-c) to specify the name of the archive.
If you use the --verbose (-v) option with --list, then tar
will print out a listing reminiscent of ls -l, showing owner, file size, and so forth. This output is described
in detail in verbose member listing.
If you had used --verbose (-v) mode, the example above would look like:
$ tar --list --verbose --file=collection.tar folk -rw-r--r-- myself/user 62 1990-05-23 10:55 folk |
It is important to notice that the output of tar --list --verbose does not necessarily match that produced
by tar --create --verbose while creating the archive. It is because GNU tar
,
unless told explicitly not to do so, removes some directory prefixes from file names before storing them in the archive
(See section Absolute File Names, 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.
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. See section Wildcards Patterns and Matching,
for a detailed discussion of globbing patterns and related tar
command line options.
Listing the Contents of a Stored Directory |
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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.
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Creating an archive is only half the jobthere 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.
2.8.1 Extracting an Entire Archive | ||
2.8.2 Extracting Specific Files | ||
2.8.3 Extracting Files that are Directories | ||
2.8.4 Extracting Archives from Untrusted Sources | ||
2.8.5 Commands That Will Fail |
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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 |
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To extract specific archive members, give their exact member names as arguments, as printed by --list (-t). If you had mistakenly deleted one of the files you had placed in the archive collection.tar earlier (say, blues), you can extract it from the archive without changing the archive's structure. Its contents will be identical to the original file blues that you deleted.
First, make sure you are in the practice directory, and list the files in the directory. Now, delete the file, blues, and list the files in the directory again.
You can now extract the member blues from the archive file collection.tar like this:
$ tar --extract --file=collection.tar blues |
If you list the files in the directory again, you will see that the file blues has been restored, with its
original permissions, data modification times, and owner.(1) (These parameters will be
identical to those which the file had when you originally placed it in the archive; any changes you may have made before
deleting the file from the file system, however, will not have been made to the archive member.) The archive file,
collection.tar, is the same as it was before you extracted blues. You can confirm this by
running tar
with --list (-t).
Remember that as with other operations, specifying the exact member name is important. tar --extract --file=bfiles.tar birds will fail, because there is no member named birds. To extract the member named ./birds, you must specify tar --extract --file=bfiles.tar ./birds. If you don't remember the exact member names, use --list (-t) option (see section How to List Archives). You can also extract those members that match a specific globbing pattern. For example, to extract from bfiles.tar all files that begin with b, no matter their directory prefix, you could type:
$ tar -x -f bfiles.tar --wildcards --no-anchored 'b*' |
Here, --wildcards instructs tar
to treat command line arguments as globbing patterns and
--no-anchored informs it that the patterns apply to member names after any / delimiter. The
use of globbing patterns is discussed in detail in See section Wildcards Patterns and Matching.
You can extract a file to standard output by combining the above options with the --to-stdout (-O) option (see section Writing to Standard Output).
If you give the --verbose option, then --extract will print the names of the archive members as it extracts them.
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Extracting directories which are members of an archive is similar to extracting other files. The main difference to be
aware of is that if the extracted directory has the same name as any directory already in the working directory, then files
in the extracted directory will be placed into the directory of the same name. Likewise, if there are files in the pre-existing
directory with the same names as the members which you extract, the files from the extracted archive will replace the files
already in the working directory (and possible subdirectories). This will happen regardless of whether or not the files
in the working directory were more recent than those extracted (there exist, however, special options that alter this behavior
see section Changing How tar
Writes Files).
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.
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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).
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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.
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(This message will disappear, once this node revised.)
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tar
This chapter is about how one invokes the GNU tar
command, from the command synopsis
(see section General Synopsis of tar
). There are numerous options, and many styles for
writing them. One mandatory option specifies the operation tar
should perform (see section
Operations), other options are meant to detail how this operation should be performed (see section
tar
Options). Non-option arguments are not always interpreted the same way, depending on what the operation
is.
You will find in this chapter everything about option styles and rules for writing them (see section
The Three Option Styles). On the other hand, operations and options are fully described elsewhere, in other chapters.
Here, you will find only synthetic descriptions for operations and options, together with pointers to other parts of the
tar
manual.
Some options are so special they are fully described right in this chapter. They have the effect of inhibiting the normal
operation of tar
or else, they globally alter the amount of feedback the user receives about what is going
on. These are the --help and --version (see section GNU
tar
documentation), --verbose (see section Checking tar
progress)
and --interactive options (see section Asking for Confirmation During Operations).
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tar
The GNU tar
program is invoked as either one of:
tar option [name] tar letter [argument] [option] [name] |
The second form is for when old options are being used.
You can use tar
to store files in an archive, to extract them from an archive, and to do other types of
archive manipulation. The primary argument to tar
, which is called the operation, specifies which
action to take. The other arguments to tar
are either options, which change the way tar
performs an operation, or file names or archive members, which specify the files or members tar
is to act on.
You can actually type in arguments in any order, even if in this manual the options always precede the other arguments,
to make examples easier to understand. Further, the option stating the main operation mode (the tar
main command)
is usually given first.
Each name in the synopsis above is interpreted as an archive member name when the main command is one of --compare
(--diff, -d), --delete, --extract (--get, -x),
--list (-t) or --update (-u). When naming archive members, you
must give the exact name of the member in the archive, as it is printed by --list. For --append
(-r) and --create (-c), these name arguments specify the names of
either files or directory hierarchies to place in the archive. These files or hierarchies should already exist in the file
system, prior to the execution of the tar
command.
tar
interprets relative file names as being relative to the working directory. tar
will make
all file names relative (by removing leading slashes when archiving or restoring files), unless you specify otherwise (using
the --absolute-names option). See section Absolute File Names, for more information
about --absolute-names.
If you give the name of a directory as either a file name or a member name, then tar
acts recursively on
all the files and directories beneath that directory. For example, the name / identifies all the files in the
file system to tar
.
The distinction between file names and archive member names is especially important when shell globbing is used, and
sometimes a source of confusion for newcomers. See section Wildcards Patterns and Matching, 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:
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 (see section
Creating and Reading Compressed Archives) and the external compressor program failed. Another example
is rmt
failure during backup to the remote device (see section Remote Tape Server).
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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
The Three Most Frequently Used Operations and The Five Advanced tar
Operations).
Depending on circumstances, you may also wish to customize how the chosen operating mode behaves. For example, you may wish
to change the way the output looks, or the format of the files that you wish to archive may require you to do something
special in order to make the archive look right.
You can customize and control tar
's performance by running tar
with one or more options (such
as --verbose (-v), which we used in the tutorial). As we said in the tutorial, options
are arguments to tar
which are (as their name suggests) optional. Depending on the operating mode, you may
specify one or more options. Different options will have different effects, but in general they all change details of the
operation, such as archive format, archive name, or level of user interaction. Some options make sense with all operating
modes, while others are meaningful only with particular modes. You will likely use some options frequently, while you will
only use others infrequently, or not at all. (A full list of options is available in see section All
tar
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.
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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(2). 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.
3.3.1 Long Option Style | ||
3.3.2 Short Option Style | ||
3.3.3 Old Option Style | ||
3.3.4 Mixing Option Styles |
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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.
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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(3).
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.
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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(4). 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.
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All three styles may be intermixed in a single tar
command, so long as the rules for each style are fully
respected(5). 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.
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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.
3.4.1 Operations | ||
3.4.2 tar Options |
||
3.4.3 Short Options Cross Reference |
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tar
archives to the end of the archive. See section Combining Archives
with --concatenate.
tar
archive. See section How to Create Archives.
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tar
Options tar
strips an initial / from member names. This option
disables that behavior. See section Absolute File Names.
--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.
tar
will back them up using simple or numbered backups,
depending upon backup-type. See section Backup options.
tar
prints error messages for read errors with the block number in the archive
file. See block-number.
tar
uses to blocking x 512 bytes per record. See section
The Blocking Factor of an Archive.
tar
to read or write archives through bzip2
. See section
Creating and Reading Compressed Archives.
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 Checkpoints.
tar
to execute an action upon hitting a breakpoint. Here we give only a brief outline. See
section Checkpoints, for a complete description.
The action argument can be one of the following:
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.
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
(6).
See section Hard Links.
tar
will use the compress
program when reading or writing the archive. This allows you
to directly act on archives while saving space. See section Creating and Reading Compressed Archives.
tar
accesses the file that a symbolic link points to,
rather than the symlink itself. See section Symbolic Links.
tar
will change its current directory to dir before performing
any operations. When this option is used during archive creation, it is order sensitive. See section
Changing the Working Directory.
tar
will skip files that match pattern. See section
Excluding Some Files.
tar
will use the list of patterns in the file file.
See section Excluding Some Files.
See section exclude-caches.
See section Excluding Some Files.
See section exclude-vcs.
tar
will use the file archive as the tar
archive it performs operations on,
rather than tar
's compilation dependent default. See section The --file
Option.
tar
will use the contents of file as a list of archive members or files to operate on, in
addition to those specified on the command-line. See section Reading Names from a File.
tar
to interpret the file name given to --file as a local file, even if it looks
like a remote tape drive name. See local and remote archives.
tar
. tar
version 1.12 or earlier. See section Controlling the Archive Format, for a detailed discussion of these formats.
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 (see section The --verbose Option).
tar
archive will have a group ID of group, rather
than the group from the source file. group is first decoded as a group symbolic name, but if this interpretation
fails, it has to be a decimal numeric group ID. See section Overriding File Metadata.
Also see the comments for the --owner=user option.
tar
to read or write archives through gzip
, allowing tar
to directly operate on several kinds of compressed archives transparently. See section Creating and
Reading Compressed Archives.
See section Hard Links.
tar
will print out a short message summarizing the operations and options to tar
and exit.
See section GNU tar
documentation.
tar
will ignore zeroed blocks in the archive, which normally signals EOF. See section
Options to Help Read Archives.
tar
that it is working with an old GNU-format incremental backup archive.
It is intended primarily for backwards compatibility only. See section Using tar
to Perform
Incremental Dumps, for a detailed discussion of incremental archives.
tar
is performing multi-tape backups, script-file is run at the end of each tape. If
script-file exits with nonzero status, tar
fails immediately. See
info-script, for a detailed discussion of script-file.
tar
should ask the user for confirmation before performing potentially destructive options,
such as overwriting files. See section Asking for Confirmation During Operations.
tar
to write name as a name record in the archive. When
extracting or listing archives, tar
will only operate on archives that have a label matching the pattern
specified in name. See section Tape Files.
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. See level=0, for details and examples.
The use of this option is valid only in conjunction with the --listed-incremental option. See section
Using tar
to Perform Incremental Dumps, for a detailed description.
tar
creates is a new
GNU-format incremental backup, using snapshot-file to determine which files to backup.
With other operations, informs tar
that the archive is in incremental format. See section
Using tar
to Perform Incremental Dumps.
tar
to read or write archives through lzip
. See section
Creating and Reading Compressed Archives.
tar
to read or write archives through lzma
. See section
Creating and Reading Compressed Archives. tar
to read or write archives through lzop
. See section
Creating and Reading Compressed Archives.
tar
will use permissions for the archive members, rather
than the permissions from the files. permissions can be specified either as an octal number or as symbolic
permissions, like with chmod
. See section Overriding File Metadata.
tar
will use date as the modification time of members when
creating archives, instead of their actual modification times. The value of date can be either a textual
date representation (see section Date input formats) or a name of the existing file, starting
with / or .. In the latter case, the modification time of that file is used. See section
Overriding File Metadata.
tar
that it should create or otherwise operate on a multi-volume tar
archive.
See section Using Multiple Tapes.
tar
will only add files that have changed since date. If
date begins with / or ., it is taken to be the name of a file whose data modification
time specifies the date. See section Operating Only on New Files.
tar
will not recurse into directories. See section Descending into
Directories.
tar
archive. This
the default behavior for ordinary users.
tar
determines automatically
whether the archive can be seeked or not. Use this option to disable this mechanism.
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. See section NUL
-Terminated File Names.
tar
that it should use numeric user and group IDs when creating a tar
file, rather than names. See section Handling File Attributes. 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.
See section Changes, for more information.
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.
tar
from recursing into directories that are on different file
systems from the current directory.
tar
should use user as the owner of members when creating archives, instead
of the user associated with the source file. user is first decoded as a user symbolic name, but if this interpretation
fails, it has to be a decimal numeric user ID. See section Overriding File Metadata.
This option does not affect extraction from archives.
tar
and POSIX tar
) and modifies
the way tar
handles the extended header keywords. Keyword-list is a comma-separated list of
keyword options. See section Controlling Extended Header Keywords, for a detailed discussion.
tar
is extracting an archive, it normally subtracts the users' umask from the permissions specified
in the archive and uses that number as the permissions to create the destination file. Specifying this option instructs
tar
that it should use the permissions directly from the archive. See section Setting
Access Permissions.
literal
, shell
, shell-always
, c
,
escape
, locale
, and clocale
. Default quoting style is escape
, unless
overridden while configuring the package.
tar
should reblock its input, for reading from pipes on systems with buggy implementations.
See section Options to Help Read Archives.
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. See
size-suffixes, for a list of valid suffixes. See section The Blocking
Factor of an Archive, for a detailed description of this option.
tar
recurses into directories (default). See section Descending
into Directories.
tar
to remove the source file from the file system after appending it to an archive. See section
Removing Files.
tar
options. Currently this option disables shell invocation
from multi-volume menu (see section Using Multiple Tapes).
tar
that it should use cmd instead of the default /usr/libexec/rmt (see
section Remote Tape Server).
tar
that is should use cmd to communicate with remote devices. See section
Device Selection and Switching.
tar
when running on machines with small amounts of memory. It informs
tar
that the list of file arguments has already been sorted to match the order of files in the archive.
See section Options to Help Read Archives.
tar
will attempt to preserve the owner specified in the tar
archive with this option present. This is the default behavior for the superuser; this option has an effect only for
ordinary users. See section Handling File Attributes.
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).
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.
tar
to mention the directories it is skipping when operating on a tar
archive.
See show-omitted-dirs.
tar
to list the member names stored in the archive, as opposed to the actual file names. See
listing member and file names.
tar
will skip extracting files in the archive until it finds one
that matches name. See section Coping with Scarce Resources.
tar --extract --file archive.tar --strip-components=2 |
would extract this file to file name.
tar
uses when backing up files from the default ~. See section
Backup options.
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. See size-suffixes, for a list of allowed suffixes. See section
Using Multiple Tapes, for a detailed discussion of this option.
tar
will pipe extracted files to the standard input of command. See section
Writing to an External Program.
tar
will extract files to stdout rather than to the file system. See section
Writing to Standard Output.
tar
. See totals.
sed
replacement expression sed-expr. For example,
$ tar cf archive.tar --transform 's,^\./,usr/,' . |
will add to archive files from the current working directory, replacing initial ./ prefix with usr/. For the detailed discussion, See section Modifying File and Member Names.
To see transformed member names in verbose listings, use --show-transformed-names option (see show-transformed-names).
tar
to remove the corresponding file from the file system before extracting it from the archive.
See section Unlink First.
tar
to access the archive through prog, which is presumed to be a compression
program of some sort. See section Creating and Reading Compressed Archives.
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. See section
Checking tar
progress.
tar
documentation.
tar
will keep track of which volume of a multi-volume
archive it is working in file. See volno-file.
xz
for compressing or decompressing the archives. See section Creating and Reading
Compressed Archives. [ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
Here is an alphabetized list of all of the short option forms, matching them with the equivalent long option.
Short Option | Reference |
---|---|
-A | concatenate. |
-B | read-full-records. |
-C | directory. |
-F | info-script. |
-G | incremental. |
-J | xz. |
-K | starting-file. |
-L | tape-length. |
-M | multi-volume. |
-N | newer. |
-O | to-stdout. |
-P | absolute-names. |
-R | block-number. |
-S | sparse. |
-T | files-from. |
-U | unlink-first. |
-V | label. |
-W | verify. |
-X | exclude-from. |
-Z | compress. |
-b | blocking-factor. |
-c | create. |
-d | compare. |
-f | file. |
-g | listed-incremental. |
-h | dereference. |
-i | ignore-zeros. |
-j | bzip2. |
-k | keep-old-files. |
-l | check-links. |
-m | touch. |
-o | When creating, no-same-owner, when extracting
portability.
The latter usage is deprecated. It is retained for compatibility with the earlier versions of GNU
|
-p | preserve-permissions. |
-r | append. |
-s | same-order. |
-t | list. |
-u | update. |
-v | verbose. |
-w | interactive. |
-x | extract. |
-z | gzip. |
[ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
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(7).
Another thing you might want to do is checking the spelling or meaning of some particular tar
option, without
resorting to this manual, for once you have carefully read it. GNU tar
has a short help
feature, triggerable through the --help option. By using this option, tar
will print a usage
message listing all available options on standard output, then exit successfully, without doing anything else and ignoring
all other options. Even if this is only a brief summary, it may be several screens long. So, if you are not using some kind
of scrollable window, you might prefer to use something like:
$ tar --help | less |
presuming, here, that you like using less
for a pager. Other popular pagers are more
and
pg
. If you know about some keyword which interests you and do not want to read all the --help
output, another common idiom is doing:
tar --help | grep keyword |
for getting only the pertinent lines. Notice, however, that some tar
options have long description lines
and the above command will list only the first of them.
The exact look of the option summary displayed by tar --help is configurable. See section 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.
[ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
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 (see section Controlling the Archive Format), it uses standard output as the archive,
if no --file option has been given (see section The --file Option), the
default blocking factor is 20 (see section The Blocking Factor of an Archive). It also shows the default
locations where tar
will look for rmt
and rsh
binaries.
[ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
tar
progress Typically, tar
performs most operations without reporting any information to the user except error messages.
When using tar
with many options, particularly ones with complicated or difficult-to-predict behavior, it is
possible to make serious mistakes. tar
provides several options that make observing tar
easier.
These options cause tar
to print information as it progresses in its job, and you might want to use them just
for being more careful about what is going on, or merely for entertaining yourself. If you have encountered a problem when
operating on an archive, however, you may need more information than just an error message in order to solve the problem.
The following options can be helpful diagnostic tools.
Normally, the --list (-t) command to list an archive prints just the file names (one per
line) and the other commands are silent. When used with most operations, the --verbose (-v)
option causes tar
to print the name of each file or archive member as it is processed. This and the other options
which make tar
print status information can be useful in monitoring tar
.
With --create or --extract, --verbose used once just prints the names of
the files or members as they are processed. Using it twice causes tar
to print a longer listing (See
verbose member listing, for the description) for each member. Since --list
already prints the names of the members, --verbose used once with --list causes tar
to print an ls -l type listing of the files in the archive. The following examples both extract members with
long list output:
$ tar --extract --file=archive.tar --verbose --verbose $ tar xvvf archive.tar |
Verbose output appears on the standard output except when an archive is being written to the standard output, as with
tar --create --file=- --verbose (tar cfv -, or even tar cvif 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:
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(8). 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 (see section Checkpoints), for more information on it.
The --show-omitted-dirs option, when reading an archivewith --list or --extract, for examplecauses 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). See section Backup options.
[ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
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:
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.
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 (see 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
tar
version number. TAR_ARCHIVE
tar
is processing. TAR_BLOCKING_FACTOR
TAR_CHECKPOINT
TAR_SUBCOMMAND
tar
is executing. See section The Five Advanced
tar
Operations, for a complete list of subcommand options. TAR_FORMAT
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.
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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:
Multiple --warning messages accumulate.
The tables below list allowed values for keyword along with the warning messages they control.
tar
operation tar --create
tar --extract
$ 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
.
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Typically, tar
carries out a command without stopping for further instructions. In some situations however,
you may want to exclude some files and archive members from the operation (for instance if disk or storage space is tight).
You can do this by excluding certain files automatically (see section Choosing Files and Names for
tar
), 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.
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tar
Operations [ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
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.
tar
to destroy a magnetic tape with an empty archive(9). The two most common errors are:
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 :-). 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 |
tar
archive, as a pipe. 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.
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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.
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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 Preparing a Practice Directory 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:
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If you want to add files to an existing archive, you don't need to create a new archive; you can use --append (-r). The archive must already exist in order to use --append. (A related operation is the --update operation; you can use this to add newer versions of archive members to an existing archive. To learn how to do this with --update, see section Updating an Archive.)
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(10).
Thus, only the most recently archived member will end up being extracted, as it will replace the one extracted before it,
and so on.
There exists a special option that allows you to get around this behavior and extract (or list) only a particular copy
of the file. This is --occurrence option. If you run tar
with this option, it will extract only
the first copy of the file. You may also give this option an argument specifying the number of copy to be extracted. Thus,
for example if the archive archive.tar contained three copies of file myfile, then the command
tar --extract --file archive.tar --occurrence=2 myfile |
would extract only the second copy. See section occurrence, for the description of --occurrence option.
If you want to replace an archive member, use --delete to delete the member you want to remove from the archive, and then use --append to add the member you want to be in the archive. Note that you can not change the order of the archive; the most recently added member will still appear last. In this sense, you cannot truly replace one member with another. (Replacing one member with another will not work on certain types of media, such as tapes; see Removing Archive Members Using --delete and Tapes and Other Archive Media, for more information.)
4.2.2.1 Appending Files to an Archive | ||
4.2.2.2 Multiple Members with the Same Name |
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The simplest way to add a file to an already existing archive is the --append (-r) operation, which writes specified files into the archive whether or not they are already among the archived files.
When you use --append, you must specify file name arguments, as there is no default. If you specify a file that already exists in the archive, another copy of the file will be added to the end of the archive. As with other operations, the member names of the newly added files will be exactly the same as their names given on the command line. The --verbose (-v) option will print out the names of the files as they are written into the archive.
--append cannot be performed on some tape drives, unfortunately, due to deficiencies in the formats those
tape drives use. The archive must be a valid tar
archive, or else the results of using this operation will
be unpredictable. See section Tapes and Other Archive 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 |
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You can use --append (-r) to add copies of files which have been updated since the archive
was created. (However, we do not recommend doing this since there is another tar
option called --update;
See section Updating an Archive, for more information. We describe this use of --append
here for the sake of completeness.) When you extract the archive, the older version will be effectively lost. This works
because files are extracted from an archive in the order in which they were archived. Thus, when the archive is extracted,
a file archived later in time will replace a file of the same name which was archived earlier, even though the older version
of the file will remain in the archive unless you delete all versions of the file.
Supposing you change the file blues and then append the changed version to collection.tar. As you saw above, the original blues is in the archive collection.tar. If you change the file and append the new version of the file to the archive, there will be two copies in the archive. When you extract the archive, the older version of the file will be extracted first, and then replaced by the newer version when it is extracted.
You can append the new, changed copy of the file blues to the archive in this way:
$ tar --append --verbose --file=collection.tar blues blues |
Because you specified the --verbose option, tar
has printed the name of the file being appended
as it was acted on. Now list the contents of the archive:
$ tar --list --verbose --file=collection.tar -rw-r--r-- me/user 28 1996-10-18 16:31 jazz -rw-r--r-- me/user 21 1996-09-23 16:44 blues -rw-r--r-- me/user 20 1996-09-23 16:44 folk -rw-r--r-- me/user 20 1996-09-23 16:44 rock -rw-r--r-- me/user 58 1996-10-24 18:30 blues |
The newest version of blues is now at the end of the archive (note the different creation dates and file sizes). If you extract the archive, the older version of the file blues will be replaced by the newer version. You can confirm this by extracting the archive and running ls on the directory.
If you wish to extract the first occurrence of the file blues from the archive, use --occurrence option, as shown in the following example:
$ tar --extract -vv --occurrence --file=collection.tar blues -rw-r--r-- me/user 21 1996-09-23 16:44 blues |
See section Changing How tar
Writes Files, for more information on --extract
and see occurrence, for a description of --occurrence option.
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In the previous section, you learned how to use --append to add a file to an existing archive. A related
operation is --update (-u). The --update operation updates a tar
archive by comparing the date of the specified archive members against the date of the file with the same name. If the file
has been modified more recently than the archive member, then the newer version of the file is added to the archive (as
with --append).
Unfortunately, you cannot use --update with magnetic tape drives. The operation will fail.
Both --update and --append work by adding to the end of the archive. When you extract a file from the archive, only the version stored last will wind up in the file system, unless you use the --backup option. See section Multiple Members with the Same Name, for a detailed discussion.
4.2.3.1 How to Update an Archive Using --update |
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You must use file name arguments with the --update (-u) operation. If you don't specify
any files, tar
won't act on any files and won't tell you that it didn't do anything (which may end up confusing
you).
To see the --update option at work, create a new file, classical, in your practice directory,
and some extra text to the file blues, using any text editor. Then invoke tar
with the update
operation and the --verbose (-v) option specified, using the names of all the files in the
practice directory as file name arguments:
$ tar --update -v -f collection.tar blues folk rock classical blues classical $ |
Because we have specified verbose mode, tar
prints out the names of the files it is working on, which in
this case are the names of the files that needed to be updated. If you run tar --list and look at the archive,
you will see blues and classical at its end. There will be a total of two versions of the member blues;
the one at the end will be newer and larger, since you added text before updating it.
The reason tar
does not overwrite the older file when updating it is because writing to the middle of a
section of tape is a difficult process. Tapes are not designed to go backward. See section Tapes and Other
Archive 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
Performing Backups and Restoring Files.
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Sometimes it may be convenient to add a second archive onto the end of an archive rather than adding individual files to the archive. To add one or more archives to the end of another archive, you should use the --concatenate (--catenate, -A) operation.
To use --concatenate, give the first archive with --file option and name the rest of archives
to be concatenated on the command line. The members, and their member names, will be copied verbatim from those archives
to the first one(11). The new, concatenated archive will be called by the same name
as the one given with the --file option. As usual, if you omit --file, tar
will
use the value of the environment variable TAPE
, or, if this has not been set, the default archive name.
To demonstrate how --concatenate works, create two small archives called bluesrock.tar and folkjazz.tar, using the relevant files from practice:
$ tar -cvf bluesrock.tar blues rock blues rock $ tar -cvf folkjazz.tar folk jazz folk jazz |
If you like, You can run tar --list to make sure the archives contain what they are supposed to:
$ tar -tvf bluesrock.tar -rw-r--r-- melissa/user 105 1997-01-21 19:42 blues -rw-r--r-- melissa/user 33 1997-01-20 15:34 rock $ tar -tvf jazzfolk.tar -rw-r--r-- melissa/user 20 1996-09-23 16:44 folk -rw-r--r-- melissa/user 65 1997-01-30 14:15 jazz |
We can concatenate these two archives with tar
:
$ cd .. $ tar --concatenate --file=bluesrock.tar jazzfolk.tar |
If you now list the contents of the bluesrock.tar, you will see that now it also contains the archive members of jazzfolk.tar:
$ tar --list --file=bluesrock.tar blues rock folk jazz |
When you use --concatenate, the source and target archives must already exist and must have been created
using compatible format parameters. Notice, that tar
does not check whether the archives it concatenates have
compatible formats, it does not even check if the files are really tar archives.
Like --append (-r), this operation cannot be performed on some tape drives, due to deficiencies in the formats those tape drives use.
It may seem more intuitive to you to want or try to use cat
to concatenate two archives instead of using
the --concatenate operation; after all, cat
is the utility for combining files.
However, tar
archives incorporate an end-of-file marker which must be removed if the concatenated archives
are to be read properly as one archive. --concatenate removes the end-of-archive marker from the target archive
before each new archive is appended. If you use cat
to combine the archives, the result will not be a valid
tar
format archive. If you need to retrieve files from an archive that was added to using the cat
utility, use the --ignore-zeros (-i) option. See section Ignoring Blocks
of Zeros, for further information on dealing with archives improperly combined using the cat
shell utility.
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You can remove members from an archive by using the --delete option. Specify the name of the archive with
--file (-f) and then specify the names of the members to be deleted; if you list no member
names, nothing will be deleted. The --verbose option will cause tar
to print the names of the
members as they are deleted. As with --extract, you must give the exact member names when using tar
--delete. --delete will remove all versions of the named file from the archive. The --delete
operation can run very slowly.
Unlike other operations, --delete has no short form.
This operation will rewrite the archive. You can only use --delete on an archive if the archive device allows you to write to any point on the media, such as a disk; because of this, it does not work on magnetic tapes. Do not try to delete an archive member from a magnetic tape; the action will not succeed, and you will be likely to scramble the archive and damage your tape. There is no safe way (except by completely re-writing the archive) to delete files from most kinds of magnetic tape. See section Tapes and Other Archive 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
.
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The --compare (-d), or --diff operation compares specified archive members
against files with the same names, and then reports differences in file size, mode, owner, modification date and contents.
You should only specify archive member names, not file names. If you do not name any members, then tar
will compare the entire archive. If a file is represented in the archive but does not exist in the file system, tar
reports a difference.
You have to specify the record size of the archive when modifying an archive with a non-default record size.
tar
ignores files in the file system that do not have corresponding members in the archive.
The following example compares the archive members rock, blues and funk in the archive
bluesrock.tar with files of the same name in the file system. (Note that there is no file, funk;
tar
will report an error message.)
$ tar --compare --file=bluesrock.tar rock blues funk rock blues tar: funk not found in archive |
The spirit behind the --compare (--diff, -d) option is to check whether the archive represents the current state of files on disk, more than validating the integrity of the archive media. For this latter goal, see Verifying Data as It is Stored.
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The previous chapter described the basics of how to use --create (-c) to create an archive from a set of files. See section How to Create Archives. This section described advanced options to be used with --create.
4.3.1 Overriding File Metadata | ||
4.3.2 Ignore Fail Read |
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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 Controlling Extended Header Keywords, for additional ways of controlling metadata,
stored in the archive.
tar
will use permissions for the archive members, rather
than the permissions from the files. permissions can be specified either as an octal number or as symbolic
permissions, like with chmod
(See
Permissions: (fileutils)File permissions
section `File permissions' in GNU file utilities. This reference also has useful information
for those not being overly familiar with the UNIX permission system). Using latter syntax allows for more flexibility.
For example, the value a+rw adds read and write permissions for everybody, while retaining executable
bits on directories or on any other file already marked as executable:
$ tar -c -f archive.tar --mode='a+rw' . |
tar
will use date as the modification time of members when
creating archives, instead of their actual modification times. The argument date can be either a textual
date representation in almost arbitrary format (see section 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 (see section The --verbose Option)
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 |
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 . |
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. [ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
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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 How to Extract Members from an Archive for more
basic information about the --extract operation.
4.4.1 Options to Help Read Archives | ||
4.4.2 Changing How tar Writes Files |
||
4.4.3 Coping with Scarce Resources |
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Normally, tar
will request data in full record increments from an archive storage device. If the device
cannot return a full record, tar
will report an error. However, some devices do not always return full records,
or do not require the last record of an archive to be padded out to the next record boundary. To keep reading until you
obtain a full record, or to accept an incomplete record if it contains an end-of-archive marker, specify the --read-full-records
(-B) option in conjunction with the --extract or --list operations. See section
Blocking.
The --read-full-records (-B) option is turned on by default when tar
reads
an archive from standard input, or from a remote machine. This is because on BSD Unix systems, attempting
to read a pipe returns however much happens to be in the pipe, even if it is less than was requested. If this option were
not enabled, tar
would fail as soon as it read an incomplete record from the pipe.
If you're not sure of the blocking factor of an archive, you can read the archive by specifying --read-full-records (-B) and --blocking-factor=512-size (-b 512-size), using a blocking factor larger than what the archive uses. This lets you avoid having to determine the blocking factor of an archive. See section The Blocking Factor of an Archive.
Reading Full Records | ||
Ignoring Blocks of Zeros |
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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.
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tar
Writes Files (This message will disappear, once this node revised.)
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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.
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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.
[ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
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. [ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
[ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
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. [ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
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.
[ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
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).
[ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
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.
[ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
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 (see section
Using tar
to Perform 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:
TAR_OPTIONS
variable (see TAR_OPTIONS) and wish to temporarily disable
it. [ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
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.
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.
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You can instruct tar
to send the contents of each extracted file to the standard input of an external program:
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
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
TAR_FILENAME
TAR_REALNAME
TAR_UNAME
TAR_GNAME
TAR_ATIME
TAR_MTIME
TAR_CTIME
TAR_SIZE
TAR_UID
TAR_GID
Additionally, the following variables contain information about tar mode and the archive being processed:
TAR_VERSION
tar
version number. TAR_ARCHIVE
tar
is processing. TAR_BLOCKING_FACTOR
TAR_VOLUME
tar
is processing. TAR_FORMAT
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:
TAR_OPTIONS
(see TAR_OPTIONS)
and wish to temporarily cancel it. [ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
[ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
(This message will disappear, once this node revised.)
Starting File | ||
Same Order |
[ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
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 Asking for Confirmation During
Operations, and Excluding Some Files.
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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.
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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.
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:
SIMPLE_BACKUP_SUFFIX
environment variable is used. And if SIMPLE_BACKUP_SUFFIX
is not set, the default is ~, just as in Emacs. [ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
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.
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You have now seen how to use all eight of the operations available to tar
, and a number of the possible
options. The next chapter explains how to choose and change file and archive names, how to use files to store names of other
files which you can then call as arguments to tar
(this can help you save time if you expect to archive the
same list of files a number of times), and so forth.
If there are too many files to conveniently list on the command line, you can list the names in a file, and tar
will read that file. See section Reading Names from a File.
There are various ways of causing tar
to skip over some files, and not archive them. See section
Choosing Files and Names for tar
.
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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 [email protected] 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.
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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) (see section Using tar
to Perform
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.
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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:
To create an incremental backup, you would use --listed-incremental together with --create (see section How to Create Archives). 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:
TAR_OPTIONS
environment variable (see TAR_OPTIONS). There is also another way to cope with changing device numbers. It is described in detail in 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 (see section Extracting Specific Files). In this case, tar
does not need to access
snapshot file, since all the data necessary for extraction are stored in the archive itself. So, when extracting, you can
give whatever argument to --listed-incremental, the usual practice is to use --listed-incremental=/dev/null.
Alternatively, you can use --incremental, which needs no arguments. In general, --incremental
(-G) can be used as a shortcut for --listed-incremental when listing or extracting incremental
backups (for more information regarding this option, see incremental-op).
When extracting from the incremental backup GNU tar
attempts to restore the exact state
the file system had when the archive was created. In particular, it will delete those files in the file system
that did not exist in their directories when the archive was created. If you have created several levels of incremental
files, then in order to restore the exact contents the file system had when the last level was created, you will need to
restore from all backups in turn. Continuing our example, to restore the state of /usr file system, one would
do(12):
$ tar --extract \ --listed-incremental=/dev/null \ --file archive.1.tar $ tar --extract \ --listed-incremental=/dev/null \ --file archive.2.tar |
To list the contents of an incremental archive, use --list (see section How to List Archives), as usual. To obtain more information about the archive, use --listed-incremental or --incremental combined with two --verbose options(13):
tar --list --incremental --verbose --verbose archive.tar |
This command will print, for each directory in the archive, the list of files in that directory at the time the archive was created. This information is put out in a format which is both human-readable and unambiguous for a program: each file name is printed as
x file |
where x is a letter describing the status of the file: Y if the file is present in the archive, N if the file is not included in the archive, or a D if the file is a directory (and is included in the archive). See section 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.
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An archive containing all the files in the file system is called a full backup or full dump. You could insure your data by creating a full dump every day. This strategy, however, would waste a substantial amount of archive media and user time, as unchanged files are daily re-archived.
It is more efficient to do a full dump only occasionally. To back up files between full dumps, you can 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 archivesthe last weekly (full) dump and the last daily (level one) dump. The only information lost would be in files changed or created since the last daily backup. (Doing dumps more than once a day is usually not worth the trouble.)
GNU tar
comes with scripts you can use to do full and level-one (actually, even level-two
and so on) dumps. Using scripts (shell programs) to perform backups and restoration is a convenient and reliable alternative
to typing out file name lists and tar
commands by hand.
Before you use these scripts, you need to edit the file backup-specs, which specifies parameters used by the backup scripts and by the restore script. This file is usually located in /etc/backup directory. See section Setting Parameters for Backups and Restoration, for its detailed description. Once the backup parameters are set, you can perform backups or restoration by running the appropriate script.
The name of the backup script is backup
. The name of the restore script is restore
. The following
sections describe their use in detail.
Please Note: The backup and restoration scripts are designed to be used together. While it is possible to restore
files by hand from an archive which was created using a backup script, and to create an archive by hand which could then
be extracted using the restore script, it is easier to use the scripts. See section Using tar
to Perform Incremental Dumps, before making such an attempt.
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The file backup-specs specifies backup parameters for the backup and restoration scripts provided with
tar
. You must edit backup-specs to fit your system configuration and schedule before using these
scripts.
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. See also (bashref)Top section `Bash
Features' in Bash Reference Manual.
The shell variables controlling behavior of backup
and restore
are described in the following
subsections.
5.4.1 General-Purpose Variables | ||
5.4.2 Magnetic Tape Control | ||
5.4.3 User Hooks | ||
5.4.4 An Example Text of Backup-specs |
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Backup
scripts sends a backup report to this address.
This variable is used by backup
. Its value may be overridden using --time option (see
section Using the Backup Scripts).
tar
writes the archive to. If TAPE_FILE is a remote archive (see
remote-dev), backup script will suppose that your mt
is able to access remote
devices. If RSH (see RSH) is set, --rsh-command option will be added to
invocations of mt
. tar
will use when writing the dump archive. See section The Blocking
Factor of an Archive. 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
), 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.
mt
binary. 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. rsh
binary on remote machines. This will be passed via --rsh-command
option to the remote invocation of GNU tar
. This variable affects only backup
.
This variable affects only backup
.
tar
will display its built-in
prompt, and will expect confirmation from the console. For the description of the default prompt, see
change volume prompt. tar
executable. If this is not set, backup scripts will
search tar
in the current shell path. [ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
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:
MT_BEGIN=mt_begin mt_begin() { mt -f "$1" retension } |
MT_REWIND=mt_rewind mt_rewind() { mt -f "$1" rewind } |
MT_OFFLINE=mt_offline mt_offline() { mt -f "$1" offl } |
MT_STATUS=mt_status mt_status() { mt -f "$1" status } |
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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:
Following variables keep the names of user hook functions:
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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 |
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The syntax for running a backup script is:
backup --level=level --time=time |
The --level option requests the dump level. Thus, to produce a full dump, specify --level=0
(this is the default, so --level may be omitted if its value is 0
)(14).
The --time option determines when should the backup be run. Time may take three forms:
You should start a script with a tape or disk mounted. Once you start a script, it prompts you for new tapes or disks
as it needs them. Media volumes don't have to correspond to archive files a multi-volume archive can be started in the
middle of a tape that already contains the end of another multi-volume archive. The restore
script prompts
for media by its archive volume, so to avoid an error message you should keep track of which tape (or disk) contains which
volume of the archive (see section Using the Restore Script).
The backup scripts write two files on the file system. The first is a record file in /etc/tar-backup/, which is used by the scripts to store and retrieve information about which files were dumped. This file is not meant to be read by humans, and should not be deleted by them. See section Format of the Incremental 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:
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To restore files that were archived using a scripted backup, use the restore
script. Its usage is quite
straightforward. In the simplest form, invoke restore --all
, it will then restore all the file systems and
files specified in backup-specs (see section BACKUP_DIRS).
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:
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 beginningif the tape head is positioned past the beginning of the archive, the script will rewind the tape as needed. See section Tape Positions and Tape Marks, for a discussion of tape positioning.
Warning: The script will delete files from the active file system if they were not in the file system when the archive was made.
See section Using tar
to Perform Incremental Dumps, for an explanation of how the script
makes that determination.
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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.
6.1 Choosing and Naming Archive Files | Choosing the Archive's Name | |
6.2 Selecting Archive Members | ||
6.3 Reading Names from a File | ||
6.4 Excluding Some Files | ||
6.5 Wildcards Patterns and Matching | ||
6.6 Quoting Member Names | Ways of Quoting Special Characters in Names | |
6.7 Modifying File and Member Names | ||
6.8 Operating Only on New Files | ||
6.9 Descending into Directories | ||
6.10 Crossing File System Boundaries |
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By default, tar
uses an archive file name that was compiled when it was built on the system; usually this
name refers to some physical tape drive on the machine. However, the person who installed tar
on the system
may not 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.
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.
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File Name arguments specify which files in the file system tar
operates on, when creating or adding
to an archive, or which archive members tar
operates on, when reading or deleting from an archive. See section
The Five Advanced tar
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:
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.
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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.
If you give a single dash as a file name for --files-from, (i.e., you specify either --files-from=-
or -T -
), then the file names are read from standard input.
Unless you are running tar
with --create, you can not use both --files-from=-
and --file=-
(-f -
) in the same command.
Any number of -T options can be given in the command line.
The following example shows how to use find
to generate a list of files smaller than 400K in length and
put that list into a file called small-files. You can then use the -T option to tar
to specify the files from that file, small-files, to create the archive little.tgz. (The -z
option to tar
compresses the archive with gzip
; see section Creating and Reading
Compressed Archives for more information.)
$ find . -size -400 -print > small-files $ tar -c -v -z -T small-files -f little.tgz |
In the file list given by -T option, any file name beginning with - character is considered
a tar
option and is processed accordingly(15). For example, the common
use of this feature is to change to another directory by specifying -C option:
$ cat list -C/etc passwd hosts -C/lib libc.a $ tar -c -f foo.tar --files-from list |
In this example, tar
will first switch to /etc directory and add files passwd and
hosts to the archive. Then it will change to /lib directory and will archive the file libc.a.
Thus, the resulting archive foo.tar will contain:
$ tar tf foo.tar passwd hosts libc.a |
Notice that the option parsing algorithm used with -T is stricter than the one used by shell. Namely, when specifying option arguments, you should observe the following rules:
-Cdir
. --directory=dir
.
--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
.
6.3.1 NUL -Terminated File Names |
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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.
NUL
-terminated file names, instead of files that terminate in a newline. The --null option is just like the one in GNU xargs
and cpio
,
and is useful with the -print0 predicate of GNU find
. In tar
,
--null also disables special handling for file names that begin with dash.
This example shows how to use find
to generate a list of files larger than 800K in length and put that list
into a file called long-files. The -print0 option to find
is just like -print,
except that it separates files with a NUL
rather than with a newline. You can then run tar
with
both the --null and -T options to specify that tar
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.
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To avoid operating on files whose names match a particular pattern, use the --exclude or --exclude-from options.
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.
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.
As of version 1.26, the following files are excluded:
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:
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:
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 |
Problems with Using the exclude Options |
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exclude
Options Some users find exclude options confusing. Here are some common pitfalls:
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. 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 |
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. 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. [ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
Globbing is the operation by which wildcard characters, * or ? for example,
are replaced and expanded into all existing files matching the given pattern. GNU tar
can
use wildcard patterns for matching (or globbing) archive members when extracting from or listing an archive. Wildcard patterns
are also used for verifying volume labels of tar
archives. This section has the purpose of explaining wildcard
syntax for tar
.
A pattern should be written according to shell syntax, using wildcard characters to effect globbing. Most characters in the pattern stand for themselves in the matched string, and case is significant: a will match only a, and not A. The character ? in the pattern matches any single character in the matched string. The character * in the pattern matches zero, one, or more single characters in the matched string. The character \ says to take the following character of the pattern literally; it is useful when one needs to match the ?, *, [ or \ characters, themselves.
The character [, up to the matching ], introduces a character class. A character class is a list of acceptable characters for the next single character of the matched string. For example, [abcde] would match any of the first five letters of the alphabet. Note that within a character class, all of the special characters listed above other than \ lose their special meaning; for example, [-\\[*?]] would match any of the characters, -, \, [, *, ?, or ]. (Due to parsing constraints, the characters - and ] must either come first or last in a character class.)
If the first character of the class after the opening [ is ! or ^, then the meaning of the class is reversed. Rather than listing character to match, it lists those characters which are forbidden as the next single character of the matched string.
Other characters of the class stand for themselves. The special construction [a-e], using an hyphen between two letters, is meant to represent all characters between a and e, inclusive.
Periods (.) or forward slashes (/) are not considered special for wildcard matches. However, if a pattern completely matches a directory prefix of a matched string, then it matches the full matched string: thus, excluding a directory also excludes all the files beneath it.
Controlling Pattern-Matching |
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For the purposes of this section, we call exclusion members all member names obtained while processing --exclude and --exclude-from options, and inclusion members those member names that were given in the command line or read from the file specified with --files-from option.
These two pairs of member lists are used in the following operations: --diff, --extract, --list, --update.
There are no inclusion members in create mode (--create and --append), since in this mode the names obtained from the command line refer to files, not archive members.
By default, inclusion members are compared with archive members literally (16) and exclusion members are treated as globbing patterns. For example:
$ tar tf foo.tar a.c b.c a.txt [remarks] # Member names are used verbatim: $ tar -xf foo.tar -v '[remarks]' [remarks] # Exclude member names are globbed: $ tar -xf foo.tar -v --exclude '*.c' a.txt [remarks] |
This behavior can be altered by using the following options:
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 (see section Absolute File Names), 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.
The --recursion and --no-recursion options (see section Descending into Directories) 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 |
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When displaying member names, tar
takes care to avoid ambiguities caused by certain characters. This is
called name quoting. The characters in question are:
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 |
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:
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:
$ tar tf arch.tar --quoting-style=literal ./ ./a space ./a'single'quote ./a"double"quote ./a\backslash ./a tab ./a newline |
$ tar tf arch.tar --quoting-style=shell ./ './a space' './a'\''single'\''quote' './a"double"quote' './a\backslash' './a tab' './a newline' |
$ tar tf arch.tar --quoting-style=shell-always './' './a space' './a'\''single'\''quote' './a"double"quote' './a\backslash' './a tab' './a newline' |
$ tar tf arch.tar --quoting-style=c "./" "./a space" "./a'single'quote" "./a\"double\"quote" "./a\\backslash" "./a\ttab" "./a\nnewline" |
$ tar tf arch.tar --quoting-style=escape ./ ./a space ./a'single'quote ./a"double"quote ./a\\backslash ./a\ttab ./a\nnewline |
For example:
$ tar tf arch.tar --quoting-style=locale `./' `./a space' `./a\'single\'quote' `./a"double"quote' `./a\\backslash' `./a\ttab' `./a\nnewline' |
$ 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:
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:
This option is particularly useful if you have added --quote-chars to your TAR_OPTIONS
(see
TAR_OPTIONS) and wish to disable it for the current invocation.
Note, that --no-quote-chars does not disable those characters that are quoted by default in the selected quoting style.
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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 Absolute File Names.
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.
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:
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:
The expression is a sed
-like replace expression of the form:
s/regexp/replace/[flags] |
where regexp is a regular expression, replace is a replacement for each file name part that matches regexp. Both regexp and replace are described in detail in The "s" Command: (sed)The "s" Command section `The `s' Command' in GNU sed.
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:
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:
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:
$ tar --transform='s,usr/,usr/local/,' -x -f arch.tar |
$ tar --transform='s,/*[^/]*/[^/]*/,,' -x -f arch.tar |
$ tar --transform 's/.*/\L&/' -x -f arch.tar |
$ tar --transform 's,^,/prefix/,' -x -f arch.tar |
$ 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/,' |
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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.
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.
These options limit tar
to operate only on files which have been modified after the date specified. A file's
status is considered to have changed if its contents have been modified, or if its owner, permissions, and so forth, have
been changed. (For more information on how to specify a date, see Date input formats; remember that
the entire date argument must be quoted if it contains any spaces.)
Gurus would say that --after-date tests both the data modification time (mtime
, the time
the contents of the file were last modified) and the status change time (ctime
, the time the file's status
was last changed: owner, permissions, etc.) fields, while --newer-mtime tests only the mtime
field.
To be precise, --after-date checks both mtime
and ctime
and processes
the file if either one is more recent than date, while --newer-mtime only checks mtime
and disregards ctime
. Neither does it use atime
(the last time the contents of the file were looked
at).
Date specifiers can have embedded spaces. Because of this, you may need to quote date arguments to keep the shell from parsing them as separate arguments. For example, the following command will add to the archive all the files modified less than two days ago:
$ tar -cf foo.tar --newer-mtime '2 days ago' |
When any of these options is used with the option --verbose (see section The --verbose
Option) GNU tar
will try to convert the specified date back to its textual representation
and compare that with the one given with the option. If the two dates differ, tar
will print a warning saying
what date it will use. This is to help user ensure he is using the right date. For example:
$ tar -c -f archive.tar --after-date='10 days ago' . tar: Option --after-date: Treating date `10 days ago' as 2006-06-11 13:19:37.232434 |
Please Note: --after-date and --newer-mtime should not be used for incremental backups. See section Using
tar
to Perform Incremental Dumps, for proper way of creating incremental backups.
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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
(see
(find)Top section `find' in GNU
Find Manual) 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
Reading Names from a File, for more information on using find
with tar
.
tar
from recursively descending directories. tar
to recursively descend directories. This is the default. When you use --no-recursion, GNU tar
grabs directory entries themselves,
but does not descend on them recursively. Many people use find
for locating files they want to back up, and
since tar
usually recursively descends on directories, they have to use the -not -type d
test in their find
invocation (see
Type: (find)Type section `Type test'
in Finding Files), as they usually do not want all the files in a directory. They then use the --files-from
option to archive the files located via find
.
The problem when restoring files archived in this manner is that the directories themselves are not in the archive; so
the --same-permissions (--preserve-permissions, -p) option does not affect
themwhile users might really like it to. Specifying --no-recursion is a way to tell tar
to
grab only the directory entries given to it, adding no new files on its own. To summarize, if you use find
to create a list of files to be stored in an archive, use it as follows:
$ find dir tests | \ tar -cf archive -T - --no-recursion |
The --no-recursion option also applies when extracting: it causes tar
to extract only the
matched directory entries, not the files under those directories.
The --no-recursion option also affects how globbing patterns are interpreted (see section 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.
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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.
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.
6.10.1 Changing the Working Directory | Changing Directory | |
6.10.2 Absolute File Names |
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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.
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 (see section Reading Names from a File), 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.
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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:
When tar
extracts archive members from an archive, it strips any leading slashes (/) from
the member name. This causes absolute member names in the archive to be treated as relative file names. This allows you
to have such members extracted wherever you want, instead of being restricted to extracting the member in the exact directory
named in the archive. For example, if the archive member has the name /etc/passwd, tar
will extract
it as if the name were really etc/passwd.
File names containing .. can cause problems when extracting, so tar
normally warns you about
such files when creating an archive, and rejects attempts to extracts such files.
Other tar
programs do not do this. As a result, if you create an archive whose member names start with a
slash, they will be difficult for other people with a non-GNU tar
program to use. Therefore,
GNU tar
also strips leading slashes from member names when putting members into the archive.
For example, if you ask tar
to add the file /bin/ls to an archive, it will do so, but the member
name will be bin/ls(17).
If you use the --absolute-names (-P) option, tar
will do none of these transformations.
To archive or extract files relative to the root directory, specify the --absolute-names (-P) option.
Normally, tar
acts on files relative to the working directoryignoring 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.
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 |
See section Integrity, for some of the security-related implications of using this option.
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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.
7.1 General date syntax | Common rules. | |
7.2 Calendar date items | 19 Dec 1994. | |
7.3 Time of day items | 9:20pm. | |
7.4 Time zone items | EST, PDT, GMT. | |
7.5 Day of week items | Monday and others. | |
7.6 Relative items in date strings | next tuesday, 2 years ago. | |
7.7 Pure numbers in date strings | 19931219, 1440. | |
7.8 Seconds since the Epoch | @1078100502. | |
7.9 Specifying time zone rules | TZ="America/New_York", TZ="UTC0". | |
7.10 Authors of parse_datetime |
Bellovin, Eggert, Salz, Berets, et al. |
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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:
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.
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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 |
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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.
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A time zone item specifies an international time zone, indicated by a small set of letters, e.g., UTC or Z for Coordinated Universal Time. Any included periods are ignored. By following a non-daylight-saving time zone by the string DST in a separate word (that is, separated by some white space), the corresponding daylight saving time zone may be specified. Alternatively, a non-daylight-saving time zone can be followed by a time zone correction, to add the two values. This is normally done only for UTC; for example, UTC+05:30 is equivalent to +05:30.
Time zone items other than UTC and Z are obsolescent and are not recommended, because they are ambiguous; for example, EST has a different meaning in Australia than in the United States. Instead, it's better to use unambiguous numeric time zone corrections like -0500, as described in the previous section.
If neither a time zone item nor a time zone correction is supplied, time stamps are interpreted using the rules of the default time zone (see section Specifying time zone rules).
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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.
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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.
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The precise interpretation of a pure decimal number depends on the context in the date string.
If the decimal number is of the form yyyymmdd and no other calendar date item (see section 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.
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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 epocha 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.
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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. A recent catalog of location names appears
in the TWiki Date and Time Gateway. A few non-GNU
hosts require a colon before a location name in a TZ
setting, e.g., TZ=":America/New_York".
The tz database includes a wide variety of locations ranging from Arctic/Longyearbyen to
Antarctica/South_Pole, but if you are at sea and have your own private time zone, or if you are using a non-GNU
host that does not support the tz database, you may need to use a POSIX rule instead.
Simple POSIX rules like UTC0 specify a time zone without daylight saving time; other rules
can specify simple daylight saving regimes. See
(libc)TZ Variable section `Specifying
the Time Zone with TZ
' in The GNU C Library.
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parse_datetime
parse_datetime
started life as getdate
, as originally implemented by Steven M. Bellovin ([email protected])
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 ([email protected]) and Jim Berets ([email protected])
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 Franηois Pinard ([email protected]) from the parse_datetime.y source code, and then edited by K. Berry ([email protected]).
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Due to historical reasons, there are several formats of tar archives. All of them are based on the same principles, but have some subtle differences that often make them incompatible with each other.
GNU tar is able to create and handle archives in a variety of formats. The most frequently used formats are (in alphabetical order):
tar
versions up to 1.13.25. This format derived from an early
POSIX standard, adding some improvements such as sparse file handling and incremental archives. Unfortunately
these features were implemented in a way incompatible with other archive formats.
Archives in gnu format are able to hold file names of unlimited length.
tar
of versions prior to 1.12. This format has traditionally been used by Automake when producing Makefiles. This practice will change in the future,
in the meantime, however this means that projects containing file names more than 99 characters long will not be able
to use GNU tar
1.26 and Automake prior to 1.9.
star
implementation. GNU tar
is able
to read star archives but currently does not produce them. This archive format will be the default format for future versions of GNU tar
.
The following table summarizes the limitations of each of these formats:
Format | UID | File Size | File Name | Devn |
---|---|---|---|---|
gnu | 1.8e19 | Unlimited | Unlimited | 63 |
oldgnu | 1.8e19 | Unlimited | Unlimited | 63 |
v7 | 2097151 | 8GB | 99 | n/a |
ustar | 2097151 | 8GB | 256 | 21 |
posix | Unlimited | Unlimited | Unlimited | Unlimited |
The default format for GNU tar
is defined at compilation time. You may check it by running
tar --help
, and examining the last lines of its output. Usually, GNU tar
is
configured to create archives in gnu format, however, future version will switch to posix.
8.1 Using Less Space through Compression | ||
8.2 Handling File Attributes | ||
8.3 Making tar Archives More Portable |
||
8.4 Comparison of tar and cpio |
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8.1.1 Creating and Reading Compressed Archives | ||
8.1.2 Archiving Sparse Files |
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GNU tar
is able to create and read compressed archives. It supports a wide variety of
compression programs, namely: gzip
, bzip2
, lzip
, lzma
, lzop
,
xz
and traditional compress
. The latter is supported mostly for backward compatibility, and we
recommend against using it, because it is by far less effective than the other compression programs(18).
Creating a compressed archive is simple: you just specify a compression option along with the usual archive
creation commands. The compression option is -z (--gzip) to create a gzip
compressed
archive, -j (--bzip2) to create a bzip2
compressed archive, --lzip
to create an lzip compressed archive, -J (--xz) to create an XZ archive, --lzma
to create an LZMA compressed archive, --lzop to create an LSOP archive, and -Z (--compress)
to use compress
program. For example:
$ tar cfz archive.tar.gz . |
You can also let GNU tar
select the compression program based on the suffix of the archive
file name. This is done using --auto-compress (-a) command line option. For example, the following
invocation will use bzip2
for compression:
$ tar cfa archive.tar.bz2 . |
whereas the following one will use lzma
:
$ tar cfa archive.tar.lzma . |
For a complete list of file name suffixes recognized by GNU tar
, see
auto-compress.
Reading compressed archive is even simpler: you don't need to specify any additional options as GNU
tar
recognizes its format automatically. Thus, the following commands will list and extract the archive created
in previous example:
# List the compressed archive $ tar tf archive.tar.gz # Extract the compressed archive $ tar xf archive.tar.gz |
The format recognition algorithm is based on signatures, a special byte sequences in the beginning of file,
that are specific for certain compression formats. If this approach fails, tar
falls back to using archive
name suffix to determine its format (see auto-compress, for a list of recognized suffixes).
Some compression programs are able to handle different compression formats. GNU tar
uses
this, if the principal decompressor for the given format is not available. For example, if compress
is not
installed, tar
will try to use gzip
. As of version 1.26 the following alternatives are tried(19):
Format | Main decompressor | Alternatives |
---|---|---|
compress | compress | gzip |
lzma | lzma | xz |
bzip2 | bzip2 | lbzip2 |
The only case when you have to specify a decompression option while reading the archive is when reading from a pipe or
from a tape drive that does not support random access. However, in this case GNU tar
will
indicate which option you should use. For example:
$ cat archive.tar.gz | tar tf - tar: Archive is compressed. Use -z option tar: Error is not recoverable: exiting now |
If you see such diagnostics, just add the suggested option to the invocation of GNU tar
:
$ cat archive.tar.gz | tar tfz - |
Notice also, that there are several restrictions on operations on compressed archives. First of all, compressed archives
cannot be modified, i.e., you cannot update (--update, alias -u) them or delete (--delete)
members from them or add (--append, alias -r) members to them. Likewise, you cannot append
another tar
archive to a compressed archive using --concatenate (-A). Secondly,
multi-volume archives cannot be compressed.
The following options allow to select a particular compressor program:
gzip
. xz
. bzip2
. lzip
. lzma
. lzop
. compress
. When any of these options is given, GNU tar
searches the compressor binary in the current
path and invokes it. The name of the compressor program is specified at compilation time using a corresponding --with-compname
option to configure
, e.g. --with-bzip2 to select a specific bzip2
binary. See section
Using lbzip2 with GNU tar
., for a detailed discussion.
The output produced by tar --help
shows the actual compressor names along with each of these options.
You can use any of these options on physical devices (tape drives, etc.) and remote files as well as on normal files;
data to or from such devices or remote files is reblocked by another copy of the tar
program to enforce the
specified (or default) record size. The default compression parameters are used. Most compression programs allow to override
these by setting a program-specific environment variable. For example, when using gzip
you can use GZIP
as in the example below:
$ GZIP=--best tar cfz archive.tar.gz subdir |
Another way would be to use the -I option instead (see below), e.g.:
$ tar -cf archive.tar.gz -I 'gzip --best' subdir |
Finally, the third, traditional, way to achieve the same result is to use pipe:
$ tar cf - subdir | gzip --best -c - > archive.tar.gz |
About corrupted compressed archives: compressed files have no redundancy, for maximum compression. The adaptive nature of the compression scheme means that the compression tables are implicitly spread all over the archive. If you lose a few blocks, the dynamic construction of the compression tables becomes unsynchronized, and there is little chance that you could recover later in the archive.
Another compression options provide a better control over creating compressed archives. These are:
Suffix | Compression program |
---|---|
.gz | gzip |
.tgz | gzip |
.taz | gzip |
.Z | compress |
.taZ | compress |
.bz2 | bzip2 |
.tz2 | bzip2 |
.tbz2 | bzip2 |
.tbz | bzip2 |
.lz | lzip |
.lzma | lzma |
.tlz | lzma |
.lzo | lzop |
.xz | xz |
tar
does not support. There are two requirements to which prog should comply:
First, when called without options, it should read data from standard input, compress it and output it on standard output.
Secondly, if called with -d argument, it should do exactly the opposite, i.e., read the compressed data from the standard input and produce uncompressed data on the standard output.
The --use-compress-program option, in particular, lets you implement your own filters, not necessarily
dealing with compression/decompression. For example, suppose you wish to implement PGP encryption on top of compression,
using gpg
(see gpg: (gpg)Top section `gpg
- encryption and signing tool' in GNU Privacy Guard Manual). The following script does that:
#! /bin/sh case $1 in -d) gpg --decrypt - | gzip -d -c;; '') gzip -c | gpg -s;; *) echo "Unknown option $1">&2; exit 1;; esac |
Suppose you name it gpgz and save it somewhere in your PATH
. Then the following command will
create a compressed archive signed with your private key:
$ tar -cf foo.tar.gpgz -Igpgz . |
Likewise, the command below will list its contents:
$ tar -tf foo.tar.gpgz -Igpgz . |
8.1.1.1 Using lbzip2 with GNU tar . |
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tar
. Lbzip2
is a multithreaded utility for handling bzip2 compression, written by Laszlo Ersek.
It makes use of multiple processors to speed up its operation and in general works considerably faster than bzip2
.
For a detailed description of lbzip2
see http://freshmeat.net/projects/lbzip2
and lbzip2: parallel bzip2 utility.
Recent versions of lbzip2
are mostly command line compatible with bzip2
, which makes it possible
to automatically invoke it via the --bzip2 GNU tar
command line option. To
do so, GNU tar
must be configured with the --with-bzip2 command line option,
like this:
$ ./configure --with-bzip2=lbzip2 [other-options] |
Once configured and compiled this way, tar --help
will show the following:
$ tar --help | grep -- --bzip2 -j, --bzip2 filter the archive through lbzip2 |
which means that running tar --bzip2
will invoke lbzip2
.
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Files in the file system occasionally have holes. A hole in a file is a section of the file's contents
which was never written. The contents of a hole reads as all zeros. On many operating systems, actual disk storage is not
allocated for holes, but they are counted in the length of the file. If you archive such a file, tar
could
create an archive longer than the original. To have tar
attempt to recognize the holes in a file, use --sparse
(-S). When you use this option, then, for any file using less disk space than would be expected from its
length, tar
searches the file for consecutive stretches of zeros. It then records in the archive for the file
where the consecutive stretches of zeros are, and only archives the real contents of the file. On extraction (using --sparse
is not needed on extraction) any such files have holes created wherever the continuous stretches of zeros were found. Thus,
if you use --sparse, tar
archives won't take more space than the original.
tar
to test each file for sparseness before attempting to archive it. If the
file is found to be sparse it is treated specially, thus allowing to decrease the amount of space used by its image
in the archive.
This option is meaningful only when creating or updating archives. It has no effect on extraction.
Consider using --sparse when performing file system backups, to avoid archiving the expanded forms of files stored sparsely in the system.
Even if your system has no sparse files currently, some may be created in the future. If you use --sparse
while making file system backups as a matter of course, you can be assured the archive will never take more space on the
media than the files take on disk (otherwise, archiving a disk filled with sparse files might take hundreds of tapes). See
section Using tar
to Perform Incremental Dumps.
However, be aware that --sparse option presents a serious drawback. Namely, in order to determine if the
file is sparse tar
has to read it before trying to archive it, so in total the file is read twice.
So, always bear in mind that the time needed to process all files with this option is roughly twice the time needed to archive
them without it.
When using POSIX archive format, GNU tar
is able to store sparse files
using in three distinct ways, called sparse formats. A sparse format is identified by its number, consisting,
as usual of two decimal numbers, delimited by a dot. By default, format 1.0 is used. If, for some reason,
you wish to use an earlier format, you can select it using --sparse-version option.
Using --sparse-format option implies --sparse.
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When tar
reads files, it updates their access times. To avoid this, use the --atime-preserve[=METHOD]
option, which can either reset the access time retroactively or avoid changing it in the first place.
--atime-preserve=replace works on most systems, but it also restores the data modification time and
updates the status change time. Hence it doesn't interact with incremental dumps nicely (see section
Using tar
to Perform Incremental Dumps), and it can set access or data modification
times incorrectly if other programs access the file while tar
is running.
--atime-preserve=system avoids changing the access time in the first place, if the operating system
supports this. Unfortunately, this may or may not work on any given operating system or file system. If tar
knows for sure it won't work, it complains right away.
Currently --atime-preserve with no operand defaults to --atime-preserve=replace, but this is intended to change to --atime-preserve=system when the latter is better-supported.
When this option is used, tar
leaves the data modification times of the files it extracts as the times
when the files were extracted, instead of setting it to the times recorded in the archive.
This option is meaningless with --list (-t).
This is the default behavior for the superuser, so this option is meaningful only for non-root users, when
tar
is executed on those systems able to give files away. This is considered as a security flaw by many people,
at least because it makes quite difficult to correctly account users for the disk space they occupy. Also, the
suid
or sgid
attributes of files are easily and silently lost when files are given away.
When writing an archive, tar
writes the user ID and user name separately. If it can't
find a user name (because the user ID is not in /etc/passwd), then it does not write one.
When restoring, it tries to look the name (if one was written) up in /etc/passwd. If it fails, then it uses
the user ID stored in the archive instead.
This is useful in certain circumstances, when restoring a backup from an emergency floppy with different passwd/group files for example. It is otherwise impossible to extract files with the right ownerships if the password file in use during the extraction does not match the one belonging to the file system(s) being extracted. This occurs, for example, if you are restoring your files after a major crash and had booted from an emergency floppy with no password file or put your disk into another machine to do the restore.
The numeric ids are always saved into tar
archives. The identifying names are added at create
time when provided by the system, unless --format=oldgnu is used. Numeric ids could be used when moving
archives between a collection of machines using a centralized management for attribution of numeric ids to users and
groups. This is often made through using the NIS capabilities.
When making a tar
file for distribution to other sites, it is sometimes cleaner to use a single owner
for all files in the distribution, and nicer to specify the write permission bits of the files as stored in the archive
independently of their actual value on the file system. The way to prepare a clean distribution is usually to have some
Makefile rule creating a directory, copying all needed files in that directory, then setting ownership and permissions
as wanted (there are a lot of possible schemes), and only then making a tar
archive out of this directory,
before cleaning everything out. Of course, we could add a lot of options to GNU tar
for fine tuning permissions and ownership. This is not the good way, I think. GNU tar
is already crowded with options and moreover, the approach just explained gives you a great deal of control already.
This option causes tar
to set the modes (access permissions) of extracted files exactly as recorded
in the archive. If this option is not used, the current umask
setting limits the permissions on extracted
files. This option is by default enabled when tar
is executed by a superuser.
This option is meaningless with --list (-t).
This option is deprecated, and will be removed in GNU tar
version 1.23.
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tar
Archives More Portable Creating a tar
archive on a particular system that is meant to be useful later on many other machines and
with other versions of tar
is more challenging than you might think. tar
archive formats have
been evolving since the first versions of Unix. Many such formats are around, and are not always compatible with each other.
This section discusses a few problems, and gives some advice about making tar
archives more portable.
One golden rule is simplicity. For example, limit your tar
archives to contain only regular files and directories,
avoiding other kind of special files. Do not attempt to save sparse files or contiguous files as such. Let's discuss a few
more problems, in turn.
8.3.1 Portable Names | ||
8.3.2 Symbolic Links | ||
8.3.3 Hard Links | ||
8.3.4 Old V7 Archives | ||
8.3.5 Ustar Archive Format | Ustar Archives | |
8.3.6 GNU and old GNU
tar format |
GNU and old GNU format archives. | |
8.3.7 GNU tar and POSIX
tar |
POSIX archives | |
8.3.8 Checksumming Problems | ||
8.3.9 Large or Negative Values | Large files, negative time stamps, etc. | |
8.3.10 How to Extract GNU-Specific Data Using Other tar
Implementations |
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Use portable file and member names. A name is portable if it contains only ASCII letters and digits, /, ., _, and -; it cannot be empty, start with - or //, or contain /-. Avoid deep directory nesting. For portability to old Unix hosts, limit your file name components to 14 characters or less.
If you intend to have your tar
archives to be read under MSDOS, you should not rely on case distinction
for file names, and you might use the GNU doschk
program for helping you further diagnosing
illegal MSDOS names, which are even more limited than System V's.
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Normally, when tar
archives a symbolic link, it writes a block to the archive naming the target of the link.
In that way, the tar
archive is a faithful record of the file system contents. When --dereference
(-h) is used with --create (-c), tar
archives the files symbolic
links point to, instead of the links themselves.
When creating portable archives, use --dereference (-h): some systems do not support symbolic links, and moreover, your distribution might be unusable if it contains unresolved symbolic links.
When reading from an archive, the --dereference (-h) option causes tar
to
follow an already-existing symbolic link when tar
writes or reads a file named in the archive. Ordinarily,
tar
does not follow such a link, though it may remove the link before writing a new file. See section
Options Controlling the Overwriting of Existing Files.
The --dereference option is unsafe if an untrusted user can modify directories while tar
is running. See section Security.
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Normally, when tar
archives a hard link, it writes a block to the archive naming the target of the link
(a 1 type block). In that way, the actual file contents is stored in file only once. For example, consider
the following two files:
$ ls -l -rw-r--r-- 2 gray staff 4 2007-10-30 15:11 one -rw-r--r-- 2 gray staff 4 2007-10-30 15:11 jeden |
Here, jeden is a link to one. When archiving this directory with a verbose level 2, you will get an output similar to the following:
$ tar cfvv ../archive.tar . drwxr-xr-x gray/staff 0 2007-10-30 15:13 ./ -rw-r--r-- gray/staff 4 2007-10-30 15:11 ./jeden hrw-r--r-- gray/staff 0 2007-10-30 15:11 ./one link to ./jeden |
The last line shows that, instead of storing two copies of the file, tar
stored it only once, under the
name jeden, and stored file one as a hard link to this file.
It may be important to know that all hard links to the given file are stored in the archive. For example, this may be necessary for exact reproduction of the file system. The following option does that:
For example, trying to archive only file jeden with this option produces the following diagnostics:
$ tar -c -f ../archive.tar -l jeden tar: Missing links to `jeden'. |
Although creating special records for hard links helps keep a faithful record of the file system contents and makes archives more compact, it may present some difficulties when extracting individual members from the archive. For example, trying to extract file one from the archive created in previous examples produces, in the absense of file jeden:
$ tar xf archive.tar ./one tar: ./one: Cannot hard link to `./jeden': No such file or directory tar: Error exit delayed from previous errors |
The reason for this behavior is that tar
cannot seek back in the archive to the previous member (in this
case, one), to extract it(20). If you wish to avoid such problems at the
cost of a bigger archive, use the following option:
For example, trying this option on our two sample files, we get two copies in the archive, each of which can then be extracted independently of the other:
$ tar -c -vv -f ../archive.tar --hard-dereference . drwxr-xr-x gray/staff 0 2007-10-30 15:13 ./ -rw-r--r-- gray/staff 4 2007-10-30 15:11 ./jeden -rw-r--r-- gray/staff 4 2007-10-30 15:11 ./one |
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Certain old versions of tar
cannot handle additional information recorded by newer tar
programs.
To create an archive in V7 format (not ANSI), which can be read by these old versions, specify the --format=v7
option in conjunction with the --create (-c) (tar
also accepts --portability
or --old-archive for this option). When you specify it, tar
leaves out information about directories,
pipes, fifos, contiguous files, and device files, and specifies file ownership by group and user IDs instead of group and
user names.
When updating an archive, do not use --format=v7 unless the archive was created using this option.
In most cases, a new format archive can be read by an old tar
program without serious
trouble, so this option should seldom be needed. On the other hand, most modern tar
s are able to read old format
archives, so it might be safer for you to always use --format=v7 for your distributions. Notice, however,
that ustar format is a better alternative, as it is free from many of v7's drawbacks.
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Archive format defined by POSIX.1-1988 specification is called ustar
. Although it is
more flexible than the V7 format, it still has many restrictions (see section ustar, for the detailed
description of ustar
format). Along with V7 format, ustar
format is a good choice for archives
intended to be read with other implementations of tar
.
To create archive in ustar
format, use --format=ustar option in conjunction with the --create
(-c).
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tar
format GNU tar
was based on an early draft of the POSIX 1003.1 ustar
standard. GNU extensions to tar
, such as the support for file names longer than 100 characters,
use portions of the tar
header record which were specified in that POSIX draft as unused.
Subsequent changes in POSIX have allocated the same parts of the header record for other purposes. As
a result, GNU tar
format is incompatible with the current POSIX specification,
and with tar
programs that follow it.
In the majority of cases, tar
will be configured to create this format by default. This will change in future
releases, since we plan to make POSIX format the default.
To force creation a GNU tar
archive, use option --format=gnu.
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tar
and POSIX tar
Starting from version 1.14 GNU tar
features full support for POSIX.1-2001
archives.
A POSIX conformant archive will be created if tar
was given --format=posix
(--format=pax) option. No special option is required to read and extract from a POSIX
archive.
8.3.7.1 Controlling Extended Header Keywords |
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pax
utility. Keyword-list is a comma-separated list of keyword options, each keyword option taking one of the following forms:
delete=pattern
tar
to omit from extended header
records that it produces any keywords matching the string pattern.
When used in extract or list mode, this option instructs tar to ignore any keywords matching the given pattern in the extended header records. In both cases, matching is performed using the pattern matching notation described in POSIX 1003.2, 3.13 (see section Wildcards Patterns and Matching). For example:
--pax-option delete=security.* |
would suppress security-related information.
exthdr.name=string
Meta-character | Replaced By |
---|---|
%d | The directory name of the file, equivalent to the result of the dirname utility on the translated
file name. |
%f | The name of the file with the directory information stripped, equivalent to the result of the basename
utility on the translated file name. |
%p | The process ID of the tar process. |
%% | A % character. |
Any other % characters in string produce undefined results.
If no option exthdr.name=string is specified, tar
will use the following default value:
%d/PaxHeaders.%p/%f |
exthdr.mtime=value
globexthdr.name=string
Meta-character | Replaced By |
---|---|
%n | An integer that represents the sequence number of the global extended header record in the archive, starting at 1. |
%p | The process ID of the tar process. |
%% | A % character. |
Any other % characters in string produce undefined results.
If no option globexthdr.name=string is specified, tar
will use the following default
value:
$TMPDIR/GlobalHead.%p.%n |
where $TMPDIR represents the value of the TMPDIR environment variable. If TMPDIR
is not set, tar
uses /tmp.
globexthdr.mtime=value
tar
was
invoked. keyword=value
tar
will
behave as if it has encountered these keyword/value pairs at the beginning of the archive in a global extended header
record. keyword:=value
When used with one of archive-reading commands, tar
will behave as if these keyword/value pairs were
included as records at the end of each extended header; thus, they will override any global or file-specific extended
header record keywords of the same names. For example, in the command:
tar --format=posix --create \ --file archive --pax-option gname:=user . |
the group name will be forced to a new value for all files stored in the archive.
In any of the forms described above, the value may be a string enclosed in curly braces. In that case, the string between the braces is understood either as a textual time representation, as described in 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.
For example, to set all modification times to the current date, you use the following option:
--pax-option='mtime:={now}' |
Note quoting of the option's argument.
As another example, here is the option that ensures that any two archives created using it, will be binary equivalent if they have the same contents:
--pax-option=exthdr.name=%d/PaxHeaders/%f,atime:=0 |
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SunOS and HP-UX tar
fail to accept archives created using GNU tar
and containing
non-ASCII file names, that is, file names having characters with the eight bit set, because they use
signed checksums, while GNU tar
uses unsigned checksums while creating archives, as per
POSIX standards. On reading, GNU tar
computes both checksums and accepts
any. It is somewhat worrying that a lot of people may go around doing backup of their files using faulty (or at least non-standard)
software, not learning about it until it's time to restore their missing files with an incompatible file extractor, or vice
versa.
GNU tar
computes checksums both ways, and accept any on read, so GNU
tar can read Sun tapes even with their wrong checksums. GNU tar
produces the standard checksum,
however, raising incompatibilities with Sun. That is to say, GNU tar
has not been modified
to produce incorrect archives to be read by buggy tar
's. I've been told that more recent Sun
tar
now read standard archives, so maybe Sun did a similar patch, after all?
The story seems to be that when Sun first imported tar
sources on their system, they recompiled it without
realizing that the checksums were computed differently, because of a change in the default signing of char
's
in their compiler. So they started computing checksums wrongly. When they later realized their mistake, they merely decided
to stay compatible with it, and with themselves afterwards. Presumably, but I do not really know, HP-UX has chosen that
their tar
archives to be compatible with Sun's. The current standards do not favor Sun tar
format.
In any case, it now falls on the shoulders of SunOS and HP-UX users to get a tar
able to read the good archives
they receive.
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(This message will disappear, once this node revised.)
The above sections suggest to use oldest possible archive format if in doubt. However, sometimes it is
not possible. If you attempt to archive a file whose metadata cannot be represented using required format, GNU
tar
will print error message and ignore such a file. You will than have to switch to a format that is able
to handle such values. The format summary table (see section Controlling the Archive Format) will
help you to do so.
In particular, when trying to archive files larger than 8GB or with timestamps not in the range 1970-01-01 00:00:00 through
2242-03-16 12:56:31 UTC, you will have to chose between GNU and POSIX
archive formats. When considering which format to choose, bear in mind that the GNU format uses two's-complement
base-256 notation to store values that do not fit into standard ustar range. Such archives can generally
be read only by a GNU tar
implementation. Moreover, they sometimes cannot be correctly restored
on another hosts even by GNU tar
. For example, using two's complement representation for
negative time stamps that assumes a signed 32-bit time_t
generates archives that are not portable to hosts
with differing time_t
representations.
On the other hand, POSIX archives, generally speaking, can be extracted by any tar implementation that understands older ustar format. The only exception are files larger than 8GB.
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tar
Implementations In previous sections you became acquainted with various quirks necessary to make your archives portable. Sometimes you
may need to extract archives containing GNU-specific members using some third-party tar
implementation or an
older version of GNU tar
. Of course your best bet is to have GNU
tar
installed, but if it is for some reason impossible, this section will explain how to cope without it.
When we speak about GNU-specific members we mean two classes of them: members split between the volumes of a multi-volume archive and sparse members. You will be able to always recover such members if the archive is in PAX format. In addition split members can be recovered from archives in old GNU format. The following subsections describe the required procedures in detail.
8.3.10.1 Extracting Members Split Between Volumes | Members Split Between Volumes | |
8.3.10.2 Extracting Sparse Members | Sparse Members |
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If a member is split between several volumes of an old GNU format archive most third party tar
implementation
will fail to extract it. To extract it, use tarcat
program (see section Concatenate Volumes
into a Single Archive). This program is available from
GNU tar
home page. It concatenates several archive volumes into a single valid archive.
For example, if you have three volumes named from vol-1.tar to vol-3.tar, you can do the following
to extract them using a third-party tar
:
$ tarcat vol-1.tar vol-2.tar vol-3.tar | tar xf - |
You could use this approach for most (although not all) PAX format archives as well. However, extracting split members
from a PAX archive is a much easier task, because PAX volumes are constructed in such a way that each part of a split member
is extracted to a different file by tar
implementations that are not aware of GNU extensions. More specifically,
the very first part retains its original name, and all subsequent parts are named using the pattern:
%d/GNUFileParts.%p/%f.%n |
where symbols preceeded by % are macro characters that have the following meaning:
Meta-character | Replaced By |
---|---|
%d | The directory name of the file, equivalent to the result of the dirname utility on its full name.
|
%f | The file name of the file, equivalent to the result of the basename utility on its full name.
|
%p | The process ID of the tar process that created the archive. |
%n | Ordinal number of this particular part. |
For example, if the file var/longfile was split during archive creation between three volumes, and the creator
tar
process had process ID 27962, then the member names will be:
var/longfile var/GNUFileParts.27962/longfile.1 var/GNUFileParts.27962/longfile.2 |
When you extract your archive using a third-party tar
, these files will be created on your disk, and the
only thing you will need to do to restore your file in its original form is concatenate them in the proper order, for example:
$ cd var $ cat GNUFileParts.27962/longfile.1 \ GNUFileParts.27962/longfile.2 >> longfile $ rm -f GNUFileParts.27962 |
Notice, that if the tar
implementation you use supports PAX format archives, it will probably emit warnings
about unknown keywords during extraction. They will look like this:
Tar file too small Unknown extended header keyword 'GNU.volume.filename' ignored. Unknown extended header keyword 'GNU.volume.size' ignored. Unknown extended header keyword 'GNU.volume.offset' ignored. |
You can safely ignore these warnings.
If your tar
implementation is not PAX-aware, you will get more warnings and more files generated on your
disk, e.g.:
$ tar xf vol-1.tar var/PaxHeaders.27962/longfile: Unknown file type 'x', extracted as normal file Unexpected EOF in archive $ tar xf vol-2.tar tmp/GlobalHead.27962.1: Unknown file type 'g', extracted as normal file GNUFileParts.27962/PaxHeaders.27962/sparsefile.1: Unknown file type 'x', extracted as normal file |
Ignore these warnings. The PaxHeaders.* directories created will contain files with extended header keywords describing the extracted files. You can delete them, unless they describe sparse members. Read further to learn more about them.
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Any tar
implementation will be able to extract sparse members from a PAX archive. However, the extracted
files will be condensed, i.e., any zero blocks will be removed from them. When we restore such a condensed file
to its original form, by adding zero blocks (or holes) back to their original locations, we call this process
expanding a compressed sparse file.
To expand a file, you will need a simple auxiliary program called xsparse
. It is available in source form
from GNU tar
home page.
Let's begin with archive members in sparse format version 1.0(21), which are the easiest to expand. The condensed file will contain both file map and file data, so no additional data will be needed to restore it. If the original file name was dir/name, then the condensed file will be named dir/GNUSparseFile.n/name, where n is a decimal number(22).
To expand a version 1.0 file, run xsparse
as follows:
$ xsparse cond-file |
where cond-file is the name of the condensed file. The utility will deduce the name for the resulting expanded file using the following algorithm:
In the unlikely case when this algorithm does not suit your needs, you can explicitly specify output file name as a second argument to the command:
$ xsparse cond-file out-file |
It is often a good idea to run xsparse
in dry run mode first. In this mode, the command does not
actually expand the file, but verbosely lists all actions it would be taking to do so. The dry run mode is enabled by -n
command line argument:
$ xsparse -n /home/gray/GNUSparseFile.6058/sparsefile Reading v.1.0 sparse map Expanding file `/home/gray/GNUSparseFile.6058/sparsefile' to `/home/gray/sparsefile' Finished dry run |
To actually expand the file, you would run:
$ xsparse /home/gray/GNUSparseFile.6058/sparsefile |
The program behaves the same way all UNIX utilities do: it will keep quiet unless it has simething important to tell you (e.g. an error condition or something). If you wish it to produce verbose output, similar to that from the dry run mode, use -v option:
$ xsparse -v /home/gray/GNUSparseFile.6058/sparsefile Reading v.1.0 sparse map Expanding file `/home/gray/GNUSparseFile.6058/sparsefile' to `/home/gray/sparsefile' Done |
Additionally, if your tar
implementation has extracted the extended headers for this file, you
can instruct xstar
to use them in order to verify the integrity of the expanded file. The option -x
sets the name of the extended header file to use. Continuing our example:
$ xsparse -v -x /home/gray/PaxHeaders.6058/sparsefile \ /home/gray/GNUSparseFile.6058/sparsefile Reading extended header file Found variable GNU.sparse.major = 1 Found variable GNU.sparse.minor = 0 Found variable GNU.sparse.name = sparsefile Found variable GNU.sparse.realsize = 217481216 Reading v.1.0 sparse map Expanding file `/home/gray/GNUSparseFile.6058/sparsefile' to `/home/gray/sparsefile' Done |
An extended header is a special tar
archive header that precedes an archive member and contains
a set of variables, describing the member properties that cannot be stored in the standard ustar
header.
While optional for expanding sparse version 1.0 members, the use of extended headers is mandatory when expanding sparse
members in older sparse formats: v.0.0 and v.0.1 (The sparse formats are described in detail in Storing
Sparse Files.) So, for these formats, the question is: how to obtain extended headers from the archive?
If you use a tar
implementation that does not support PAX format, extended headers for each member will
be extracted as a separate file. If we represent the member name as dir/name, then the
extended header file will be named dir/PaxHeaders.n/name, where n
is an integer number.
Things become more difficult if your tar
implementation does support PAX headers, because in this case you
will have to manually extract the headers. We recommend the following algorithm:
tar
implementation for an option that prints block numbers
along with the archive listing (analogous to GNU tar
's -R option). For
example, star
has -block-number. star
on our archive we obtain:
$ star -t -v -block-number -f arc.tar star: Unknown extended header keyword 'GNU.sparse.size' ignored. star: Unknown extended header keyword 'GNU.sparse.numblocks' ignored. star: Unknown extended header keyword 'GNU.sparse.name' ignored. star: Unknown extended header keyword 'GNU.sparse.map' ignored. block 56: 425984 -rw-r--r-- gray/users Jun 25 14:46 2006 GNUSparseFile.28124/sparsefile block 897: 65391 -rw-r--r-- gray/users Jun 24 20:06 2006 README |
(as usual, ignore the warnings about unknown keywords.)
N = Bs - Bn - size/512 - 2 |
This number gives the size of the extended header part in tar blocks. In our example, this formula gives:
897 - 56 - 425984 / 512 - 2 = 7
.
dd
to extract the headers:
dd if=archive of=hname bs=512 skip=Bs count=N |
where archive is the archive name, hname is a name of the file to store the extended header in, Bs and N are computed in previous steps.
In our example, this command will be
$ dd if=arc.tar of=xhdr bs=512 skip=56 count=7 |
Finally, you can expand the condensed file, using the obtained header:
$ xsparse -v -x xhdr GNUSparseFile.6058/sparsefile Reading extended header file Found variable GNU.sparse.size = 217481216 Found variable GNU.sparse.numblocks = 208 Found variable GNU.sparse.name = sparsefile Found variable GNU.sparse.map = 0,2048,1050624,2048, Expanding file `GNUSparseFile.28124/sparsefile' to `sparsefile' Done |
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tar
and cpio
(This message will disappear, once this node revised.)
The cpio
archive formats, like tar
, do have maximum file name lengths. The binary and old
ASCII formats have a maximum file length of 256, and the new ASCII and CRC
ASCII formats have a max file length of 1024. GNU cpio
can read and write archives
with arbitrary file name lengths, but other cpio
implementations may crash unexplainedly trying to read them.
tar
handles symbolic links in the form in which it comes in BSD; cpio
doesn't
handle symbolic links in the form in which it comes in System V prior to SVR4, and some vendors may have added symlinks
to their system without enhancing cpio
to know about them. Others may have enhanced it in a way other than
the way I did it at Sun, and which was adopted by AT&T (and which is, I think, also present in the cpio
that
Berkeley picked up from AT&T and put into a later BSD releaseI think I gave them my changes).
(SVR4 does some funny stuff with tar
; basically, its cpio
can handle tar
format
input, and write it on output, and it probably handles symbolic links. They may not have bothered doing anything to enhance
tar
as a result.)
cpio
handles special files; traditional tar
doesn't.
tar
comes with V7, System III, System V, and BSD source; cpio
comes only
with System III, System V, and later BSD (4.3-tahoe and later).
tar
's way of handling multiple hard links to a file can handle file systems that support 32-bit i-numbers
(e.g., the BSD file system); cpio
s way requires you to play some games (in its binary
format, i-numbers are only 16 bits, and in its portable ASCII format, they're 18 bitsit would have
to play games with the "file system ID" field of the header to make sure that the file system
ID/i-number pairs of different files were always different), and I don't know which cpio
s,
if any, play those games. Those that don't might get confused and think two files are the same file when they're not, and
make hard links between them.
tar
s way of handling multiple hard links to a file places only one copy of the link on the tape, but the
name attached to that copy is the only one you can use to retrieve the file; cpio
s way puts one copy
for every link, but you can retrieve it using any of the names.
What type of check sum (if any) is used, and how is this calculated.
See the attached manual pages for tar
and cpio
format. tar
uses a checksum which
is the sum of all the bytes in the tar
header for a file; cpio
uses no checksum.
If anyone knows why
cpio
was made whentar
was present at the unix scene,
It wasn't. cpio
first showed up in PWB/UNIX 1.0; no generally-available version of UNIX had tar
at the time. I don't know whether any version that was generally available within AT&T had tar
, or,
if so, whether the people within AT&T who did cpio
knew about it.
On restore, if there is a corruption on a tape tar
will stop at that point, while cpio
will
skip over it and try to restore the rest of the files.
The main difference is just in the command syntax and header format.
tar
is a little more tape-oriented in that everything is blocked to start on a record boundary.
Is there any differences between the ability to recover crashed archives between the two of them. (Is there any chance of recovering crashed archives at all.)
Theoretically it should be easier under tar
since the blocking lets you find a header with some variation
of dd skip=nn. However, modern cpio
's and variations have an option to just search
for the next file header after an error with a reasonable chance of resyncing. However, lots of tape driver software won't
allow you to continue past a media error which should be the only reason for getting out of sync unless a file changed sizes
while you were writing the archive.
If anyone knows why
cpio
was made whentar
was present at the unix scene, please tell me about this too.
Probably because it is more media efficient (by not blocking everything and using only the space needed for the headers
where tar
always uses 512 bytes per file header) and it knows how to archive special files.
You might want to look at the freely available alternatives. The major ones are afio
, GNU
tar
, and pax
, each of which have their own extensions with some backwards compatibility.
Sparse files were tar
red as sparse files (which you can easily test, because the resulting archive gets
smaller, and GNU cpio
can no longer read it).
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(This message will disappear, once this node revised.)
A few special cases about tape handling warrant more detailed description. These special cases are discussed below.
Many complexities surround the use of tar
on tape drives. Since the creation and manipulation of archives
located on magnetic tape was the original purpose of tar
, it contains many features making such manipulation
easier.
Archives are usually written on dismountable mediatape cartridges, mag tapes, or floppy disks.
The amount of data a tape or disk holds depends not only on its size, but also on how it is formatted. A 2400 foot long reel of mag tape holds 40 megabytes of data when formatted at 1600 bits per inch. The physically smaller EXABYTE tape cartridge holds 2.3 gigabytes.
Magnetic media are re-usableonce the archive on a tape is no longer needed, the archive can be erased and the tape or disk used over. Media quality does deteriorate with use, however. Most tapes or disks should be discarded when they begin to produce data errors. EXABYTE tape cartridges should be discarded when they generate an error count (number of non-usable bits) of more than 10k.
Magnetic media are written and erased using magnetic fields, and should be protected from such fields to avoid damage to stored data. Sticking a floppy disk to a filing cabinet using a magnet is probably not a good idea.
9.1 Device Selection and Switching | Device selection and switching | |
9.2 Remote Tape Server | ||
9.3 Some Common Problems and their Solutions | ||
9.4 Blocking | ||
9.5 Many Archives on One Tape | Many archives on one tape | |
9.6 Using Multiple Tapes | ||
9.7 Including a Label in the Archive | ||
9.8 Verifying Data as It is Stored | ||
9.9 Write Protection |
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(This message will disappear, once this node revised.)
This option is used to specify the file name of the archive tar
works on.
If the file name is -, tar
reads the archive from standard input (when listing or extracting),
or writes it to standard output (when creating). If the - file name is given when updating an archive,
tar
will read the original archive from its standard input, and will write the entire new archive to its standard
output.
If the file name contains a :, it is interpreted as hostname:file name. If the hostname
contains an at sign (@), it is treated as user@hostname:file name. In either case,
tar
will invoke the command rsh
(or remsh
) to start up an /usr/libexec/rmt
on the remote machine. If you give an alternate login name, it will be given to the rsh
. Naturally, the remote
machine must have an executable /usr/libexec/rmt
. This program is free software from the University of California,
and a copy of the source code can be found with the sources for tar
; it's compiled and installed by default.
The exact path to this utility is determined when configuring the package. It is prefix/libexec/rmt,
where prefix stands for your installation prefix. This location may also be overridden at runtime by using the
--rmt-command=command option (See section rmt-command, for detailed description
of this option. See section Remote Tape Server, for the description of rmt
command).
If this option is not given, but the environment variable TAPE
is set, its value is used; otherwise, old
versions of tar
used a default archive name (which was picked when tar
was compiled). The default
is normally set up to be the first tape drive or other transportable I/O medium on the system.
Starting with version 1.11.5, GNU tar
uses standard input and standard output as the
default device, and I will not try anymore supporting automatic device detection at installation time. This was failing
really in too many cases, it was hopeless. This is now completely left to the installer to override standard input and standard
output for default device, if this seems preferable. Further, I think most actual usages of tar
are
done with pipes or disks, not really tapes, cartridges or diskettes.
Some users think that using standard input and output is running after trouble. This could lead to a nasty surprise on your screen if you forget to specify an output file nameespecially if you are going through a network or terminal server capable of buffering large amounts of output. We had so many bug reports in that area of configuring default tapes automatically, and so many contradicting requests, that we finally consider the problem to be portably intractable. We could of course use something like /dev/tape as a default, but this is also running after various kind of trouble, going from hung processes to accidental destruction of real tapes. After having seen all this mess, using standard input and output as a default really sounds like the only clean choice left, and a very useful one too.
GNU tar
reads and writes archive in records, I suspect this is the main reason why block
devices are preferred over character devices. Most probably, block devices are more efficient too. The installer could also
check for DEFTAPE in <sys/mtio.h>.
rsh
. This option exists so that people who use something other
than the standard rsh
(e.g., a Kerberized rsh
) can access a remote device.
When this command is not used, the shell command found when the tar
program was installed is used instead.
This is the first found of /usr/ucb/rsh, /usr/bin/remsh, /usr/bin/rsh, /usr/bsd/rsh
or /usr/bin/nsh. The installer may have overridden this by defining the environment variable RSH
at installation time.
This option causes tar
to write a multi-volume archiveone that may be larger than will fit
on the medium used to hold it. See section Archives Longer than One Tape or Disk.
Suffix | Units | Byte Equivalent |
---|---|---|
b | Blocks | size x 512 |
B | Kilobytes | size x 1024 |
c | Bytes | size |
G | Gigabytes | size x 1024^3 |
K | Kilobytes | size x 1024 |
k | Kilobytes | size x 1024 |
M | Megabytes | size x 1024^2 |
P | Petabytes | size x 1024^5 |
T | Terabytes | size x 1024^4 |
w | Words | size x 2 |
Table 9.1: Size Suffixes
This option might be useful when your tape drivers do not properly detect end of physical tapes. By being slightly conservative on the maximum tape length, you might avoid the problem entirely.
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In order to access the tape drive on a remote machine, tar
uses the remote tape server written at the University
of California at Berkeley. The remote tape server must be installed as prefix/libexec/rmt on any machine
whose tape drive you want to use. tar
calls rmt
by running an rsh
or remsh
to the remote machine, optionally using a different login name if one is supplied.
A copy of the source for the remote tape server is provided. It is Copyright © 1983 by the Regents of the University of California, but can be freely distributed. It is compiled and installed by default.
Unless you use the --absolute-names (-P) option, GNU tar
will not allow you to create an archive that contains absolute file names (a file name beginning with /.)
If you try, tar
will automatically remove the leading / from the file names it stores in the
archive. It will also type a warning message telling you what it is doing.
When reading an archive that was created with a different tar
program, GNU tar
automatically extracts entries in the archive which have absolute file names as if the file names were not absolute. This
is an important feature. A visitor here once gave a tar
tape to an operator to restore; the operator used Sun
tar
instead of GNU tar
, and the result was that it replaced large portions
of our /bin and friends with versions from the tape; needless to say, we were unhappy about having to recover
the file system from backup tapes.
For example, if the archive contained a file /usr/bin/computoy, GNU tar
would
extract the file to usr/bin/computoy, relative to the current directory. If you want to extract the files in
an archive to the same absolute names that they had when the archive was created, you should do a cd / before
extracting the files from the archive, or you should either use the --absolute-names option, or use the command
tar -C /
.
Some versions of Unix (Ultrix 3.1 is known to have this problem), can claim that a short write near the end of a tape succeeded, when it actually failed. This will result in the -M option not working correctly. The best workaround at the moment is to use a significantly larger blocking factor than the default 20.
In order to update an archive, tar
must be able to backspace the archive in order to reread or rewrite a
record that was just read (or written). This is currently possible only on two kinds of files: normal disk files (or any
other file that can be backspaced with lseek), and industry-standard 9-track magnetic tape (or any other
kind of tape that can be backspaced with the MTIOCTOP
ioctl
).
This means that the --append, --concatenate, and --delete commands will not work on any other kind of file. Some media simply cannot be backspaced, which means these commands and options will never be able to work on them. These non-backspacing media include pipes and cartridge tape drives.
Some other media can be backspaced, and tar
will work on them once tar
is modified to do so.
Archives created with the --multi-volume, --label, and --incremental (-G)
options may not be readable by other version of tar
. In particular, restoring a file that was split over a
volume boundary will require some careful work with dd
, if it can be done at all. Other versions of tar
may also create an empty file whose name is that of the volume header. Some versions of tar
may create normal
files instead of directories archived with the --incremental (-G) option.
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errors from system:
permission denied
no such file or directory
not owner
errors from
|
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Block and record terminology is rather confused, and it is also confusing to the expert reader. On the other hand, readers who are new to the field have a fresh mind, and they may safely skip the next two paragraphs, as the remainder of this manual uses those two terms in a quite consistent way.
John Gilmore, the writer of the public domain tar
from which GNU tar
was
originally derived, wrote (June 1995):
The nomenclature of tape drives comes from IBM, where I believe they were invented for the IBM 650 or so. On IBM mainframes, what is recorded on tape are tape blocks. The logical organization of data is into records. There are various ways of putting records into blocks, including
F
(fixed sized records),V
(variable sized records),FB
(fixed blocked: fixed size records, n to a block),VB
(variable size records, n to a block),VSB
(variable spanned blocked: variable sized records that can occupy more than one block), etc. TheJCL
DD RECFORM= parameter specified this to the operating system.The Unix man page on
tar
was totally confused about this. When I wrotePD TAR
, I used the historically correct terminology (tar
writes data records, which are grouped into blocks). It appears that the bogus terminology made it into POSIX (no surprise here), and now Franηois has migrated that terminology back into the source code too.
The term physical block means the basic transfer chunk from or to a device, after which reading or writing may
stop without anything being lost. In this manual, the term block usually refers to a disk physical block, assuming
that each disk block is 512 bytes in length. It is true that some disk devices have different physical blocks, but
tar
ignore these differences in its own format, which is meant to be portable, so a tar
block is always
512 bytes in length, and block always mean a tar
block. The term logical block often represents
the basic chunk of allocation of many disk blocks as a single entity, which the operating system treats somewhat atomically;
this concept is only barely used in GNU tar
.
The term physical record is another way to speak of a physical block, those two terms are somewhat interchangeable.
In this manual, the term record usually refers to a tape physical block, assuming that the tar
archive is kept on magnetic tape. It is true that archives may be put on disk or used with pipes, but nevertheless,
tar
tries to read and write the archive one record at a time, whatever the medium in use. One record
is made up of an integral number of blocks, and this operation of putting many disk blocks into a single tape block is called
reblocking, or more simply, blocking. The term logical record refers to the logical organization
of many characters into something meaningful to the application. The term unit record describes a small set of
characters which are transmitted whole to or by the application, and often refers to a line of text. Those two last terms
are unrelated to what we call a record in GNU tar
.
When writing to tapes, tar
writes the contents of the archive in chunks known as records. To change
the default blocking factor, use the --blocking-factor=512-size (-b 512-size)
option. Each record will then be composed of 512-size blocks. (Each tar
block is 512 bytes. See
section Basic Tar Format.) Each file written to the archive uses at least one full record. As a result,
using a larger record size can result in more wasted space for small files. On the other hand, a larger record size can
often be read and written much more efficiently.
Further complicating the problem is that some tape drives ignore the blocking entirely. For these, a larger record size can still improve performance (because the software layers above the tape drive still honor the blocking), but not as dramatically as on tape drives that honor blocking.
When reading an archive, tar
can usually figure out the record size on itself. When this is the case, and
a non-standard record size was used when the archive was created, tar
will print a message about a non-standard
blocking factor, and then operate normally. On some tape devices, however, tar
cannot figure out the record
size itself. On most of those, you can specify a blocking factor (with --blocking-factor) larger than the
actual blocking factor, and then use the --read-full-records (-B) option. (If you specify
a blocking factor with --blocking-factor and don't use the --read-full-records option, then
tar
will not attempt to figure out the recording size itself.) On some devices, you must always specify the
record size exactly with --blocking-factor when reading, because tar
cannot figure it out. In
any case, use --list (-t) before doing any extractions to see whether tar
is
reading the archive correctly.
tar
blocks are all fixed size (512 bytes), and its scheme for putting them into records is to put a whole
number of them (one or more) into each record. tar
records are all the same size; at the end of the file there's
a block containing all zeros, which is how you tell that the remainder of the last record(s) are garbage.
In a standard tar
file (no options), the block size is 512 and the record size is 10240, for a blocking
factor of 20. What the --blocking-factor option does is sets the blocking factor, changing the record size
while leaving the block size at 512 bytes. 20 was fine for ancient 800 or 1600 bpi reel-to-reel tape drives; most tape drives
these days prefer much bigger records in order to stream and not waste tape. When writing tapes for myself, some tend to
use a factor of the order of 2048, say, giving a record size of around one megabyte.
If you use a blocking factor larger than 20, older tar
programs might not be able to read the archive, so
we recommend this as a limit to use in practice. GNU tar
, however, will support arbitrarily
large record sizes, limited only by the amount of virtual memory or the physical characteristics of the tape device.
9.4.1 Format Variations | ||
9.4.2 The Blocking Factor of an Archive |
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Format parameters specify how an archive is written on the archive media. The best choice of format parameters will vary depending on the type and number of files being archived, and on the media used to store the archive.
To specify format parameters when accessing or creating an archive, you can use the options described in the following
sections. If you do not specify any format parameters, tar
uses default parameters. You cannot modify a compressed
archive. If you create an archive with the --blocking-factor option specified (see section
The Blocking Factor of an Archive), you must specify that blocking-factor when operating on the archive.
See section Controlling the Archive Format, for other examples of format parameter considerations.
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The data in an archive is grouped into blocks, which are 512 bytes. Blocks are read and written in whole number multiples called records. The number of blocks in a record (i.e., the size of a record in units of 512 bytes) is called the blocking factor. The --blocking-factor=512-size (-b 512-size) option specifies the blocking factor of an archive. The default blocking factor is typically 20 (i.e., 10240 bytes), but can be specified at installation. To find out the blocking factor of an existing archive, use tar --list --file=archive-name. This may not work on some devices.
Records are separated by gaps, which waste space on the archive media. If you are archiving on magnetic tape, using a
larger blocking factor (and therefore larger records) provides faster throughput and allows you to fit more data on a tape
(because there are fewer gaps). If you are archiving on cartridge, a very large blocking factor (say 126 or more) greatly
increases performance. A smaller blocking factor, on the other hand, may be useful when archiving small files, to avoid
archiving lots of nulls as tar
fills out the archive to the end of the record. In general, the ideal record
size depends on the size of the inter-record gaps on the tape you are using, and the average size of the files you are archiving.
See section How to Create Archives, for information on writing archives.
Archives with blocking factors larger than 20 cannot be read by very old versions of tar
, or by some newer
versions of tar
running on old machines with small address spaces. With GNU tar
,
the blocking factor of an archive is limited only by the maximum record size of the device containing the archive, or by
the amount of available virtual memory.
Also, on some systems, not using adequate blocking factors, as sometimes imposed by the device drivers, may yield unexpected diagnostics. For example, this has been reported:
Cannot write to /dev/dlt: Invalid argument |
In such cases, it sometimes happen that the tar
bundled by the system is aware of block size idiosyncrasies,
while GNU tar
requires an explicit specification for the block size, which it cannot guess.
This yields some people to consider GNU tar
is misbehaving, because by comparison,
the bundle tar
works OK. Adding -b 256, for example, might resolve the problem.
If you use a non-default blocking factor when you create an archive, you must specify the same blocking factor when you
modify that archive. Some archive devices will also require you to specify the blocking factor when reading that archive,
however this is not typically the case. Usually, you can use --list (-t) without specifying
a blocking factortar
reports a non-default record size and then lists the archive members as it would normally.
To extract files from an archive with a non-standard blocking factor (particularly if you're not sure what the blocking
factor is), you can usually use the --read-full-records (-B) option while specifying a blocking
factor larger then the blocking factor of the archive (i.e., tar --extract --read-full-records --blocking-factor=300).
See section How to List Archives, for more information on the --list (-t)
operation. See section Options to Help Read Archives, for a more detailed explanation of that option.
Device blocking
This option is used to specify a blocking factor for the archive. When reading or writing the archive,
tar
, will do reads and writes of the archive in records of block*512 bytes. This is
true even when the archive is compressed. Some devices requires that all write operations be a multiple of a certain
size, and so, tar
pads the archive out to the next record boundary.
The default blocking factor is set when tar
is compiled, and is typically 20. Blocking factors larger
than 20 cannot be read by very old versions of tar
, or by some newer versions of tar
running
on old machines with small address spaces.
With a magnetic tape, larger records give faster throughput and fit more data on a tape (because there are fewer inter-record gaps). If the archive is in a disk file or a pipe, you may want to specify a smaller blocking factor, since a large one will result in a large number of null bytes at the end of the archive.
When writing cartridge or other streaming tapes, a much larger blocking factor (say 126 or more) will greatly increase performance. However, you must specify the same blocking factor when reading or updating the archive.
Apparently, Exabyte drives have a physical block size of 8K bytes. If we choose our blocksize as a multiple of 8k bytes, then the problem seems to disappear. Id est, we are using block size of 112 right now, and we haven't had the problem since we switched
With GNU tar
the blocking factor is limited only by the maximum record size of the
device containing the archive, or by the amount of available virtual memory.
However, deblocking or reblocking is virtually avoided in a special case which often occurs in practice, but which requires all the following conditions to be simultaneously true:
tar
invocation. If the output goes directly to a local disk, and not through stdout, then the last write is not extended to a full record size. Otherwise, reblocking occurs. Here are a few other remarks on this topic:
gzip
will complain about trailing garbage if asked to uncompress a compressed archive on tape,
there is an option to turn the message off, but it breaks the regularity of simply having to use prog
-d for decompression. It would be nice if gzip was silently ignoring any number of trailing zeros. I'll
ask Jean-loup Gailly, by sending a copy of this message to him. compress
does not show this problem, but as Jean-loup pointed out to Michael, compress -d
silently adds garbage after the result of decompression, which tar ignores because it already recognized its end-of-file
indicator. So this bug may be safely ignored. tar
might ignore the exit status returned, but I hate doing that, as
it weakens the protection tar
offers users against other possible problems at decompression time. If
gzip
was silently skipping trailing zeros and also avoiding setting the exit status in this
innocuous case, that would solve this situation. tar
should become more solid at not stopping to read a pipe at the first null block encountered.
This inelegantly breaks the pipe. tar
should rather drain the pipe out before exiting itself. The --ignore-zeros (-i) option causes tar
to ignore blocks of zeros in
the archive. Normally a block of zeros indicates the end of the archive, but when reading a damaged archive, or one
which was created by concatenating several archives together, this option allows tar
to read the entire
archive. This option is not on by default because many versions of tar
write garbage after the zeroed blocks.
Note that this option causes tar
to read to the end of the archive file, which may sometimes avoid problems
when multiple files are stored on a single physical tape.
If --read-full-records is used, tar
will not panic if an attempt to read a record from
the archive does not return a full record. Instead, tar
will keep reading until it has obtained a full
record.
This option is turned on by default when tar
is reading an archive from standard input, or from a remote
machine. This is because on BSD Unix systems, a read of a pipe will return however much happens to
be in the pipe, even if it is less than tar
requested. If this option was not used, tar
would
fail as soon as it read an incomplete record from the pipe.
This option is also useful with the commands for updating an archive.
Tape blocking
When handling various tapes or cartridges, you have to take care of selecting a proper blocking, that is, the number of disk blocks you put together as a single tape block on the tape, without intervening tape gaps. A tape gap is a small landing area on the tape with no information on it, used for decelerating the tape to a full stop, and for later regaining the reading or writing speed. When the tape driver starts reading a record, the record has to be read whole without stopping, as a tape gap is needed to stop the tape motion without losing information.
Using higher blocking (putting more disk blocks per tape block) will use the tape more efficiently as there will be less
tape gaps. But reading such tapes may be more difficult for the system, as more memory will be required to receive at once
the whole record. Further, if there is a reading error on a huge record, this is less likely that the system will succeed
in recovering the information. So, blocking should not be too low, nor it should be too high. tar
uses by default
a blocking of 20 for historical reasons, and it does not really matter when reading or writing to disk. Current tape technology
would easily accommodate higher blockings. Sun recommends a blocking of 126 for Exabytes and 96 for DATs. We were told that
for some DLT drives, the blocking should be a multiple of 4Kb, preferably 64Kb (-b 128) or 256 for decent performance.
Other manufacturers may use different recommendations for the same tapes. This might also depends of the buffering techniques
used inside modern tape controllers. Some imposes a minimum blocking, or a maximum blocking. Others request blocking to
be some exponent of two.
So, there is no fixed rule for blocking. But blocking at read time should ideally be the same as blocking used at write time. At one place I know, with a wide variety of equipment, they found it best to use a blocking of 32 to guarantee that their tapes are fully interchangeable.
I was also told that, for recycled tapes, prior erasure (by the same drive unit that will be used to create the archives) sometimes lowers the error rates observed at rewriting time.
I might also use --number-blocks instead of --block-number, so --block will then expand to --blocking-factor unambiguously.
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Most tape devices have two entries in the /dev directory, or entries that come in pairs, which differ only in the minor number for this device. Let's take for example /dev/tape, which often points to the only or usual tape device of a given system. There might be a corresponding /dev/nrtape or /dev/ntape. The simpler name is the rewinding version of the device, while the name having nr in it is the no rewinding version of the same device.
A rewinding tape device will bring back the tape to its beginning point automatically when this device is opened or closed.
Since tar
opens the archive file before using it and closes it afterwards, this means that a simple:
$ tar cf /dev/tape directory |
will reposition the tape to its beginning both prior and after saving directory contents to it, thus erasing prior tape contents and making it so that any subsequent write operation will destroy what has just been saved.
So, a rewinding device is normally meant to hold one and only one file. If you want to put more than one tar
archive on a given tape, you will need to avoid using the rewinding version of the tape device. You will also have to pay
special attention to tape positioning. Errors in positioning may overwrite the valuable data already on your tape. Many
people, burnt by past experiences, will only use rewinding devices and limit themselves to one file per tape, precisely
to avoid the risk of such errors. Be fully aware that writing at the wrong position on a tape loses all information past
this point and most probably until the end of the tape, and this destroyed information cannot be recovered.
To save directory-1 as a first archive at the beginning of a tape, and leave that tape ready for a second archive, you should use:
$ mt -f /dev/nrtape rewind $ tar cf /dev/nrtape directory-1 |
Tape marks are special magnetic patterns written on the tape media, which are later recognizable by the reading
hardware. These marks are used after each file, when there are many on a single tape. An empty file (that is to say, two
tape marks in a row) signal the logical end of the tape, after which no file exist. Usually, non-rewinding tape device drivers
will react to the close request issued by tar
by first writing two tape marks after your archive, and by backspacing
over one of these. So, if you remove the tape at that time from the tape drive, it is properly terminated. But if you write
another file at the current position, the second tape mark will be erased by the new information, leaving only one tape
mark between files.
So, you may now save directory-2 as a second archive after the first on the same tape by issuing the command:
$ tar cf /dev/nrtape directory-2 |
and so on for all the archives you want to put on the same tape.
Another usual case is that you do not write all the archives the same day, and you need to remove and store the tape between two archive sessions. In general, you must remember how many files are already saved on your tape. Suppose your tape already has 16 files on it, and that you are ready to write the 17th. You have to take care of skipping the first 16 tape marks before saving directory-17, say, by using these commands:
$ mt -f /dev/nrtape rewind $ mt -f /dev/nrtape fsf 16 $ tar cf /dev/nrtape directory-17 |
In all the previous examples, we put aside blocking considerations, but you should do the proper things for that as well. See section Blocking.
9.5.1 Tape Positions and Tape Marks | ||
9.5.2 The mt Utility |
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Just as archives can store more than one file from the file system, tapes can store more than one archive file. To keep track of where archive files (or any other type of file stored on tape) begin and end, tape archive devices write magnetic tape marks on the archive media. Tape drives write one tape mark between files, two at the end of all the file entries.
If you think of data as a series of records "rrrr"'s, and tape marks as "*"'s, a tape might look like the following:
rrrr*rrrrrr*rrrrr*rr*rrrrr**------------------------- |
Tape devices read and write tapes using a read/write tape heada physical part of the device which can only
access one point on the tape at a time. When you use tar
to read or write archive data from a tape device,
the device will begin reading or writing from wherever on the tape the tape head happens to be, regardless of which archive
or what part of the archive the tape head is on. Before writing an archive, you should make sure that no data on the tape
will be overwritten (unless it is no longer needed). Before reading an archive, you should make sure the tape head is at
the beginning of the archive you want to read. You can do it manually via mt
utility (see section
The mt
Utility). The restore
script does that automatically (see section
Using the Restore Script).
If you want to add new archive file entries to a tape, you should advance the tape to the end of the existing file entries, backspace over the last tape mark, and write the new archive file. If you were to add two archives to the example above, the tape might look like the following:
rrrr*rrrrrr*rrrrr*rr*rrrrr*rrr*rrrr**---------------- |
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mt
Utility (This message will disappear, once this node revised.)
See section The Blocking Factor of an Archive.
You can use the mt
utility to advance or rewind a tape past a specified number of archive files on the tape.
This will allow you to move to the beginning of an archive before extracting or reading it, or to the end of all the archives
before writing a new one.
The syntax of the mt
command is:
mt [-f tapename] operation [number] |
where tapename is the name of the tape device, number is the number of times an operation is performed (with a default of one), and operation is one of the following:
If you don't specify a tapename, mt
uses the environment variable TAPE
; if
TAPE
is not set, mt
will use the default device specified in your sys/mtio.h file (DEFTAPE
variable). If this is not defined, the program will display a descriptive error message and exit with code 1.
mt
returns a 0 exit status when the operation(s) were successful, 1 if the command was unrecognized, and
2 if an operation failed.
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Often you might want to write a large archive, one larger than will fit on the actual tape you are using. In such a case,
you can run multiple tar
commands, but this can be inconvenient, particularly if you are using options like
--exclude=pattern or dumping entire file systems. Therefore, tar
provides a special
mode for creating multi-volume archives.
Multi-volume archive is a single tar
archive, stored on several media volumes of fixed size. Although
in this section we will often call volume a tape, there is absolutely no requirement for multi-volume
archives to be stored on tapes. Instead, they can use whatever media type the user finds convenient, they can even be located
on files.
When creating a multi-volume archive, GNU tar
continues to fill current volume until
it runs out of space, then it switches to next volume (usually the operator is queried to replace the tape on this point),
and continues working on the new volume. This operation continues until all requested files are dumped. If GNU
tar
detects end of media while dumping a file, such a file is archived in split form. Some very big files can
even be split across several volumes.
Each volume is itself a valid GNU tar
archive, so it can be read without any special
options. Consequently any file member residing entirely on one volume can be extracted or otherwise operated upon without
needing the other volume. Sure enough, to extract a split member you would need all volumes its parts reside on.
Multi-volume archives suffer from several limitations. In particular, they cannot be compressed.
GNU tar
is able to create multi-volume archives of two formats (see section
Controlling the Archive Format): GNU and POSIX.
9.6.1 Archives Longer than One Tape or Disk | ||
9.6.2 Tape Files | ||
9.6.3 Concatenate Volumes into a Single Archive | ||
|
---|
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To create an archive that is larger than will fit on a single unit of the media, use the --multi-volume (-M) option in conjunction with the --create option (see section How to Create Archives). A multi-volume archive can be manipulated like any other archive (provided the --multi-volume option is specified), but is stored on more than one tape or file.
When you specify --multi-volume, tar
does not report an error when it comes to the end of
an archive volume (when reading), or the end of the media (when writing). Instead, it prompts you to load a new storage
volume. If the archive is on a magnetic tape, you should change tapes when you see the prompt; if the archive is on a floppy
disk, you should change disks; etc.
$ tar --create --multi-volume --file=/dev/tape files |
The method tar
uses to detect end of tape is not perfect, and fails on some operating systems or on some
devices. If tar
cannot detect the end of the tape itself, you can use --tape-length option to
inform it about the capacity of the tape:
This option selects --multi-volume automatically. For example:
$ tar --create --tape-length=41943040 --file=/dev/tape files |
or, which is equivalent:
$ tar --create --tape-length=4G --file=/dev/tape files |
When GNU tar
comes to the end of a storage media, it asks you to change the volume. The
built-in prompt for POSIX locale is(23):
Prepare volume #n for `archive' and hit return: |
where n is the ordinal number of the volume to be created and archive is archive file or device name.
When prompting for a new tape, tar
accepts any of the following responses:
tar
to explain possible responses. tar
to exit immediately. tar
to write the next volume on the file file-name. tar
to run a subshell. This option can be disabled by giving --restrict command
line option to tar
(24). tar
to begin writing the next volume. (You should only type y after you have changed the tape; otherwise tar
will write over the
volume it just finished.)
The volume number used by tar
in its tape-changing prompt can be changed; if you give the --volno-file=file-of-number
option, then file-of-number should be an non-existing file to be created, or else, a file already containing
a decimal number. That number will be used as the volume number of the first volume written. When tar
is finished,
it will rewrite the file with the now-current volume number. (This does not change the volume number written on a tape label,
as per Including a Label in the Archive, it only affects the number used in the prompt.)
If you want more elaborate behavior than this, you can write a special new volume script, that will be responsible
for changing the volume, and instruct tar
to use it instead of its normal prompting procedure:
The script-name is executed without any command line arguments. It inherits tar
's shell environment.
Additional data is passed to it via the following environment variables:
TAR_VERSION
tar
version number. TAR_ARCHIVE
tar
is processing. TAR_BLOCKING_FACTOR
TAR_VOLUME
tar
is about to start. TAR_SUBCOMMAND
tar
is executing. See section The Five Advanced
tar
Operations, for a complete list of subcommand options. TAR_FORMAT
TAR_FD
tar
. The volume script can instruct tar
to use new archive name, by writing in to file descriptor $TAR_FD
(see below for an example).
If the info script fails, tar
exits; otherwise, it begins writing the next volume.
If you want tar
to cycle through a series of files or tape drives, there are three approaches to choose
from. First of all, you can give tar
multiple --file options. In this case the specified files
will be used, in sequence, as the successive volumes of the archive. Only when the first one in the sequence needs to be
used again will tar
prompt for a tape change (or run the info script). For example, suppose someone has two
tape drives on a system named /dev/tape0 and /dev/tape1. For having GNU tar
to switch to the second drive when it needs to write the second tape, and then back to the first tape, etc., just do either
of:
$ tar --create --multi-volume --file=/dev/tape0 --file=/dev/tape1 files $ tar cMff /dev/tape0 /dev/tape1 files |
The second method is to use the n response to the tape-change prompt.
Finally, the most flexible approach is to use a volume script, that writes new archive name to the file descriptor
$TAR_FD
. For example, the following volume script will create a series of archive files, named archive-vol,
where archive is the name of the archive being created (as given by --file option) and vol
is the ordinal number of the archive being created:
#! /bin/sh echo Preparing volume $TAR_VOLUME of $TAR_ARCHIVE. name=`expr $TAR_ARCHIVE : '\(.*\)-.*'` case $TAR_SUBCOMMAND in -c) ;; -d|-x|-t) test -r ${name:-$TAR_ARCHIVE}-$TAR_VOLUME || exit 1 ;; *) exit 1 esac echo ${name:-$TAR_ARCHIVE}-$TAR_VOLUME >&$TAR_FD |
The same script can be used while listing, comparing or extracting from the created archive. For example:
# Create a multi-volume archive: $ tar -c -L1024 -f archive.tar -F new-volume . # Extract from the created archive: $ tar -x -f archive.tar -F new-volume . |
Notice, that the first command had to use -L option, since otherwise GNU tar
will end up writing everything to file archive.tar.
You can read each individual volume of a multi-volume archive as if it were an archive by itself. For example, to list the contents of one volume, use --list, without --multi-volume specified. To extract an archive member from one volume (assuming it is described that volume), use --extract, again without --multi-volume.
If an archive member is split across volumes (i.e., its entry begins on one volume of the media and ends on another),
you need to specify --multi-volume to extract it successfully. In this case, you should load the volume where
the archive member starts, and use tar --extract --multi-volumetar
will prompt for later volumes
as it needs them. See section Extracting an Entire Archive, for more information about extracting archives.
Multi-volume archives can be modified like any other archive. To add files to a multi-volume archive, you need to only mount the last volume of the archive media (and new volumes, if needed). For all other operations, you need to use the entire archive.
If a multi-volume archive was labeled using --label=archive-label (see section
Including a Label in the Archive) when it was created, tar
will not automatically label
volumes which are added later. To label subsequent volumes, specify --label=archive-label again
in conjunction with the --append, --update or --concatenate operation.
Notice that multi-volume support is a GNU extension and the archives created in this mode should be read only using
GNU tar
. If you absolutely have to process such archives using a third-party tar
implementation, read Extracting Members Split Between Volumes.
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To give the archive a name which will be recorded in it, use the --label=volume-label (-V volume-label) option. This will write a special block identifying volume-label as the name of the archive to the front of the archive which will be displayed when the archive is listed with --list. If you are creating a multi-volume archive with --multi-volume (see section Using Multiple Tapes), then the volume label will have Volume nnn appended to the name you give, where nnn is the number of the volume of the archive. If you use the --label=volume-label option when reading an archive, it checks to make sure the label on the tape matches the one you gave. See section Including a Label in the Archive.
When tar
writes an archive to tape, it creates a single tape file. If multiple archives are written to the
same tape, one after the other, they each get written as separate tape files. When extracting, it is necessary to position
the tape at the right place before running tar
. To do this, use the mt
command. For more information
on the mt
command and on the organization of tapes into a sequence of tape files, see The
mt
Utility.
People seem to often do:
--label="some-prefix `date +some-format`" |
or such, for pushing a common date in all volumes or an archive set.
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Sometimes it is necessary to convert existing GNU tar
multi-volume archive to a single
tar
archive. Simply concatenating all volumes into one will not work, since each volume carries an additional
information at the beginning. GNU tar
is shipped with the shell script tarcat
designed for this purpose.
The script takes a list of files comprising a multi-volume archive and creates the resulting archive at the standard output. For example:
tarcat vol.1 vol.2 vol.3 | tar tf - |
The script implements a simple heuristics to determine the format of the first volume file and to decide how to process
the rest of the files. However, it makes no attempt to verify whether the files are given in order or even if they are valid
tar
archives. It uses dd
and does not filter its standard error, so you will usually see lots
of spurious messages.
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To avoid problems caused by misplaced paper labels on the archive media, you can include a label entry an archive member which contains the name of the archive in the archive itself. Use the --label=archive-label (-V archive-label) option(25) in conjunction with the --create operation to include a label entry in the archive as it is being created.
If you create an archive using both --label=archive-label (-V archive-label) and --multi-volume (-M), each volume of the archive will have an archive label of the form archive-label Volume n, where n is 1 for the first volume, 2 for the next, and so on. See section Using Multiple Tapes, for information on creating multiple volume archives.
The volume label will be displayed by --list along with the file contents. If verbose display is requested, it will also be explicitly marked as in the example below:
$ tar --verbose --list --file=iamanarchive V--------- 0/0 0 1992-03-07 12:01 iamalabel--Volume Header-- -rw-r--r-- ringo/user 40 1990-05-21 13:30 iamafilename |
However, --list option will cause listing entire contents of the archive, which may be undesirable (for example, if the archive is stored on a tape). You can request checking only the volume label by specifying --test-label option. This option reads only the first block of an archive, so it can be used with slow storage devices. For example:
$ tar --test-label --file=iamanarchive iamalabel |
If --test-label is used with one or more command line arguments, tar
compares the volume
label with each argument. It exits with code 0 if a match is found, and with code 1 otherwise(26).
No output is displayed, unless you also used the --verbose option. For example:
$ tar --test-label --file=iamanarchive 'iamalabel' ⇒ 0 $ tar --test-label --file=iamanarchive 'alabel' ⇒ 1 |
When used with the --verbose option, tar
prints the actual volume label (if any), and a verbose
diagnostics in case of a mismatch:
$ tar --test-label --verbose --file=iamanarchive 'iamalabel' iamalabel ⇒ 0 $ tar --test-label --verbose --file=iamanarchive 'alabel' iamalabel tar: Archive label mismatch ⇒ 1 |
If you request any operation, other than --create, along with using --label option,
tar
will first check if the archive label matches the one specified and will refuse to proceed if it does not.
Use this as a safety precaution to avoid accidentally overwriting existing archives. For example, if you wish to add files
to archive, presumably labeled with string My volume, you will get:
$ tar -rf archive --label 'My volume' . tar: Archive not labeled to match `My volume' |
in case its label does not match. This will work even if archive is not labeled at all.
Similarly, tar
will refuse to list or extract the archive if its label doesn't match the archive-label
specified. In those cases, archive-label argument is interpreted as a globbing-style pattern which must match
the actual magnetic volume label. See section Excluding Some Files, for a precise description of how
match is attempted(27). If the switch --multi-volume (-M)
is being used, the volume label matcher will also suffix archive-label by Volume [1-9]* if the
initial match fails, before giving up. Since the volume numbering is automatically added in labels at creation time, it
sounded logical to equally help the user taking care of it when the archive is being read.
You can also use --label to get a common information on all tapes of a series. For having this information different in each series created through a single script used on a regular basis, just manage to get some date string as part of the label. For example:
$ tar cfMV /dev/tape "Daily backup for `date +%Y-%m-%d`" $ tar --create --file=/dev/tape --multi-volume \ --label="Daily backup for `date +%Y-%m-%d`" |
Some more notes about volume labels:
tar
initially attempted to write it, often soon after the operator launches tar
or types the carriage return
telling that the next tape is ready. [ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
This option causes tar
to verify the archive after writing it. Each volume is checked after it is written,
and any discrepancies are recorded on the standard error output.
Verification requires that the archive be on a back-space-able medium. This means pipes, some cartridge tape drives, and some other devices cannot be verified.
You can insure the accuracy of an archive by comparing files in the system with archive members. tar
can
compare an archive to the file system as the archive is being written, to verify a write operation, or can compare a previously
written archive, to insure that it is up to date.
To check for discrepancies in an archive immediately after it is written, use the --verify (-W)
option in conjunction with the --create operation. When this option is specified, tar
checks
archive members against their counterparts in the file system, and reports discrepancies on the standard error.
To verify an archive, you must be able to read it from before the end of the last written entry. This option is useful for detecting data errors on some tapes. Archives written to pipes, some cartridge tape drives, and some other devices cannot be verified.
One can explicitly compare an already made archive with the file system by using the --compare (--diff, -d) option, instead of using the more automatic --verify option. See section Comparing Archive Members with the File System.
Note that these two options have a slightly different intent. The --compare option checks how identical
are the logical contents of some archive with what is on your disks, while the --verify option is really
for checking if the physical contents agree and if the recording media itself is of dependable quality. So, for the --verify
operation, tar
tries to defeat all in-memory cache pertaining to the archive, while it lets the speed optimization
undisturbed for the --compare option. If you nevertheless use --compare for media verification,
you may have to defeat the in-memory cache yourself, maybe by opening and reclosing the door latch of your recording unit,
forcing some doubt in your operating system about the fact this is really the same volume as the one just written or read.
The --verify option would not be necessary if drivers were indeed able to detect dependably all write failures. This sometimes require many magnetic heads, some able to read after the writes occurred. One would not say that drivers unable to detect all cases are necessarily flawed, as long as programming is concerned.
The --verify (-W) option will not work in conjunction with the --multi-volume
(-M) option or the --append (-r), --update (-u)
and --delete operations. See section The Five Advanced tar
Operations,
for more information on these operations.
Also, since tar
normally strips leading / from file names (see section
Absolute File Names), a command like tar --verify -cf /tmp/foo.tar /etc will work as desired only if
the working directory is /, as tar
uses the archive's relative member names (e.g., etc/motd)
when verifying the archive.
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Almost all tapes and diskettes, and in a few rare cases, even disks can be write protected, to protect data on them from being changed. Once an archive is written, you should write protect the media to prevent the archive from being accidentally overwritten or deleted. (This will protect the archive from being changed with a tape or floppy driveit will not protect it from magnet fields or other physical hazards.)
The write protection device itself is usually an integral part of the physical media, and can be a two position (write enabled/write disabled) switch, a notch which can be popped out or covered, a ring which can be removed from the center of a tape reel, or some other changeable feature.
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The tar
command reads and writes files as any other application does, and is subject to the usual caveats
about reliability and security. This section contains some commonsense advice on the topic.
10.1 Reliability | ||
10.2 Security |
[ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
Ideally, when tar
is creating an archive, it reads from a file system that is not being modified, and encounters
no errors or inconsistencies while reading and writing. If this is the case, the archive should faithfully reflect what
was read. Similarly, when extracting from an archive, ideally tar
ideally encounters no errors and the extracted
files faithfully reflect what was in the archive.
However, when reading or writing real-world file systems, several things can go wrong; these include permissions problems, corruption of data, and race conditions.
10.1.1 Permissions Problems | ||
10.1.2 Data Corruption and Repair | ||
10.1.3 Race conditions |
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If tar
encounters errors while reading or writing files, it normally reports an error and exits with nonzero
status. The work it does may therefore be incomplete. For example, when creating an archive, if tar
cannot
read a file then it cannot copy the file into the archive.
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If an archive becomes corrupted by an I/O error, this may corrupt the data in an extracted file. Worse, it may corrupt the file's metadata, which may cause later parts of the archive to become misinterpreted. An tar-format archive contains a checksum that most likely will detect errors in the metadata, but it will not detect errors in the data.
If data corruption is a concern, you can compute and check your own checksums of an archive by using other programs,
such as cksum
.
When attempting to recover from a read error or data corruption in an archive, you may need to skip past the questionable data and read the rest of the archive. This requires some expertise in the archive format and in other software tools.
[ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
If some other process is modifying the file system while tar
is reading or writing files, the result may
well be inconsistent due to race conditions. For example, if another process creates some files in a directory while
tar
is creating an archive containing the directory's files, tar
may see some of the files but
not others, or it may see a file that is in the process of being created. The resulting archive may not be a snapshot of
the file system at any point in time. If an application such as a database system depends on an accurate snapshot, restoring
from the tar
archive of a live file system may therefore break that consistency and may break the application.
The simplest way to avoid the consistency issues is to avoid making other changes to the file system while tar is reading
it or writing it.
When creating an archive, several options are available to avoid race conditions. Some hosts have a way of snapshotting
a file system, or of temporarily suspending all changes to a file system, by (say) suspending the only virtual machine that
can modify a file system; if you use these facilities and have tar -c
read from a snapshot when creating an
archive, you can avoid inconsistency problems. More drastically, before starting tar
you could suspend or shut
down all processes other than tar
that have access to the file system, or you could unmount the file system
and then mount it read-only.
When extracting from an archive, one approach to avoid race conditions is to create a directory that no other process can write to, and extract into that.
[ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
In some cases tar
may be used in an adversarial situation, where an untrusted user is attempting to gain
information about or modify otherwise-inaccessible files. Dealing with untrusted data (that is, data generated by an untrusted
user) typically requires extra care, because even the smallest mistake in the use of tar
is more likely to
be exploited by an adversary than by a race condition.
10.2.1 Privacy | ||
10.2.2 Integrity | ||
10.2.3 Dealing with Live Untrusted Data | ||
10.2.4 Security Rules of Thumb |
[ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
Standard privacy concerns apply when using tar
. For example, suppose you are archiving your home directory
into a file /archive/myhome.tar. Any secret information in your home directory, such as your SSH secret keys,
are copied faithfully into the archive. Therefore, if your home directory contains any file that should not be read by some
other user, the archive itself should be not be readable by that user. And even if the archive's data are inaccessible to
untrusted users, its metadata (such as size or last-modified date) may reveal some information about your home directory;
if the metadata are intended to be private, the archive's parent directory should also be inaccessible to untrusted users.
One precaution is to create /archive so that it is not accessible to any user, unless that user also has permission to access all the files in your home directory.
Similarly, when extracting from an archive, take care that the permissions of the extracted files are not more generous than what you want. Even if the archive itself is readable only to you, files extracted from it have their own permissions that may differ.
[ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
When creating archives, take care that they are not writable by a untrusted user; otherwise, that user could modify the archive, and when you later extract from the archive you will get incorrect data.
When tar
extracts from an archive, by default it writes into files relative to the working directory. If
the archive was generated by an untrusted user, that user therefore can write into any file under the working directory.
If the working directory contains a symbolic link to another directory, the untrusted user can also write into any file
under the referenced directory. When extracting from an untrusted archive, it is therefore good practice to create an empty
directory and run tar
in that directory.
When extracting from two or more untrusted archives, each one should be extracted independently, into different empty directories. Otherwise, the first archive could create a symbolic link into an area outside the working directory, and the second one could follow the link and overwrite data that is not under the working directory. For example, when restoring from a series of incremental dumps, the archives should have been created by a trusted process, as otherwise the incremental restores might alter data outside the working directory.
If you use the --absolute-names (-P) option when extracting, tar
respects
any file names in the archive, even file names that begin with / or contain ... As this lets the archive
overwrite any file in your system that you can write, the --absolute-names (-P) option should
be used only for trusted archives.
Conversely, with the --keep-old-files (-k) option, tar
refuses to replace
existing files when extracting; and with the --no-overwrite-dir option, tar
refuses to replace
the permissions or ownership of already-existing directories. These options may help when extracting from untrusted archives.
[ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
Extra care is required when creating from or extracting into a file system that is accessible to untrusted users. For
example, superusers who invoke tar
must be wary about its actions being hijacked by an adversary who is reading
or writing the file system at the same time that tar
is operating.
When creating an archive from a live file system, tar
is vulnerable to denial-of-service attacks. For example,
an adversarial user could create the illusion of an indefinitely-deep directory hierarchy d/e/f/g/... by creating
directories one step ahead of tar
, or the illusion of an indefinitely-long file by creating a sparse file but
arranging for blocks to be allocated just before tar
reads them. There is no easy way for tar
to distinguish these scenarios from legitimate uses, so you may need to monitor tar
, just as you'd need to
monitor any other system service, to detect such attacks.
While a superuser is extracting from an archive into a live file system, an untrusted user might replace a directory
with a symbolic link, in hopes that tar
will follow the symbolic link and extract data into files that the
untrusted user does not have access to. Even if the archive was generated by the superuser, it may contain a file such as
d/etc/passwd that the untrusted user earlier created in order to break in; if the untrusted user replaces the
directory d/etc with a symbolic link to /etc while tar
is running, tar
will
overwrite /etc/passwd. This attack can be prevented by extracting into a directory that is inaccessible to untrusted
users.
Similar attacks via symbolic links are also possible when creating an archive, if the untrusted user can modify an ancestor
of a top-level argument of tar
. For example, an untrusted user that can modify /home/eve can hijack
a running instance of tar -cf - /home/eve/Documents/yesterday by replacing /home/eve/Documents
with a symbolic link to some other location. Attacks like these can be prevented by making sure that untrusted users cannot
modify any files that are top-level arguments to tar
, or any ancestor directories of these files.
[ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
This section briefly summarizes rules of thumb for avoiding security pitfalls.
$ chmod go-rwx . $ mkdir -m go-rwx dir $ cd dir $ tar -xvf /archives/got-it-off-the-net.tar.gz |
As a corollary, do not do an incremental restore from an untrusted archive.
tar
. For
example, while you are executing tar -cf /archive/u-home.tar /u/home, do not let an untrusted user modify
/, /archive, or /u. tar
. tar
to detect denial-of-service attacks.
[ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
This appendix lists some important user-visible changes between version GNU tar
1.26
and previous versions. An up-to-date version of this document is available at
the GNU tar
documentation
page.
$ tar xf foo.tar '*.c' |
would extract all files whose names end in .c. This behavior was not documented and was incompatible with traditional tar implementations. Therefore, starting from version 1.15.91, GNU tar no longer uses globbing by default. For example, the above invocation is now interpreted as a request to extract from the archive the file named *.c.
To facilitate transition to the new behavior for those users who got used to the previous incorrect one, tar
will print a warning if it finds out that a requested member was not found in the archive and its name looks like a
globbing pattern. For example:
$ tar xf foo.tar '*.c' tar: Pattern matching characters used in file names. Please, tar: use --wildcards to enable pattern matching, or --no-wildcards to tar: suppress this warning. tar: *.c: Not found in archive tar: Error exit delayed from previous errors |
To treat member names as globbing patterns, use the --wildcards option. If you want to tar to mimic
the behavior of versions prior to 1.15.91, add this option to your TAR_OPTIONS
variable.
See section Wildcards Patterns and Matching, for the detailed discussion of the use of globbing
patterns by GNU tar
.
tar
understood -o command line option as
a synonym for --old-archive.
GNU tar
starting from version 1.13.90 understands this option as a synonym for --no-same-owner.
This is compatible with UNIX98 tar
implementations.
However, to facilitate transition, -o option retains its old semantics when it is used with one of archive-creation commands. Users are encouraged to use --format=oldgnu instead.
It is especially important, since versions of GNU Automake up to and including 1.8.4 invoke tar with this option to produce distribution tarballs. See section v7, for the detailed discussion of this issue and its implications.
See tar-formats: (automake)Options
section `Changing Automake's Behavior' in GNU Automake, for a description on how to use various archive
formats with automake
.
Future versions of GNU tar
will understand -o only as a synonym for
--no-same-owner.
tar
understood -l option as a synonym for
--one-file-system. Since such usage contradicted to UNIX98 specification and harmed compatibility with
other implementations, it was declared deprecated in version 1.14. However, to facilitate transition to its new semantics,
it was supported by versions 1.15 and 1.15.90. The present use of -l as a short variant of --check-links
was introduced in version 1.15.91. [ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
Running tar --help displays the short tar
option summary (see section
GNU tar
documentation). This summary is organized by groups of semantically
close options. The options within each group are printed in the following order: a short option, eventually followed by
a list of corresponding long option names, followed by a short description of the option. For example, here is an excerpt
from the actual tar --help output:
Main operation mode: -A, --catenate, --concatenate append tar files to an archive -c, --create create a new archive -d, --diff, --compare find differences between archive and file system --delete delete from the archive
The exact visual representation of the help output is configurable via ARGP_HELP_FMT
environment variable.
The value of this variable is a comma-separated list of format variable assignments. There are two kinds of format
variables. An offset variable keeps the offset of some part of help output text from the leftmost column on the
screen. A boolean variable is a flag that toggles some output feature on or off. Depending on the type of the corresponding
variable, there are two kinds of assignments:
variable=value |
where variable is the variable name, and value is a numeric value to be assigned to the variable.
true
value to a variable, simply put this variable name. To assign false
value,
prefix the variable name with no-. For example:
# Assign |
Following variables are declared:
-f ARCHIVE, --file=ARCHIVE use archive file or device ARCHIVE |
If false, then if an option has both short and long forms, the argument is only shown with the long one, for example:
-f, --file=ARCHIVE use archive file or device ARCHIVE |
and a message indicating that the argument is applicable to both forms is printed below the options. This message
can be disabled using dup-args-note
(see below).
The default is false.
Mandatory or optional arguments to long options are also mandatory or optional for any corresponding short options.
Setting no-dup-args-note
inhibits this message. Normally, only one of variables dup-args
or dup-args-note
should be set.
$ tar --help|grep ARCHIVE -f, --file=ARCHIVE use archive file or device ARCHIVE $ ARGP_HELP_FMT=short-opt-col=6 tar --help|grep ARCHIVE -f, --file=ARCHIVE use archive file or device ARCHIVE |
$ tar --help|grep ARCHIVE -f, --file=ARCHIVE use archive file or device ARCHIVE $ ARGP_HELP_FMT=long-opt-col=16 tar --help|grep ARCHIVE -f, --file=ARCHIVE use archive file or device ARCHIVE |
-H, --format=FORMAT create archive of the given format. FORMAT is one of the following: gnu GNU tar 1.13.x format oldgnu GNU format as per tar <= 1.12 pax POSIX 1003.1-2001 (pax) format posix same as pax ustar POSIX 1003.1-1988 (ustar) format v7 old V7 tar format |
the format names are doc options. Thus, if you set ARGP_HELP_FMT=doc-opt-col=6 the above part of the help output will look as follows:
-H, --format=FORMAT create archive of the given format. FORMAT is one of the following: gnu GNU tar 1.13.x format oldgnu GNU format as per tar <= 1.12 pax POSIX 1003.1-2001 (pax) format posix same as pax ustar POSIX 1003.1-1988 (ustar) format v7 old V7 tar format |
$ tar --help|grep ARCHIVE -f, --file=ARCHIVE use archive file or device ARCHIVE $ ARGP_HELP_FMT=opt-doc-col=19 tar --help|grep ARCHIVE -f, --file=ARCHIVE use archive file or device ARCHIVE $ ARGP_HELP_FMT=opt-doc-col=9 tar --help|grep ARCHIVE -f, --file=ARCHIVE use archive file or device ARCHIVE |
Notice, that the description starts on a separate line if opt-doc-col
value is too small.
Main operation mode: -A, --catenate, --concatenate append tar files to an archive -c, --create create a new archive
Main operation mode: is the group header.
The default value is 1.
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Sometimes device numbers can change after upgrading your kernel version or reconfiguring the hardware. Reportedly this
is the case with some newer Linux kernels, when using LVM. In majority of cases this change is
unnoticed by the users. However, it influences tar
incremental backups: the device number is stored in tar
snapshot files (see section Format of the Incremental Snapshot Files) and is used to determine whether
the file has changed since the last backup. If the device numbers change for some reason, the next backup you run will be
a full backup.
To minimize the impact in these cases, GNU tar
comes with the tar-snapshot-edit
utility for
inspecting and updating device numbers in snapshot files. The utility, written by Dustin J. Mitchell, is available from
GNU tar
home page.
To obtain the device numbers used in the snapshot file, run
$ tar-snapshot-edit snapfile |
where snapfile is the name of the snapshot file (you can supply as many files as you wish in a single command line).
To update all occurrences of the given device number in the file, use -r option. It takes a single argument of the form olddev-newdev, where olddev is the device number used in the snapshot file, and newdev is the corresponding new device number. Both numbers may be specified in hex (e.g., 0xfe01), decimal (e.g., 65025), or as a major:minor number pair (e.g., 254:1). To change several device numbers at once, specify them in a single comma-separated list, as in -r 0x3060-0x4500,0x307-0x4600.
Before updating the snapshot file, it is a good idea to create a backup copy of it. This is accomplished by -b option. The name of the backup file is obtained by appending ~ to the original file name.
An example session:
$ tar-snapshot-edit /var/backup/snap.a file version 2 /tmp/snap: Device 0x0306 occurs 634 times. $ tar-snapshot-edit -b -r 0x0306-0x4500 /var/backup/snap.a file version 2 |
[ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
Basic Tar Format | ||
GNU Extensions to the Archive Format | ||
Storing Sparse Files | ||
Format of the Incremental Snapshot Files | ||
Dumpdir |
[ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
(This message will disappear, once this node revised.)
While an archive may contain many files, the archive itself is a single ordinary file. Like any other file, an archive
file can be written to a storage device such as a tape or disk, sent through a pipe or over a network, saved on the active
file system, or even stored in another archive. An archive file is not easy to read or manipulate without using the
tar
utility or Tar mode in GNU Emacs.
Physically, an archive consists of a series of file entries terminated by an end-of-archive entry, which consists of
two 512 blocks of zero bytes. A file entry usually describes one of the files in the archive (an archive member),
and consists of a file header and the contents of the file. File headers contain file names and statistics, checksum information
which tar
uses to detect file corruption, and information about file types.
Archives are permitted to have more than one member with the same member name. One way this situation can occur is if more than one version of a file has been stored in the archive. For information about adding new versions of a file to an archive, see Updating an Archive.
In addition to entries describing archive members, an archive may contain entries which tar
itself uses
to store information. See section Including a Label in the Archive, for an example of such an archive
entry.
A tar
archive file contains a series of blocks. Each block contains BLOCKSIZE
bytes. Although
this format may be thought of as being on magnetic tape, other media are often used.
Each file archived is represented by a header block which describes the file, followed by zero or more blocks which give
the contents of the file. At the end of the archive file there are two 512-byte blocks filled with binary zeros as an end-of-file
marker. A reasonable system should write such end-of-file marker at the end of an archive, but must not assume that such
a block exists when reading an archive. In particular GNU tar
always issues a warning if
it does not encounter it.
The blocks may be blocked for physical I/O operations. Each record of n blocks (where n
is set by the --blocking-factor=512-size (-b 512-size) option to
tar
) is written with a single write () operation. On magnetic tapes, the result of such a write
is a single record. When writing an archive, the last record of blocks should be written at the full size, with blocks after
the zero block containing all zeros. When reading an archive, a reasonable system should properly handle an archive whose
last record is shorter than the rest, or which contains garbage records after a zero block.
The header block is defined in C as follows. In the GNU tar
distribution, this is part
of file src/tar.h:
/* tar Header Block, from POSIX 1003.1-1990. */ /* POSIX header. */ struct posix_header { /* byte offset */ char name[100]; /* 0 */ char mode[8]; /* 100 */ char uid[8]; /* 108 */ char gid[8]; /* 116 */ char size[12]; /* 124 */ char mtime[12]; /* 136 */ char chksum[8]; /* 148 */ char typeflag; /* 156 */ char linkname[100]; /* 157 */ char magic[6]; /* 257 */ char version[2]; /* 263 */ char uname[32]; /* 265 */ char gname[32]; /* 297 */ char devmajor[8]; /* 329 */ char devminor[8]; /* 337 */ char prefix[155]; /* 345 */ /* 500 */ }; #define TMAGIC "ustar" /* ustar and a null */ #define TMAGLEN 6 #define TVERSION "00" /* 00 and no null */ #define TVERSLEN 2 /* Values used in typeflag field. */ #define REGTYPE '0' /* regular file */ #define AREGTYPE '\0' /* regular file */ #define LNKTYPE '1' /* link */ #define SYMTYPE '2' /* reserved */ #define CHRTYPE '3' /* character special */ #define BLKTYPE '4' /* block special */ #define DIRTYPE '5' /* directory */ #define FIFOTYPE '6' /* FIFO special */ #define CONTTYPE '7' /* reserved */ #define XHDTYPE 'x' /* Extended header referring to the next file in the archive */ #define XGLTYPE 'g' /* Global extended header */ /* Bits used in the mode field, values in octal. */ #define TSUID 04000 /* set UID on execution */ #define TSGID 02000 /* set GID on execution */ #define TSVTX 01000 /* reserved */ /* file permissions */ #define TUREAD 00400 /* read by owner */ #define TUWRITE 00200 /* write by owner */ #define TUEXEC 00100 /* execute/search by owner */ #define TGREAD 00040 /* read by group */ #define TGWRITE 00020 /* write by group */ #define TGEXEC 00010 /* execute/search by group */ #define TOREAD 00004 /* read by other */ #define TOWRITE 00002 /* write by other */ #define TOEXEC 00001 /* execute/search by other */ /* tar Header Block, GNU extensions. */ /* In GNU tar, SYMTYPE is for to symbolic links, and CONTTYPE is for contiguous files, so maybe disobeying the `reserved' comment in POSIX header description. I suspect these were meant to be used this way, and should not have really been `reserved' in the published standards. */ /* *BEWARE* *BEWARE* *BEWARE* that the following information is still boiling, and may change. Even if the OLDGNU format description should be accurate, the so-called GNU format is not yet fully decided. It is surely meant to use only extensions allowed by POSIX, but the sketch below repeats some ugliness from the OLDGNU format, which should rather go away. Sparse files should be saved in such a way that they do *not* require two passes at archive creation time. Huge files get some POSIX fields to overflow, alternate solutions have to be sought for this. */ /* Descriptor for a single file hole. */ struct sparse { /* byte offset */ char offset[12]; /* 0 */ char numbytes[12]; /* 12 */ /* 24 */ }; /* Sparse files are not supported in POSIX ustar format. For sparse files with a POSIX header, a GNU extra header is provided which holds overall sparse information and a few sparse descriptors. When an old GNU header replaces both the POSIX header and the GNU extra header, it holds some sparse descriptors too. Whether POSIX or not, if more sparse descriptors are still needed, they are put into as many successive sparse headers as necessary. The following constants tell how many sparse descriptors fit in each kind of header able to hold them. */ #define SPARSES_IN_EXTRA_HEADER 16 #define SPARSES_IN_OLDGNU_HEADER 4 #define SPARSES_IN_SPARSE_HEADER 21 /* Extension header for sparse files, used immediately after the GNU extra header, and used only if all sparse information cannot fit into that extra header. There might even be many such extension headers, one after the other, until all sparse information has been recorded. */ struct sparse_header { /* byte offset */ struct sparse sp[SPARSES_IN_SPARSE_HEADER]; /* 0 */ char isextended; /* 504 */ /* 505 */ }; /* The old GNU format header conflicts with POSIX format in such a way that POSIX archives may fool old GNU tar's, and POSIX tar's might well be fooled by old GNU tar archives. An old GNU format header uses the space used by the prefix field in a POSIX header, and cumulates information normally found in a GNU extra header. With an old GNU tar header, we never see any POSIX header nor GNU extra header. Supplementary sparse headers are allowed, however. */ struct oldgnu_header { /* byte offset */ char unused_pad1[345]; /* 0 */ char atime[12]; /* 345 Incr. archive: atime of the file */ char ctime[12]; /* 357 Incr. archive: ctime of the file */ char offset[12]; /* 369 Multivolume archive: the offset of the start of this volume */ char longnames[4]; /* 381 Not used */ char unused_pad2; /* 385 */ struct sparse sp[SPARSES_IN_OLDGNU_HEADER]; /* 386 */ char isextended; /* 482 Sparse file: Extension sparse header follows */ char realsize[12]; /* 483 Sparse file: Real size*/ /* 495 */ }; /* OLDGNU_MAGIC uses both magic and version fields, which are contiguous. Found in an archive, it indicates an old GNU header format, which will be hopefully become obsolescent. With OLDGNU_MAGIC, uname and gname are valid, though the header is not truly POSIX conforming. */ #define OLDGNU_MAGIC "ustar " /* 7 chars and a null */ /* The standards committee allows only capital A through capital Z for user-defined expansion. Other letters in use include: 'A' Solaris Access Control List 'E' Solaris Extended Attribute File 'I' Inode only, as in 'star' 'N' Obsolete GNU tar, for file names that do not fit into the main header. 'X' POSIX 1003.1-2001 eXtended (VU version) */ /* This is a dir entry that contains the names of files that were in the dir at the time the dump was made. */ #define GNUTYPE_DUMPDIR 'D' /* Identifies the *next* file on the tape as having a long linkname. */ #define GNUTYPE_LONGLINK 'K' /* Identifies the *next* file on the tape as having a long name. */ #define GNUTYPE_LONGNAME 'L' /* This is the continuation of a file that began on another volume. */ #define GNUTYPE_MULTIVOL 'M' /* This is for sparse files. */ #define GNUTYPE_SPARSE 'S' /* This file is a tape/volume header. Ignore it on extraction. */ #define GNUTYPE_VOLHDR 'V' /* Solaris extended header */ #define SOLARIS_XHDTYPE 'X' /* Jφrg Schilling star header */ struct star_header { /* byte offset */ char name[100]; /* 0 */ char mode[8]; /* 100 */ char uid[8]; /* 108 */ char gid[8]; /* 116 */ char size[12]; /* 124 */ char mtime[12]; /* 136 */ char chksum[8]; /* 148 */ char typeflag; /* 156 */ char linkname[100]; /* 157 */ char magic[6]; /* 257 */ char version[2]; /* 263 */ char uname[32]; /* 265 */ char gname[32]; /* 297 */ char devmajor[8]; /* 329 */ char devminor[8]; /* 337 */ char prefix[131]; /* 345 */ char atime[12]; /* 476 */ char ctime[12]; /* 488 */ /* 500 */ }; #define SPARSES_IN_STAR_HEADER 4 #define SPARSES_IN_STAR_EXT_HEADER 21 struct star_in_header { char fill[345]; /* 0 Everything that is before t_prefix */ char prefix[1]; /* 345 t_name prefix */ char fill2; /* 346 */ char fill3[8]; /* 347 */ char isextended; /* 355 */ struct sparse sp[SPARSES_IN_STAR_HEADER]; /* 356 */ char realsize[12]; /* 452 Actual size of the file */ char offset[12]; /* 464 Offset of multivolume contents */ char atime[12]; /* 476 */ char ctime[12]; /* 488 */ char mfill[8]; /* 500 */ char xmagic[4]; /* 508 "tar" */ }; struct star_ext_header { struct sparse sp[SPARSES_IN_STAR_EXT_HEADER]; char isextended; }; |
All characters in header blocks are represented by using 8-bit characters in the local variant of ASCII. Each field within the structure is contiguous; that is, there is no padding used within the structure. Each character on the archive medium is stored contiguously.
Bytes representing the contents of files (after the header block of each file) are not translated in any way and are
not constrained to represent characters in any character set. The tar
format does not distinguish text files
from binary files, and no translation of file contents is performed.
The name
, linkname
, magic
, uname
, and gname
are null-terminated
character strings. All other fields are zero-filled octal numbers in ASCII. Each numeric field of width w contains
w minus 1 digits, and a null.
The name
field is the file name of the file, with directory names (if any) preceding the file name, separated
by slashes.
The mode
field provides nine bits specifying file permissions and three bits to specify the Set
UID, Set GID, and Save Text (sticky) modes. Values for these bits are defined above.
When special permissions are required to create a file with a given mode, and the user restoring files from the archive
does not hold such permissions, the mode bit(s) specifying those special permissions are ignored. Modes which are not supported
by the operating system restoring files from the archive will be ignored. Unsupported modes should be faked up when creating
or updating an archive; e.g., the group permission could be copied from the other permission.
The uid
and gid
fields are the numeric user and group ID of the file owners,
respectively. If the operating system does not support numeric user or group IDs, these fields should
be ignored.
The size
field is the size of the file in bytes; linked files are archived with this field specified as
zero.
The mtime
field is the data modification time of the file at the time it was archived. It is the ASCII representation
of the octal value of the last time the file's contents were modified, represented as an integer number of seconds since
January 1, 1970, 00:00 Coordinated Universal Time.
The chksum
field is the ASCII representation of the octal value of the simple sum of all bytes in the header
block. Each 8-bit byte in the header is added to an unsigned integer, initialized to zero, the precision of which shall
be no less than seventeen bits. When calculating the checksum, the chksum
field is treated as if it were all
blanks.
The typeflag
field specifies the type of file archived. If a particular implementation does not recognize
or permit the specified type, the file will be extracted as if it were a regular file. As this action occurs, tar
issues a warning to the standard error.
The atime
and ctime
fields are used in making incremental backups; they store, respectively,
the particular file's access and status change times.
The offset
is used by the --multi-volume (-M) option, when making a multi-volume
archive. The offset is number of bytes into the file that we need to restart at to continue the file on the next tape, i.e.,
where we store the location that a continued file is continued at.
The following fields were added to deal with sparse files. A file is sparse if it takes in unallocated blocks
which end up being represented as zeros, i.e., no useful data. A test to see if a file is sparse is to look at the number
blocks allocated for it versus the number of characters in the file; if there are fewer blocks allocated for the file than
would normally be allocated for a file of that size, then the file is sparse. This is the method tar
uses to
detect a sparse file, and once such a file is detected, it is treated differently from non-sparse files.
Sparse files are often dbm
files, or other database-type files which have data at some points and emptiness
in the greater part of the file. Such files can appear to be very large when an ls -l is done on them, when
in truth, there may be a very small amount of important data contained in the file. It is thus undesirable to have
tar
think that it must back up this entire file, as great quantities of room are wasted on empty blocks, which can
lead to running out of room on a tape far earlier than is necessary. Thus, sparse files are dealt with so that these empty
blocks are not written to the tape. Instead, what is written to the tape is a description, of sorts, of the sparse file:
where the holes are, how big the holes are, and how much data is found at the end of the hole. This way, the file takes
up potentially far less room on the tape, and when the file is extracted later on, it will look exactly the way it looked
beforehand. The following is a description of the fields used to handle a sparse file:
The sp
is an array of struct sparse
. Each struct sparse
contains two 12-character
strings which represent an offset into the file and a number of bytes to be written at that offset. The offset is absolute,
and not relative to the offset in preceding array element.
The header can hold four of these struct sparse
at the moment; if more are needed, they are not stored in
the header.
The isextended
flag is set when an extended_header
is needed to deal with a file. Note that
this means that this flag can only be set when dealing with a sparse file, and it is only set in the event that the description
of the file will not fit in the allotted room for sparse structures in the header. In other words, an extended_header is
needed.
The extended_header
structure is used for sparse files which need more sparse structures than can fit in
the header. The header can fit 4 such structures; if more are needed, the flag isextended
gets set and the
next block is an extended_header
.
Each extended_header
structure contains an array of 21 sparse structures, along with a similar isextended
flag that the header had. There can be an indeterminate number of such extended_header
s to describe a sparse
file.
REGTYPE
AREGTYPE
tar
, a
typeflag
value of AREGTYPE
should be silently recognized as a regular file. New archives should
be created using REGTYPE
. Also, for backward compatibility, tar
treats a regular file whose
name ends with a slash as a directory. LNKTYPE
linkname
field with a trailing null. SYMTYPE
linkname
field
with a trailing null. CHRTYPE
BLKTYPE
devmajor
and devminor
fields will contain the major and minor device numbers respectively. Operating systems may
map the device specifications to their own local specification, or may ignore the entry. DIRTYPE
name
field should end with
a slash. On systems where disk allocation is performed on a directory basis, the size
field will contain
the maximum number of bytes (which may be rounded to the nearest disk block allocation unit) which the directory may
hold. A size
field of zero indicates no such limiting. Systems which do not support limiting in this manner
should ignore the size
field. FIFOTYPE
CONTTYPE
A
Z
Other values are reserved for specification in future revisions of the P1003 standard, and should not be used by any
tar
program.
The magic
field indicates that this archive was output in the P1003 archive format. If this field contains
TMAGIC
, the uname
and gname
fields will contain the ASCII representation of the owner
and group of the file respectively. If found, the user and group IDs are used rather than the values
in the uid
and gid
fields.
For references, see ISO/IEC 9945-1:1990 or IEEE Std 1003.1-1990, pages 169-173 (section 10.1) for Archive/Interchange File Format; and IEEE Std 1003.2-1992, pages 380-388 (section 4.48) and pages 936-940 (section E.4.48) for pax - Portable archive interchange.
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(This message will disappear, once this node revised.)
The GNU format uses additional file types to describe new types of files in an archive. These are listed below.
GNUTYPE_DUMPDIR
'D'
size
field gives the total size of the associated list of files. Each file name is preceded
by either a Y (the file should be in this archive) or an N. (The file is a directory,
or is not stored in the archive.) Each file name is terminated by a null. There is an additional null after the last
file name. GNUTYPE_MULTIVOL
'M'
size
field gives the maximum
size of this piece of the file (assuming the volume does not end before the file is written out). The offset
field gives the offset from the beginning of the file where this part of the file begins. Thus size
plus
offset
should equal the original size of the file. GNUTYPE_SPARSE
'S'
GNUTYPE_VOLHDR
'V'
name
field contains
the name
given after the --label=archive-label (-V archive-label)
option. The size
field is zero. Only the first file in each volume of an archive should have this type.
You may have trouble reading a GNU format archive on a non-GNU system if the options
--incremental (-G), --multi-volume (-M), --sparse
(-S), or --label=archive-label (-V archive-label) were
used when writing the archive. In general, if tar
does not use the GNU-added fields of the
header, other versions of tar
should be able to read the archive. Otherwise, the tar
program will
give an error, the most likely one being a checksum error.
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The notion of sparse file, and the ways of handling it from the point of view of GNU tar
user have been described in detail in Archiving Sparse Files. This chapter describes the internal
format GNU tar
uses to store such files.
The support for sparse files in GNU tar
has a long history. The earliest version featuring
this support that I was able to find was 1.09, released in November, 1990. The format introduced back then is called
old GNU sparse format and in spite of the fact that its design contained many flaws, it was the only format
GNU tar
supported until version 1.14 (May, 2004), which introduced initial support for sparse
archives in PAX archives (see section GNU tar
and
POSIX tar
). This format was not free from design flaws, either and it was subsequently
improved in versions 1.15.2 (November, 2005) and 1.15.92 (June, 2006).
In addition to GNU sparse format, GNU tar
is able to read and extract sparse files archived
by star
.
The following subsections describe each format in detail.
D.0.1 Old GNU Format | ||
D.0.2 PAX Format, Versions 0.0 and 0.1 | ||
D.0.3 PAX Format, Version 1.0 |
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The format introduced in November 1990 (v. 1.09) was designed on top of standard ustar
headers in such an
unfortunate way that some of its fields overwrote fields required by POSIX.
An old GNU sparse header is designated by type S (GNUTYPE_SPARSE
) and has the following layout:
Offset | Size | Name | Data type | Contents |
---|---|---|---|---|
0 | 345 | N/A | Not used. | |
345 | 12 | atime | Number | atime of the file. |
357 | 12 | ctime | Number | ctime of the file . |
369 | 12 | offset | Number | For multivolume archives: the offset of the start of this volume. |
381 | 4 | N/A | Not used. | |
385 | 1 | N/A | Not used. | |
386 | 96 | sp | sparse_header |
(4 entries) File map. |
482 | 1 | isextended | Bool | 1 if an extension sparse header follows, 0 otherwise. |
483 | 12 | realsize | Number | Real size of the file. |
Each of sparse_header
object at offset 386 describes a single data chunk. It has the following structure:
Offset | Size | Data type | Contents |
---|---|---|---|
0 | 12 | Number | Offset of the beginning of the chunk. |
12 | 12 | Number | Size of the chunk. |
If the member contains more than four chunks, the isextended
field of the header has the value 1
and the main header is followed by one or more extension headers. Each such header has the following structure:
Offset | Size | Name | Data type | Contents |
---|---|---|---|---|
0 | 21 | sp | sparse_header |
(21 entries) File map. |
504 | 1 | isextended | Bool | 1 if an extension sparse header follows, or 0 otherwise. |
A header with isextended=0
ends the map.
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There are two formats available in this branch. The version 0.0
is the initial version of sparse format
used by tar
versions 1.141.15.1. The sparse file map is kept in extended (x
) PAX header variables:
GNU.sparse.size
GNU.sparse.numblocks
GNU.sparse.offset
GNU.sparse.numbytes
The latter two variables repeat for each data block, so the overall structure is like this:
GNU.sparse.size=size GNU.sparse.numblocks=numblocks repeat numblocks times GNU.sparse.offset=offset GNU.sparse.numbytes=numbytes end repeat |
This format presented the following two problems:
GNU.sparse.offset
and GNU.sparse.numbytes
are conflicting with the POSIX specs. tar
results in extraction of sparse files
in condensed form. If the tar
implementation in question does not support POSIX format, it will
also extract a file containing extension header attributes. This file can be used to expand the file to its original
state. However, posix-aware tar
s will usually ignore the unknown variables, which makes restoring the file
more difficult. See Extraction of sparse members in v.0.0 format, for
the detailed description of how to restore such members using non-GNU tar
s. GNU tar
1.15.2 introduced sparse format version 0.1
, which attempted to
solve these problems. As its predecessor, this format stores sparse map in the extended POSIX header. It retains GNU.sparse.size
and GNU.sparse.numblocks
variables, but instead of GNU.sparse.offset
/GNU.sparse.numbytes
pairs it uses a single variable:
GNU.sparse.map
To address the 2nd problem, the name
field in ustar
is replaced with a special name, constructed
using the following pattern:
%d/GNUSparseFile.%p/%f |
The real name of the sparse file is stored in the variable GNU.sparse.name
. Thus, those tar
implementations that are not aware of GNU extensions will at least extract the files into separate directories, giving the
user a possibility to expand it afterwards. See Extraction of sparse members
in v.0.1 format, for the detailed description of how to restore such members using non-GNU tar
s.
The resulting GNU.sparse.map
string can be very long. Although POSIX does not impose any limit
on the length of a x
header variable, this possibly can confuse some tar
s.
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The version 1.0
of sparse format was introduced with GNU tar
1.15.92. Its
main objective was to make the resulting file extractable with little effort even by non-posix aware tar
implementations.
Starting from this version, the extended header preceding a sparse member always contains the following variables that identify
the format being used:
GNU.sparse.major
GNU.sparse.minor
The name
field in ustar
header contains a special name, constructed using the following pattern:
%d/GNUSparseFile.%p/%f |
The real name of the sparse file is stored in the variable GNU.sparse.name
. The real size of the file is
stored in the variable GNU.sparse.realsize
.
The sparse map itself is stored in the file data block, preceding the actual file data. It consists of a series of octal numbers of arbitrary length, delimited by newlines. The map is padded with nulls to the nearest block boundary.
The first number gives the number of entries in the map. Following are map entries, each one consisting of two numbers giving the offset and size of the data block it describes.
The format is designed in such a way that non-posix aware tar
s and tar
s not supporting
GNU.sparse.*
keywords will extract each sparse file in its condensed form with the file map prepended and will
place it into a separate directory. Then, using a simple program it would be possible to expand the file to its original
form even without GNU tar
. See section Extracting Sparse Members,
for the detailed information on how to extract sparse members without GNU tar
.
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A snapshot file (or directory file) is created during incremental backups (see section
Using tar
to Perform Incremental Dumps). It contains the status of the file system at
the time of the dump and is used to determine which files were modified since the last backup.
GNU tar
version 1.26 supports three snapshot file formats. The first format, called
format 0, is the one used by GNU tar
versions up to 1.15.1. The second format,
called format 1 is an extended version of this format, that contains more metadata and allows for further extensions.
It was used by version 1.15.1. Starting from version 1.16 and up to 1.26, the format 2 is used.
GNU tar
is able to read all three formats, but will create snapshots only in format 2.
This appendix describes all three formats in detail.
nfsdev inode name |
where:
GNU tar-tar-version-incr-format-version |
where tar-version is the version number of GNU tar
implementation that
created this snapshot, and incr-format-version is the version number of the snapshot format (in this case
1).
Next line contains two decimal numbers, representing the time of the last backup. First number is the number of seconds, the second one is the number of nanoseconds, since the beginning of the epoch.
Lines that follow contain directory metadata, one line per directory. Each line is formatted as follows:
[nfs]mtime-sec mtime-nsec dev inode name |
where mtime-sec and mtime-nsec represent last modification time of this directory with nanosecond precision; nfs, dev, inode and name have the same meaning as with format 0.
GNU tar-1.26-2 |
This line is followed by newline. Rest of file consists of records, separated by null (ASCII 0) characters. Thus, in contrast to the previous formats, format 2 snapshot is a binary file.
First two records are decimal numbers, representing the time of the last backup. First number is the number of seconds, the second one is the number of nanoseconds, since the beginning of the epoch. These are followed by arbitrary number of directory records.
Each directory record contains a set of metadata describing a particular directory. Parts of a directory record are delimited with ASCII 0 characters. The following table describes each part. The Number type in this table stands for a decimal number in ASCII notation.
Field | Type | Description |
---|---|---|
nfs | Character | 1 if the directory is located on an NFS-mounted partition, or 0 otherwise; |
mtime-sec | Number | Modification time, seconds; |
mtime-nano | Number | Modification time, nanoseconds; |
dev-no | Number | Device number; |
i-no | Number | I-node number; |
name | String | Directory name; in contrast to the previous versions it is not quoted; |
contents | Dumpdir | Contents of the directory; See section Dumpdir, for a description of its format. |
Dumpdirs stored in snapshot files contain only records of types Y, N and D.
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Incremental archives keep information about contents of each dumped directory in special data blocks called dumpdirs.
Dumpdir is a sequence of entries of the following form:
C filename \0 |
where C is one of the control codes described below, filename is the name of the file C operates upon, and \0 represents a nul character (ASCII 0). The white space characters were added for readability, real dumpdirs do not contain them.
Each dumpdir ends with a single nul character.
The following table describes control codes and their meanings:
Codes Y, N and D require filename argument to be a relative file name to the directory this dumpdir describes, whereas codes R, T and X require their argument to be an absolute file name.
The three codes R, T and X specify a renaming operation. In the simplest case it is:
Rsource\0Tdest\0 |
which means rename file source to file dest.
However, there are cases that require using a temporary directory. For example, consider the following scenario:
a b c |
a became b b became c c became a |
This case cannot be handled by three successive renames, since renaming a to b will destroy the
existing directory. To correctly process it, GNU tar
needs a temporary directory, so it
creates the following dumpdir (newlines have been added for readability):
Xfoo\0 Rfoo/a\0T\0 Rfoo/b\0Tfoo/c\0 Rfoo/c\0Tfoo/a\0 R\0Tfoo/a\0 |
The first command, Xfoo\0, instructs the extractor to create a temporary directory in the directory foo. Second command, Rfoo/aT\0, says rename file foo/a to the temporary directory that has just been created (empty file name after a command means use temporary directory). Third and fourth commands work as usual, and, finally, the last command, R\0Tfoo/a\0 tells tar to rename the temporary directory to foo/a.
The exact placement of a dumpdir in the archive depends on the archive format (see section Controlling the Archive Format):
In PAX archives, dumpdir is stored in the extended header of the corresponding directory, in variable GNU.dumpdir
.
These formats implement special header type D, which is similar to ustar header 5 (directory), except that it precedes a data block containing the dumpdir.
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This appendix describes genfile
, an auxiliary program used in the GNU tar testsuite. If you are not interested
in developing GNU tar, skip this appendix.
Initially, genfile
was used to generate data files for the testsuite, hence its name. However, new operation
modes were being implemented as the testsuite grew more sophisticated, and now genfile
is a multi-purpose instrument.
There are three basic operation modes:
genfile
generates data files. genfile
displays status of specified files. genfile
executes the given program with --checkpoint option and executes
a set of actions when specified checkpoints are reached. E.1 Generate Mode | File Generation Mode. | |
E.2 Status Mode | File Status Mode. | |
E.3 Exec Mode | Synchronous Execution mode. |
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In this mode genfile
creates a data file for the test suite. The size of the file is given with the --length
(-l) option. By default the file contents is written to the standard output, this can be changed using --file
(-f) command line option. Thus, the following two commands are equivalent:
genfile --length 100 > outfile genfile --length 100 --file outfile |
If --length is not given, genfile
will generate an empty (zero-length) file.
The command line option --seek=N istructs genfile
to skip the given number of
bytes (N) in the output file before writing to it. It is similar to the seek=N of the
dd
utility.
You can instruct genfile
to create several files at one go, by giving it --files-from (-T)
option followed by a name of file containing a list of file names. Using dash (-) instead of the file name
causes genfile
to read file list from the standard input. For example:
# Read file names from file file.list genfile --files-from file.list # Read file names from standard input genfile --files-from - |
The list file is supposed to contain one file name per line. To use file lists separated by ASCII NUL character, use --null (-0) command line option:
genfile --null --files-from file.list |
The default data pattern for filling the generated file consists of first 256 letters of ASCII code, repeated enough times to fill the entire file. This behavior can be changed with --pattern option. This option takes a mandatory argument, specifying pattern name to use. Currently two patterns are implemented:
If no file name was given, the program exits with the code 0
. Otherwise, it exits with 0
only
if it was able to create a file of the specified length.
Special option --sparse (-s) instructs genfile
to create a sparse file. Sparse
files consist of data fragments, separated by holes or blocks of zeros. On many operating systems, actual
disk storage is not allocated for holes, but they are counted in the length of the file. To create a sparse file,
genfile
should know where to put data fragments, and what data to use to fill them. So, when --sparse
is given the rest of the command line specifies a so-called file map.
The file map consists of any number of fragment descriptors. Each descriptor is composed of two values: a number, specifying fragment offset from the end of the previous fragment or, for the very first fragment, from the beginning of the file, and contents string, i.e., a string of characters, specifying the pattern to fill the fragment with. File offset can be suffixed with the following quantifiers:
For each letter in contents string genfile
will generate a block of data, filled with this letter
and will write it to the fragment. The size of block is given by --block-size option. It defaults to 512.
Thus, if the string consists of n characters, the resulting file fragment will contain n*block-size
of data.
Last fragment descriptor can have only file offset part. In this case genfile
will create a hole at the
end of the file up to the given offset.
For example, consider the following invocation:
genfile --sparse --file sparsefile 0 ABCD 1M EFGHI 2000K |
It will create 3101184-bytes long file of the following structure:
Offset | Length | Contents |
0 | 4*512=2048 | Four 512-byte blocks, filled with letters A, B, C and D. |
2048 | 1046528 | Zero bytes |
1050624 | 5*512=2560 | Five blocks, filled with letters E, F, G, H, I. |
1053184 | 2048000 | Zero bytes |
The exit code of genfile --status
command is 0
only if created file is actually sparse.
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In status mode, genfile
prints file system status for each file specified in the command line. This mode
is toggled by --stat (-S) command line option. An optional argument to this option specifies
output format: a comma-separated list of struct stat
fields to be displayed. This list can contain
following identifiers :
--stat mode.777
will preserve lower nine bits of it. Notice, that you can use any punctuation
character in place of .. Modification times are displayed in UTC as UNIX timestamps, unless suffixed with
H (for human-readable), as in ctimeH, in which case usual tar tv
output format
is used.
The default output format is: name,dev,ino,mode, nlink,uid,gid,size,blksize,blocks,atime,mtime,ctime.
For example, the following command will display file names and corresponding times of last access for each file in the current working directory:
genfile --stat=name,atime * |
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This mode is designed for testing the behavior of paxutils
commands when some of the files change during
archiving. It is an experimental mode.
The Exec Mode is toggled by --run command line option (or its alias -r).
The non-optional arguments to getopt
give the command line to be executed. Normally, it should contain at least
the --checkpoint option.
A set of options is provided for defining checkpoint values and actions to be executed upon reaching them. Checkpoint values are introduced with the --checkpoint command line option. Argument to this option is the number of checkpoint in decimal.
Any number of actions may be specified after a checkpoint. Available actions are
Option --verbose instructs genfile
to print on standard output notifications about checkpoints
being executed and to verbosely describe exit status of the command.
While the command is being executed its standard output remains connected to descriptor 1. All messages it prints to file descriptor 2, except checkpoint notifications, are forwarded to standard error.
Genfile
exits with the exit status of the executed command.
For compatibility with previous genfile
versions, the --run option takes an optional argument.
If used this way, its argument supplies the command line to be executed. There should be no non-optional arguments in the
genfile
command line.
The actual command line is constructed by inserting the --checkpoint option between the command name and
its first argument (if any). Due to this, the argument to --run may not use traditional tar
option syntax, i.e., the following is wrong:
# Wrong! genfile --run='tar cf foo bar' |
Use the following syntax instead:
genfile --run='tar -cf foo bar' actions... |
The above command line is equivalent to
genfile actions... -- tar -cf foo bar |
Notice, that the use of compatibility mode is deprecated.
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The biggest deficiency in the free software community today is not in the softwareit is the lack of good free documentation that we can include with the free software. Many of our most important programs do not come with free reference manuals and free introductory texts. Documentation is an essential part of any software package; when an important free software package does not come with a free manual and a free tutorial, that is a major gap. We have many such gaps today.
Consider Perl, for instance. The tutorial manuals that people normally use are non-free. How did this come about? Because the authors of those manuals published them with restrictive termsno copying, no modification, source files not availablewhich exclude them from the free software world.
That wasn't the first time this sort of thing happened, and it was far from the last. Many times we have heard a GNU user eagerly describe a manual that he is writing, his intended contribution to the community, only to learn that he had ruined everything by signing a publication contract to make it non-free.
Free documentation, like free software, is a matter of freedom, not price. The problem with the non-free manual is not that publishers charge a price for printed copiesthat in itself is fine. (The Free Software Foundation sells printed copies of manuals, too.) The problem is the restrictions on the use of the manual. Free manuals are available in source code form, and give you permission to copy and modify. Non-free manuals do not allow this.
The criteria of freedom for a free manual are roughly the same as for free software. Redistribution (including the normal kinds of commercial redistribution) must be permitted, so that the manual can accompany every copy of the program, both on-line and on paper.
Permission for modification of the technical content is crucial too. When people modify the software, adding or changing features, if they are conscientious they will change the manual tooso they can provide accurate and clear documentation for the modified program. A manual that leaves you no choice but to write a new manual to document a changed version of the program is not really available to our community.
Some kinds of limits on the way modification is handled are acceptable. For example, requirements to preserve the original author's copyright notice, the distribution terms, or the list of authors, are ok. It is also no problem to require modified versions to include notice that they were modified. Even entire sections that may not be deleted or changed are acceptable, as long as they deal with nontechnical topics (like this one). These kinds of restrictions are acceptable because they don't obstruct the community's normal use of the manual.
However, it must be possible to modify all the technical content of the manual, and then distribute the result in all the usual media, through all the usual channels. Otherwise, the restrictions obstruct the use of the manual, it is not free, and we need another manual to replace it.
Please spread the word about this issue. Our community continues to lose manuals to proprietary publishing. If we spread the word that free software needs free reference manuals and free tutorials, perhaps the next person who wants to contribute by writing documentation will realize, before it is too late, that only free manuals contribute to the free software community.
If you are writing documentation, please insist on publishing it under the GNU Free Documentation License or another free documentation license. Remember that this decision requires your approvalyou don't have to let the publisher decide. Some commercial publishers will use a free license if you insist, but they will not propose the option; it is up to you to raise the issue and say firmly that this is what you want. If the publisher you are dealing with refuses, please try other publishers. If you're not sure whether a proposed license is free, write to [email protected].
You can encourage commercial publishers to sell more free, copylefted manuals and tutorials by buying them, and particularly by buying copies from the publishers that paid for their writing or for major improvements. Meanwhile, try to avoid buying non-free documentation at all. Check the distribution terms of a manual before you buy it, and insist that whoever seeks your business must respect your freedom. Check the history of the book, and try reward the publishers that have paid or pay the authors to work on it.
The Free Software Foundation maintains a list of free documentation published by other publishers, at http://www.fsf.org/doc/other-free-books.html.
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Version 1.3, 3 November 2008
Copyright © 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc. http://fsf.org/ Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed. |
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This appendix contains an index of all GNU tar
long command line options. The options
are listed without the preceding double-dash. For a cross-reference of short command line options, see
Short Options Cross Reference.
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