--- The Detailed Node Listing ---
Introduction
tar Does
tar Archives are Named
tar Authors
Tutorial Introduction to tar
tar Subcommands and Options
Two Frequently Used Options
How to Create Archives
How to List Archives
How to Extract Members from an Archive
GNU tar Subcommands
How to Add Files to Existing Archives: --append
Updating an Archive
Options Used by --extract
tar Writes Files
Options to Help Read Archives
Changing How tar Writes Files
Options to Prevent Overwriting Files
Coping with Scarce Resources
Performing Backups and Restoring Files
tar to Perform Full Dumps
tar to Perform Incremental Dumps
Setting Parameters for Backups and Restoration
Backup-specs
Backup-specs
Choosing Files and Names for tar
Reading Names from a File
Excluding Some Files
Crossing Filesystem Boundaries
Date Input Formats
Controlling the Archive Format
tar Archives More Portable
tar and cpio
Making tar Archives More Portable
Using Less Space through Compression
Tapes and Other Archive Media
Blocking
Many Archives on One Tape
mt Utility
Using Multiple Tapes
Invoking GNU tar
The Three Option Styles
All tar Options
Documentation and Troubleshooting
Welcome to the GNU tar manual. GNU tar is used to create
and manipulate files (archives) which are actually collections of
many other files; the program provides users with an organized and
systematic method for controlling a large amount of data.
tar Does
tar Archives are Named
tar Authors
The first part of this chapter introduces you to various terms that will
recur throughout the book. It also tells you who has worked on GNU
tar and its documentation, and where you should send bug reports
or comments.
The second chapter is a tutorial (see Tutorial) which provides a
gentle introduction for people who are new to using tar. It is
meant to be self-contained, not requiring any reading from subsequent
chapters to make sense. It moves from topic to topic in a logical,
progressive order, building on information already explained.
Although the tutorial is paced and structured to allow beginners to
learn how to use tar, it is not intended solely for beginners.
The tutorial explains how to use the three most frequently used
subcommands (create, list, and extract) as well as
two frequently used options (file and verbose). The other
chapters do not refer to the tutorial frequently; however, if a section
discusses something which is a complex variant of a basic concept, there
may be a cross reference to that basic concept. (The entire book,
including the tutorial, assumes that the reader understands some basic
concepts of using a Unix-type operating system; see Tutorial.)
The third chapter presents the remaining five subcommands, and information
about using tar options and option syntax. The other chapters are
meant to be used as a reference. Each chapter presents everything that
needs to be said about a specific topic.
One of the chapters (see Date input formats) exists in its entirety
in other GNU manuals, and is mostly self-contained. In addition, one
section of this manual (see Standard) contains a big quote which is
taken directly from tar sources.
In general, we give both the long and short (abbreviated) option names at least once in each section where the relevant option is covered, so that novice readers will become familiar with both styles. (A few options have no short versions, and the relevant sections will indicate this.)
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 forth1. You can use tar
to create a new archive in a specified directory.
The files inside an archive are called members. Within this
manual, we use the term file to refer only to files accessible in
the normal ways (by ls, cat, and so forth), and the term
member to refer only to the members of an archive. Similarly, a
file name is the name of a file, as it resides in the filesystem,
and a member name is the name of an archive member within the
archive.
The term extraction refers to the process of copying an archive
member (or multiple members) into a file in the filesystem. Extracting
all the members of an archive is often called extracting the
archive. The term unpack can also be used to refer to the
extraction of many or all the members of an archive. Extracting an
archive does not destroy the archive's structure, just as creating an
archive does not destroy the copies of the files that exist outside of
the archive. You may also list the members in a given archive
(this is often thought of as "printing" them to the standard output,
or the command line), or append members to a pre-existing archive.
All of these subcommands can be peformed using tar.
tar DoesThe tar program provides the ability to create tar
archives, as well as various other kinds of manipulation. For example,
you can use tar on previously created archives to extract files,
to store additional files, or to update or list files which were already
stored.
Initially, tar archives were used to store files conveniently on
magnetic tape. The name tar comes from this use; it stands for
tape archiver. Despite the utility's name, tar can
direct its output to available devices, files, or other programs (using
pipes). tar may even access remote devices or files (as archives).
You can use tar archives in many ways. We want to stress a few
of them: storage, backup, and transportation.
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 filesystem.
tar Archives are NamedConventionally, 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.
We make some of our recommendations throughout this book for one
reason in addition to what we think of as "good sense". The main
additional reason for a recommendation is to be compliant with the
POSIX standards. If you set the shell environment variable
POSIXLY_CORRECT, GNU tar will force you to adhere to
these standards. Therefore, if this variable is set and you violate
one of the POSIX standards in the way you phrase a command, for
example, GNU tar will not allow the command and will signal an
error. You would then have to reorder the options or rephrase
the command to comply with the POSIX standards.
There is a chance in the future that, if you set this environment
variable, your archives will be forced to comply with POSIX standards,
also. No GNU tar extensions will be allowed.
tar AuthorsGNU tar was originally written by John Gilmore, and modified by
many people. The GNU enhancements were written by Jay Fenlason, then
Joy Kendall, and the whole package has been further maintained by
Thomas Bushnell, n/BSG, and finally François Pinard, with
the help of numerous and kind users.
We wish to stress that tar is a collective work, and owes much to
all those people who reported problems, offered solutions and other
insights, or shared their thoughts and suggestions. An impressive, yet
partial list of those contributors can be found in the THANKS
file from the GNU tar distribution.
Jay Fenlason put together a draft of a GNU tar manual, borrowing
notes from the original man page from John Gilmore. This draft has been
distributed in tar versions 1.04 through 1.10, then withdrawn in
version 1.11. Michael Innis Bushnell (later known as Thomas) and Amy
Gorin worked on a tutorial and manual for GNU tar. François
Pinard put version 1.11.8 of the manual together by taking information
from all these sources and merging them. Melissa Weisshaus revised many
parts of the manual and reorganised its structure, around version 1.12,
in view of publication by the FSF. Some people especially contributed to
specific aspects of the manual: Karl Berry (dates), Dan Hagerty (backups),
Eli Zaretskii (MS-DOS), and surely others.
If you find problems or have suggestions about this program or manual,
please report them to tar-bugs@iro.umontreal.ca.
tarThis chapter guides you through some basic examples of three tar
subcommands: --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.
tar Subcommands and Options
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 subcommands. 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 path names,
we will assume that those paths are relative to your home directory.
For example, my home directory path is /home/fsf/melissa. All of
my examples are in a subdirectory of the directory named by that path
name; the subdirectory is called practice.
tar archives with tape drives.
tar Subcommands and Optionstar can take a wide variety of arguments which specify and define
the actions it will perform on the particular set
of files or the archive. The main types of arguments to tar fall
into one of two classes: subcommands, and options.
Some arguments fall into a class called subcommands; 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 them as
operating modes. You are in a particular operating mode when you
have specified the subcommand which calls for that mode; there are eight
subcommands 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 subcommands and options in any of
three forms: long (mnemonic) form, short form, and old style. Some of
the subcommands and options have no short or "old" forms; however, the
subcommands and options which we will cover in this tutorial have
corresponding abbreviations. We will indicate those abbreviations appropriately to get
you used to seeing them. (Note that the "old style" option forms
exist in GNU tar for compatibility with Unix tar. We
present a full discussion of this way of writing options and subcommands
in Old Options, and we discuss the other two styles of
writing options in Mnemonic Options and Short Options.)
In the examples and in the text of this tutorial, we usually use the
long forms of subcommands 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 Styles. 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 "subcommands"
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 subcommands". However,
experienced tar users often refer to these by shorthand terms
such as, "short and long forms of options". This term assumes that
the "subcommands" 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".
The term, tar commands, refers to the various
subcommands. 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 whole line,
not the tar subcommand. Again, use context to figure out
which of the meanings the speaker intends.
Here are the three most frequently used subcommands (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 subcommands in detail. We will present the rest of the subcommands in the next chapter.
tar archive.
To understand how to run tar in the three operating modes listed
previously, you also need to understand how to use two of the options to
tar: --file (which takes an archive file as an argument)
and --verbose. (You are usually not required to specify
either of these options when you run tar, but they can be very
useful in making things more clear and helping you avoid errors.)
--file OptionYou can specify an argument for the --file=archive-name (-f archive-name) option whenever you
use tar; this option determines the name of the archive file
that tar will work on.
If you don't specify this argument, then tar will use a default,
usually standard output or standard input, or some physical tape drive
attached to your machine. If there is no tape drive attached, or the
default is not meaningful, then tar will print an error message.
The error message might look roughly like one of the following:
tar: can't open /dev/rmt8 : No such device or address tar: can't open /dev/rsmt0 : I/O error
To avoid confusion, we recommend that you always specfiy an archive file
name by using --file=archive-name (-f archive-name) when writing your tar commands.
For more information on using the --file=archive-name (-f archive-name) option, see
file.
--verbose Optiontar is running.
tar. This can be especially useful when the results might not be
obvious. For example, if you want to see the progress of tar as
it writes files into the archive, you can use the --verbose
option. In the beginning, you may find it useful to use
--verbose at all times; when you are more accustomed to
tar, you will likely want to use it at certain times but not at
others. We will use --verbose at times to help make something
clear, and we will give many examples both using and not using
--verbose to show the differences.
Sometimes, a single instance of --verbose on the command line
will show a full, ls style listing of an archive or files,
giving sizes, owners, and similar information. Other times,
--verbose will only show files or members that the particular
subcommand is operating on at the time. In the latter case, you can
use --verbose twice in a command to get a listing such as that
in the former case. For example, instead of saying
tar -cvf afiles.tar apple angst aspic
above, you might say
tar -cvvf afiles.tar apple angst aspic
This works equally well using short or long forms of options. Using long forms, you would simply write out the mnemonic form of the option twice, like this:
$ tar --create --verbose --verbose ...
Note that you must double the hyphens properly each time.
Later in the tutorial, we will give examples using --verbose --verbose.
--help Option--help option to tar prints out a very brief list of
all subcommands and options available for the current version of
tar available on your system.
(This message will disappear, once this node revised.)
One of the basic subcommands 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
subcommands, 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.
To follow along with this and future examples, create a new directory
called practice containing files called blues, folk
and jazz. The files can contain any information you like:
ideally, they should contain information which relates to their names,
and be of different lengths. Our examples assume that practice
is a subdirectory of your home directory.
Now cd to the directory named practice; practice
is now your working directory. (Please note: Although
the full path name of this directory is
/homedir/practice, in our examples we will refer to
this directory as practice; the homedir is presumed.
In general, you should check that the files to be archived exist where
you think they do (in the working directory) by running ls.
Because you just created the directory and the files and have changed to
that directory, you probably don't need to do that this time.
It is very important to make sure there isn't already a file in the
working directory with the archive name you intend to use (in this case,
collection.tar), or that you don't care about its contents.
Whenever you use create, tar will erase the current
contents of the file named by --file=archive-name (-f archive-name) if it exists. tar
will not tell you if you are about to overwrite a file unless you
specify an option which does this . To add files to an existing archive, you need to use a
different option, such as --append (-r); see append for
information on how to do this.
To place the files blues, folk, and jazz into an
archive named collection.tar, use the following command:
$ tar --create --file=collection.tar blues folk jazz
The order of the arguments is not very important, when using long option forms. You could also say:
$ tar blues --create folk --file=collection.tar jazz
However, you can see that this order is harder to understand; this is
why we will list the arguments in the order that makes the commands
easiest to understand (and we encourage you to do the same when you use
tar, to avoid errors).
Note that the part of the command which says, --file=collection.tar is considered to be one argument. If you substituted any other string of characters for `collection.tar', then that string would become the name of the archive file you create.
The order of the options becomes more important when you begin to use short forms. With short forms, if you type commands in the wrong order (even if you type them correctly in all other ways), you may end up with results you don't expect. For this reason, it is a good idea to get into the habit of typing options in the order that makes inherent sense. See short create for more information on this.
In this example, you type the command as shown above: --create
is the subcommand 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 subcommand)
. Now that they
are are in the archive, they are called archive members, not
files .
When you create an archive, you must specify which files you want
placed in the archive. If you do not specify any archive members, GNU
tar will complain.
If you now list the contents of the working directory (ls), you will find the archive file listed as well as the files you saw previously:
$ ls blues collection.tar folk jazz
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 any subcommand, tar will not
run and will prompt you for one. If you don't name any files, tar
will complain. You must have write access to the working directory,
or else you will not be able to create an archive in that directory.
Caution: Do not attempt to use --create (-c) to add files to an existing archive; it will delete the archive and write a new one. Use --append (-r) instead. See append.
--create with --verboseIf 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 what tar does.
createAs we said before, the --create (-c) subcommand is one of the most
basic uses of tar, and you will use it countless times.
Eventually, you will probably want to use abbreviated (or "short")
forms of options. A full discussion of the three different forms that
options can take appears in Styles; for now, here is what the
previous example (including the --verbose (-v) option) looks like
using short option forms:
$ tar -cvf collection.tar blues folk jazz blues folk jazz
As you can see, the system responds the same no matter whether you use long or short option forms.
One difference between using short and long option forms is that, although the exact placement of arguments following options is no more specific when using short forms, it is easier to become confused and make a mistake when using short forms. For example, suppose you attempted the above example in the following way:
$ tar -cfv collection.tar blues folk jazz tar: Cannot add file collection.tar: No such file or directory tar: Error exit delayed from previous errors $ ls blues folk jazz v $
You got error messages and a file called v. What happened is
that tar tried to 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 tried
to add a file called collection.tar to the v archive file;
if the file collection.tar did not already exist, tar will
report an error indicating that this file does not exist. If the file
collection.tar does already exist (e.g., from a previous command
you may have run), then tar will add this file to the archive.
Because the -v option did not get registered, tar will not
run under verbose mode, and will not report its progress.
The end result is that you may be quite confused about what happened, and possibly overwrite a file. To illustrate this further, we will show you how an example we showed previously would look using short forms.
This example,
$ tar blues --create folk --file=collection.tar jazz
is confusing as it is. When shown using short forms, however, it becomes much more so:
$ tar blues -c folk -f collection.tar jazz
It would be very easy to put the wrong string of characters
immediately following the -f, but doing that could sacrifice
valuable data.
For this reason, we recommend that you pay very careful attention to
the order of options and placement of file and archive names,
especially when using short option forms. Not having the option name
written out mnemonically can affect how well you remember which option
does what, and therefore where different names have to be placed.
(Placing options in an unusual order can also cause tar to
report an error if you have set the shell environment variable,
POSIXLY_CORRECT; see posix compliance for more information
on this.)
You can archive a directory by specifying its directory name as a
file name argument to tar. The files in the directory will be
archived relative to the working directory, and the directory will be
re-created along with its contents when the archive is extracted.
To archive a directory, first move to its superior directory. If you have followed the previous instructions in this tutorial, you should type:
$ cd .. $
This will put you into the directory which contains practice,
i.e. your home directory. Once in the superior directory, you can
specify the subdirectory, practice, as a file name argument. To
store practice in the new archive file music.tar, type:
$ tar --create --verbose --file=music.tar practice
tar should output:
practice/ practice/blues practice/folk practice/jazz practice/collection.tar
Note that the archive thus created is not in the subdirectory
practice, but rather in the current working directory--the
directory from which tar was invoked. Before trying to archive a
directory from its superior directory, you should make sure you have
write access to the superior directory itself, not only the directory
you are trying archive with tar. For example, you will probably
not be able to store your home directory in an archive by invoking
tar from the root directory; See absolute. (Note
also that collection.tar, the original archive file, has itself
been archived. tar will accept any file as a file to be
archived, regardless of its content. When music.tar is
extracted, the archive file collection.tar will be re-written
into the file system).
If you give tar a command such as
$ tar --create --file=foo.tar .
tar will report tar: foo.tar is the archive; not dumped.
This happens because tar creates the archive foo.tar
in the current directory before putting any files into it. Then, when
tar attempts to add all the files in the directory . to
the archive, it notices that the file foo.tar is the same as the
archive, 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: Some other
versions of tar are not so clever; they might enter an infinite
loop when this happens. It is safer, in general, to not depend on GNU
tar behaviour, by making sure that the archive is not created
inside a directory being itself dumped.) Some MS-Windows file systems
(notably, Windows 95 VFAT disks) don't support the feature which is
used by GNU tar to detect that the archive is included in itself,
so the archive will be included on those file systems.
Sometimes the directory structure you want an archive to create when you unpack it is different from the directory structure you built it in.
This is often true when you are using a tar production in a
Makefile, to build distribution archives in a source-code directory.
The most convenient way to do this would be with a command like the
following:
$ tar --create --file=distribution.tar *
But this makes an archive that will unpack as a lot of individual files, which is usually not what you want. Assuming your build directory is named `project', you might do something like
$ tar --directory=.. --create --file=distribution.tar project
But this means you will be stuck with the name of your build directory as the top-level name of the distribution archive. Often you would like your distribution archive to unpack into a single directory, but with a synthetic top-level directory name (perhaps including a version number).
You can get this effect with the --name-prefix=prefix option. Its argument is a name prefix which gets prepended to each file name entry in an archive as it is added to the archive.
For example, if you say
$ tar --name-prefix='project-5.23/' --create --file=collection.tar *
you will make an archive of all the files and directories in the
current directory, but it will extract into the single subdirectory
project-5.23.
Note that you will almost always want the argument of --name-prefix=prefix to end with a directory delimiter (forward slash under Unix-like operating systems, forward or back-slash under DOS). Otherwise you will make an archive in which each file has a prefix welded directly to the first segment of its name. While this capability is there for full generality, it is probably not what you want.
Frequently, you will find yourself wanting to determine exactly what a
particular archive contains. You can use the --list (-t) subcommand
to get the member names as they currently appear in the archive, as well
as various attributes of the files at the time they were archived. For
example, you can examine the archive collection.tar that you
created in the last section with the command,
$ tar --list --file=collection.tar
The output of tar would then be:
blues folk jazz
The archive bfiles.tar would list as follows:
./birds baboon ./box
Be sure to use a --file=archive-name (-f archive-name) option just as with --create (-c) to specify the name of the archive.
If you use the --verbose (-v) option with --list, then
tar will print out a listing reminiscent of ls -l,
showing owner, file size, and so forth.
If you had used --verbose (-v) mode, the example above would look like:
$ tar --list --verbose --file=collection.tar -rw-rw-rw- myself user 42 1990-05-21 13:29 blues -rw-rw-rw- myself user 62 1990-05-23 10:55 folk -rw-rw-rw- myself user 40 1990-05-21 13:30 jazz
You can specify one or more individual member names as arguments when
using list. In this case, tar will only list the
names of members you identify. For example, tar --list --file=afiles.tar apple would only print apple.
Because tar preserves paths, file names must be specified as they
appear in the archive (ie., relative to the directory from which the
archive was created). Therefore, it is essential when specifying member
names to tar that you give the exact member names. For example,
tar --list --file=bfiles birds would produce an error message
something like tar: birds: Not found in archive, because there is
no member named birds, only one named ./birds. While the
names birds and ./birds name the same file, member
names are compared using a simplistic name comparison, in which an exact
match is necessary. See absolute.
However, tar --list --file=collection.tar folk would respond
with folk, because folk is in the archive file
collection.tar. If you are not sure of the exact file name, try
listing all the files in the archive and searching for the one you
expect to find; remember that if you use --list with no file
names as arguments, tar will print the names of all the members
stored in the specified archive.
As an exception to member name preservation when creating the archive,
tar on MS-DOS/MS-Windows mirrors the DOS-style backslashes into
Unix-style forward slashes. This is because the bulk of tar code
is deeply entrenched in the Unix world and assumes that forward slashes
are used in too many places. Converting slashes automatically lets the
users specify files as they are used to on those systems. But it also
means that you need to remember about this conversion, so you won't be
surprised when tar lists the files differently than what you
typed on the command line. For example:
$ tar -cvf c:\files\backup\today.tar d:\data\may97 d:\data\june97 data/may97 data/june97 $ tar -xvf c:\files\backup\today.tar data\may97 data/may97
(This message will disappear, once this node revised.)
To get information about the contents of an archived directory, use the directory name as a file name argument in conjunction with --list (-t). To find out file attributes, include the --verbose (-v) option.
For example, to find out about files in the directory practice, in
the archive file music.tar, type:
$ tar --list --verbose --file=music.tar practice
tar responds:
drwxrwxrwx myself user 0 1990-05-31 21:49 practice/ -rw-rw-rw- myself user 42 1990-05-21 13:29 practice/blues -rw-rw-rw- myself user 62 1990-05-23 10:55 practice/folk -rw-rw-rw- myself user 40 1990-05-21 13:30 practice/jazz -rw-rw-rw- myself user 10240 1990-05-31 21:49 practice/collection.tar
When you use a directory name as a file name argument, tar acts on
all the files (including sub-directories) in that directory.
(This message will disappear, once this node revised.)
Creating an archive is only half the job--there is no point in storing files in an archive if you can't retrieve them. The act of retrieving members from an archive so they can be used and manipulated as unarchived files again is called extraction. To extract files from an archive, use the --extract (--get, -x) subcommand. As with --create (-c), specify the name of the archive with --file=archive-name (-f archive-name). Extracting an archive does not modify the archive in any way; you can extract it multiple times if you want or need to.
Using --extract, you can extract an entire archive, or specific
files. The files can be directories containing other files, or not. As
with --create (-c) and --list (-t), you may use the short or the
long form of the subcommand without affecting the performance.
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:
jazz blues folk
To extract specific archive members, give their exact member names as
arguments, as printed by --list (-t). If you had mistakenly deleted
one of the files you had placed in the archive collection.tar
earlier (say, blues), you can extract it from the archive without
changing the archive's structure. It will be identical to the original
file blues that you deleted.
First, make sure you are in the practice directory, and list the
files in the directory. Now, delete the file, blues, and list
the files in the directory again.
You can now extract the member blues from the archive file
collection.tar like this:
$ tar --extract --file=collection.tar blues
If you list the files in the directory again, you will see that the file
blues has been restored, with its original permissions, creation
times, and owner.
(These parameters will be identical to those which
the file had when you originally placed it in the archive; any changes
you may have made before deleting the file from the file system,
however, will not have been made to the archive member.) The
archive file, collection.tar, is the same as it was before you
extracted blues. You can confirm this by running tar with
--list (-t).
Remember that as with other subcommands, specifying the exact member name
is important. tar --extract --file=bfiles.tar birds will fail,
because there is no member named birds. To extract the member named
./birds, you must specify tar --extract --file=bfiles.tar ./birds. To find the exact member names of the members of an archive,
use --list (-t) (see list).
If you give the --verbose (-v) option, then --extract (--get, -x) will print the names of the archive members as it extracts them.
Extracting directories which are members of an archive is similar to extracting other files. The main difference to be aware of is that if the extracted directory has the same name as any directory already in the working directory, then files in the extracted directory will be placed into the directory of the same name. Likewise, if there are files in the pre-existing directory with the same names as the members which you extract, the files from the extracted archive will overwrite the files already in the working directory (and possible subdirectories). This will happen regardless of whether or not the files in the working directory were more recent than those extracted.
If a file was stored with a directory name as part of its file name, and
that directory does not exist under the working directory when the file
is extracted, tar will create the directory.
We can demonstrate how to use --extract to extract a directory
file with an example. Change to the practice directory if you
weren't there, and remove the files folk and jazz. Then,
go back to the parent directory and extract the archive
music.tar. You may either extract the entire archive, or you may
extract only the files you just deleted. To extract the entire archive,
don't give any file names as arguments after the archive name
music.tar. To extract only the files you deleted, use the
following command:
$ tar -xvf music.tar practice/folk practice/jazz
Because you created the directory with practice as part of the
file names of each of the files by archiving the practice
directory as practice, you must give practice as part
of the file names when you extract those files from the archive.
Here are some sample commands you might try which will not work, and why they won't work.
If you try to use this command,
$ tar -xvf music.tar folk jazz
you will get the following response:
tar: folk: Not found in archive tar: jazz: Not found in archive $
This is because these files were not originally in the parent
directory .., where the archive is located; they were in the
practice directory, and their file names reflect this:
$ tar -tvf music.tar practice/folk practice/jazz practice/rock
Likewise, if you try to use this command,
$ tar -tvf music.tar folk jazz
you would get a similar response. Members with those names are not in the archive. You must use the correct member names in order to extract the files from the archive.
If you have forgotten the correct names of the files in the archive, use tar --list --verbose to list them correctly.
tar Subcommands(This message will disappear, once this node revised.)
In the last chapter, you learned about the first three subcommands to
tar. This chapter presents the remaining five subcommands to
tar: --append, --update, --concatenate,
--delete, and --compare.
You are not likely to use these subcommands as frequently as those
covered in the last chapter; however, since they perform specialized
functions, they are quite useful when you do need to use them. We
will give examples using the same directory and files that you created
in the last chapter. As you may recall, the directory is called
practice, the files are jazz, blues, folk,
rock, and the two archive files you created are
collection.tar and music.tar.
We will also use the archive files afiles.tar and
bfiles.tar. afiles.tar contains the members apple,
angst, and aspic. bfiles.tar contains the members
./birds, baboon, and ./box.
Unless we state otherwise, all practicing you do and examples you follow
in this chapter will take place in the practice directory that
you created in the previous chapter; see prepare for examples.
(Below in this section, we will remind you of the state of the examples
where the last chapter left them.)
The five subcommands that we will cover in this chapter are:
tar OperationsThe 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 archive2. 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 ot each other on the QWERTY keyboard.
Instead of being unpacked, the archive then gets wholly destroyed.
When users speak about exploding an archive, they usually mean
something else... Ahem!
file, when the intent was to create
an archive with a single file in it. This error is likely because a
tired user can easily add the f key to the cluster of option
letters, by the mere force of habit, without realizing the full
consequence of doing so. The usual consequence is that the single
file, which was meant to be saved, is rather destroyed.
So, recognizing the likelihood and the catastrophical nature of these
errors, GNU tar now takes some distance from elegance, and
cowardly refuses to create an archive when --create (-c) option is
given, there are no arguments besides options, and --files-from=file-of-names (-T file-of-names)
option is not used. Observe:
$ tar c tar: Cowardly refusing to create an empty archive Try `tar --help' for more information. $ tar cf foo tar: Cowardly refusing to create an empty archive Try `tar --help' for more information.
To get around the cautiousness of GNU
tar and nevertheless create an archive with nothing in it,
one may still use, as the value for the --files-from=file-of-names (-T file-of-names) option,
a file with no names in it, as shown in the following commands:
tar --create --file=empty-archive.tar --files-from=/dev/null tar cfT empty-archive.tar /dev/null
tar archive, as a pipe.
When archives created on Unix are extracted on MS-DOS and MS-Windows
3.x, several problems pop up due to their incompatibilities with the
Unix filesystem. GNU tar has several trick up its sleeve, to
overcome such problems:
GNU tar tries to repair such names automatically, by replacing
the offending characters with legal ones. For example, .emacs is
exracted as _emacs, foo.bar.c will be converted to
foo_bar.c, lost+found will become lost_found, etc.
(tar even tries to be intelligent by treating some cases
specially; for example, files which end with .c++ are extracted
as .cxx.)
When the file is thus renamed, tar will print a message to that
effect when operating verbosely (see verbose). Note that if a
directory needs to be renamed, tar will print such a message for
all the files in that directory.
CON, the printer
device is PRN, the serial port device is AUX, etc.
These names are special on DOS, in that files with these names, if they
exist in any directory, are inaccessible: the device driver will
intercept any file-related call before the filesystem ever sees it.
(That's because MS-DOS wants to create an illusion that the devices are
present in every directory; in contrast, on Unix the devices live under
a special directory called /dev.)
When archives created on Unix are unpacked on MS-DOS, they might include
files with these "forbidden" names, such as prn.txt or
aux.c. Trying to extract such files will lead to crashes, system
wedges, and other atrocities, as the device drivers will try to
interpret the data they get and act on it (e.g., some devices are
read-only, and don't know how to handle incoming data).
Therefore, tar checks every file name it is about to extract, and
if it refers to a character device, tar changes that file's name
by prepending an underscore _ to it; thus, aux.c will be
extracted as _aux.c. (Actually, tar tries to prepend up
to 2 underscore characters, each time checking if the new name is a
regular file. If both attempts are unsuccessful, tar will print
an error message and refuse to extract that file.)
As with illegal characters, tar announces each renamed file when
under verbose operation.
tar now shows dates as 1996-11-09, while it used to
show them as Nov 11 1996. (One can revert to the old behavior by
defining USE_OLD_CTIME in src/list.c before reinstalling.)
But preferrably, people you should get used to ISO 8601 dates. Local
American dates should be made available again with full date localisation
support, once ready. In the meantime, programs not being localisable
for dates should prefer international dates, that's really the way to go.
Look up <http://www.ft.uni-erlangen.de/~mskuhn/iso-time.html> if you
are curious, it contains a detailed explanation of the ISO 8601 standard.
Currently, the listing of the directory using ls is as follows:
$ ls blues collection.tar folk jazz
The archive file collection.tar looks like this:
$ tar -tvf collection.tar jazz blues folk
The archive file ../music.tar, in the parent directory, looks
like this:
$ tar -tvf ../music.tar practice/folk practice/jazz practice/rock
--append(This message will disappear, once this node revised.)
If you want to add files to an existing archive, you don't need to
create a new archive; you can use --append (-r). The archive must
already exist in order to use --append. (A related subcommand
is the --update subcommand; you can use this to add newer
versions of archive members to an existing archive. To learn how to
do this with --update, refer to update.)
If you use --append (-r) to add a file to an archive which already
contains a member with the same name, then the
old member is not deleted. What does happen, however, is somewhat
complex. tar allows you to have infinite numbers of
archive members
with the same name. Some subcommands treat these same-named members no
differently than any other set of archive members: for example, if you
view an archive with --list (-t), you will see all of those members
listed, with their modification times, owners, etc.
Other subcommands don't deal with these members as perfectly as you might
prefer; if you were to use --extract (--get, -x) to extract the archive,
only the most recently added copy of a member with the same name as
other members would end up in the working directory. This is because
--extract extracts an archive in the order the members appeared
in the archive; the most recently archived members will be extracted
last. Additionally, an extracted member will overwrite a file of
the same name which existed in the directory already, and tar
will not prompt you about this. Thus, only the most recently archived
member will end up being extracted, as it will overwrite the one
extracted before it, and so on. (One way of extracting all of the
members without overwriting them is to use the --backup
option, see backup.)
Other than using --backup, there are a few additional ways to get around this.
If you want to replace an archive member, use --delete to
delete the member you want to remove from the archive, and then use
--append to add the member you want to be in the archive.
(Replacing one member with another will not work on certain types of
media, such as tapes; see delete and Media, for more
information.) Note that you can not change the order of the archive;
the most recently added member will still appear last. In this sense,
you cannot truely "replace" one member with another.
(This message will disappear, once this node revised.)
The simplest way to add a file to an already existing archive is the
--append (-r) subcommand, 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 subcommands, the member names of the newly added
files will be exactly the same as their names given on the command line
(except that backslashes in MS-DOS file names are mirrored to Unix-style
forward slashes). 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
subcommand will be unpredictable. See Media.
To demonstrate use of --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) subcommand, you will see that
rock has been added to the archive:
$ tar --list --file=collection.tar jazz blues folk rock
You can use --append (-r) to add copies of files which have been
updated since the archive was created. (However, we do not recommend
doing this since there is another tar option called
--update; see update for more information. We describe this
use of --append here for the sake of completeness.)
When you extract the archive, the older version will be effectively lost. This works because files are extracted from an archive in the order in which they were archived. Thus, when the archive is extracted, a file archived later in time will overwrite a file of the same name which was archived earlier, even though the older version of the file will remain in the archive unless you delete all versions of the file.
Suppose you change the file blues and then append the changed
version to collection.tar. As you saw above, the original
blues is in the archive collection.tar. If you change the
file and append the new version of the file to the archive, there will
be two copies in the archive. When you extract the archive, the older
version of the file will be extracted first, and then overwritten by the
newer version when it is extracted.
You can append the new, changed copy of the file blues to the
archive in this way:
$ tar --append --verbose --file=collection.tar blues blues
Because you specified the --verbose option, tar has
printed the name of the file being appended as it was acted on. Now
list the contents of the archive:
$ tar --list --verbose --file=collection.tar -rw-rw-rw- me user 28 1996-10-18 16:31 jazz -rw-rw-rw- me user 21 1996-09-23 16:44 blues -rw-rw-rw- me user 20 1996-09-23 16:44 folk -rw-rw-rw- me user 20 1996-09-23 16:44 rock -rw-rw-rw- me user 58 1996-10-24 18:30 blues
The newest version of blues is now at the end of the archive
(note the different creation dates and file sizes). If you extract
the archive, the older version of the file blues will be
overwritten by the newer version. You can confirm this by extracting
the archive and running ls on the directory. See Writing
for more information. (Please note: This is the case unless
you employ the --backup option; .)
(This message will disappear, once this node revised.)
In the previous section, you learned how to use --append (-r) to add
a file to an existing archive. A related subcommand is
--update (-u). The --update subcommand updates a tar
archive by comparing the date of the specified archive members against
the date of the file with the same name. If the file has been modified
more recently than the archive member, then the newer version of the
file is added to the archive (as with --append (-r)).
Unfortunately, you cannot use --update with magnetic tape drives.
The command 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 ().
--updateYou must use file name arguments with the --update (-u) subcommand.
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 might end up confusing
you, who knows!).
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 subcommand 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 get data inserted in their middle;
doing so would destroy any data after the place you write. See Media
for more information about tapes.)
--update (-u) is not suitable for performing backups for two
reasons: it does not change directory content entries, and it lengthens
the archive every time it is used. The GNU tar options intended
specifically for backups are more efficient. If you need to run
backups, please consult Backups.
--concatenateSometimes 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 the contents of one or more archives to the end of another archive, you should use the --concatenate (--catenate, -A) subcommand.
To use --concatenate, name two archives to be concatenated on the
command line. (Nothing happens if you don't list any.) The members,
and their member names, will be copied verbatim from those archives. If
this causes multiple members to have the same name, it does not delete
any members; all the members with the same name coexist. For
information on how this affects reading the archive, .
To demonstrate how --concatenate works, create two small archives
called bluesrock.tar and folkjazz.tar, using the relevant
files from practice:
$ tar -cvf bluesrock.tar blues rock blues classical $ tar -cvf folkjazz.tar folk jazz folk jazz
If you like, You can run tar --list to make sure the archives
contain what they are supposed to:
$ tar -tvf bluesrock.tar -rw-rw-rw- melissa user 105 1997-01-21 19:42 blues -rw-rw-rw- melissa user 33 1997-01-20 15:34 rock $ tar -tvf folkjazz.tar -rw-rw-rw- melissa user 20 1996-09-23 16:44 folk -rw-rw-rw- melissa user 65 1997-01-30 14:15 jazz
We can concatenate these two archives with tar:
$ 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 jazz folk
When you use --concatenate, the source and target archives must
already exist and must have been created using compatible format parameters
(). The new, concatenated archive
will be called by the same name as the first archive listed on the command
line.
Like --append (-r), this subcommand 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
subcommand; after all, cat is the utility for combining files.
However, tar archives incorporate an end-of-file marker which
must be removed if the concatenated archives are to be read properly as
one archive. --concatenate removes the end-of-archive marker
from the target archive before each new archive is appended. If you use
cat to combine the archives, the result will not be a valid
tar format archive. If you need to retrieve files from an
archive that was added to using the cat utility, use the
--ignore-zeros (-i) option. See Ignore Zeros for further
information on dealing with archives improperly combined using the
cat utility.
You must specify the source archives using --file=archive-name (-f archive-name)
(see file). If you do not specify the target archive, tar
uses the value of the environment variable TAPE, or, if this has
not been set, the default archive name. If the default archive name is
the standard output, tar will print an error message.
--delete(This message will disappear, once this node revised.)
You can remove members from an archive by using the --delete
option. Specify the name of the archive with --file=archive-name (-f archive-name) and then
specify the names of the members to be deleted; if you list no member
names, nothing will be deleted. The --verbose (-v) option will
cause tar to print the names of the members as they are deleted.
As with --extract (--get, -x), you must give the exact member names when
using tar --delete. --delete will remove all versions of
the named file from the archive. The --delete subcommand can run
very slowly.
Unlike other subcommands, --delete has no short form.
The --delete subcommand will rewrite the archive. You can only
use --delete on an archive if the archive device allows you to
write to any point on the media, such as a disk; because of this, it
does not work on magnetic tapes. Do not try to delete an archive member
from a magnetic tape; the action will not succeed, and you will be
likely to scramble the archive and damage your tape. There is no safe
way (except by completely re-writing the archive) to delete files from
most kinds of magnetic tape. See Media.
To delete all versions of the file blues from the archive
collection.tar in the practice directory, make sure you
are in that directory, and then,
$ tar --list --file=collection.tar jazz blues folk rock blues blues classical $ tar --delete --file=collection.tar blues $ tar --list --file=collection.tar jazz folk rock classical $
The --delete option has been reported to work properly when
tar acts as a filter from stdin to stdout.
(This message will disappear, once this node revised.)
The --compare (-d), or --diff subcommand 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 funk: File does not exist
If the file in the archive is a link, tar may have a different
error message, such as:
funk: Does not exist
The spirit behind the --compare (--diff, -d) option is to check whether the archive represents the current state of files on disk, more than validating the integrity of the archive media. For this later goal, See verify.
Some filesystems, such as the FAT and VFAT used on MS-DOS and
MS-Windows, only maintain file modification times up to the nearest even
second. tar compiled for such systems knows about this, and
doesn't report difference in mod times unless it is larger than one
second. But if you run tar on Unix and your files reside on a
FAT disk (via some network interface), you could have some false alarms
in this case.
--extract(This message will disappear, once this node revised.)
The previous chapter showed how to use --extract (--get, -x) to extract
an archive into the filesystem. Various options cause tar to
extract more information than just file contents, such as the owner,
the permissions, the modification date, and so forth. This section
presents options to be used with --extract when certain special
considerations arise. You may review the information presented in
extract for more basic information about the
--extract subcommand.
tar Writes Files
(This message will disappear, once this node revised.)
Normally, tar will request data in full record increments from
an archive storage device. If the device cannot return a full record,
tar will report an error. However, some devices do not always
return full records, or do not require the last record of an archive to
be padded out to the next record boundary. To keep reading until you
obtain a full record, or to accept an incomplete record if it contains
an end-of-archive marker, specify the --read-full-records (-B) option
in conjunction with the --extract (--get, -x) or --list (-t) subcommands.
See Blocking.
The --read-full-records (-B) option is turned on by default when
tar reads an archive from standard input, or from a remote
machine. This is because on BSD Unix systems, attempting to read a
pipe returns however much happens to be in the pipe, even if it is
less than was requested. If this option were not enabled, tar
would fail as soon as it read an incomplete record from the pipe.
If you're not sure of the blocking factor of an archive, you can read the archive by specifying --read-full-records (-B) and --blocking-factor=blocks (-b blocks), using a blocking factor larger than what the archive uses. This lets you avoid having to determine the blocking factor of an archive. See Blocking Factor.
Normally, tar stops reading when it encounters a block of zeros
between file entries (which usually indicates the end of the archive).
--ignore-zeros (-i) allows tar to completely read an archive
which contains a block of zeros before the end (i.e. a damaged
archive, or one which was created by cat-ing several archives
together).
The --ignore-zeros (-i) option is turned off by default because many
versions of tar write garbage after the end-of-archive entry,
since that part of the media is never supposed to be read. GNU
tar does not write after the end of an archive, but seeks to
maintain compatablity among archiving utilities.
tar Writes Files(This message will disappear, once this node revised.)
Normally, tar writes extracted files into the file system without
regard to the files already on the system; i.e., files with the same
names as archive members are overwritten when the archive is extracted.
If the name of a corresponding file name is a symbolic link, the file
pointed to by the symbolic link will be overwritten instead of the
symbolic link itself (if this is possible). Moreover, special devices,
empty directories and even symbolic links are automatically removed if
they are found to be in the way of the proper extraction.
To prevent tar from extracting an archive member from an archive
if doing so will overwrite a file in the file system, use
--keep-old-files (-k) in conjunction with --extract. When
this option is specified, tar will report an error stating the
name of the files in conflict instead of overwriting the file with the
corresponding extracted archive member.
The --unlink-first (-U) option removes existing files, symbolic links, empty directories, devices, etc., prior to extracting over them. This could take care of several situations where overwriting files is undesirable.
One such situation is when the archive includes a regular file whereas
the filesystem has a symbolic link by the same name. Using this option
will prevent tar from overwriting the file to which the symbolic
link points by the contents of an extracted file, since the link itself
is removed prior to the extraction.
Another situation is with overwriting a running program's executable file. On some systems, the backing store for the executable is the original program file; overwriting it while the program runs could lead to segmentation violation and other crashes. You could use the --unlink-first (-U) option to prevent such woes when extracting arbitrary executables over currently running copies. Note that if something goes wrong with the extraction and you did use this option, you might end up with no file at all. Without this option, if something goes wrong with the extraction, the existing file is not overwritten and preserved. (If you want to be safe either way, use both --unlink-first (-U) and --backup; See backup.)
If you specify the --recursive-unlink option, tar removes
anything that keeps you from extracting a file as far as current
permissions will allow it. This could include removal of the contents
of a full directory hierarchy. For example, someone using this feature
may be very surprised at the results when extracting something over a
directory entry from the archive. This option can be dangerous; be very
aware of what you are doing if you choose to use it.
Without --recursive-unlink, GNU tar silently overwrites
non-directories, but will never overwrite existing non-empty
directories.
One especially nasty subtlety with both --unlink-first (-U) and
--recursive-unlink options happens on filesystems which
silently truncate the names of the files in the archive when they are
extracted to the file system. (MS-DOS and MS-Windows 3.X are two
notorious examples.) In such case, tar might incidentally delete
a file or a directory which has nothing to do with the extracted file,
only because their names are the same after truncation. We recommend
not to use these two options on such filesystems. In fact, we recommend
that if you unpack on MS-DOS archives which were created on Unix, you
always use either --keep-old-files (-k) or --backup,
to prevent overwriting existing files due to file name truncation.
See backup.
tar from over-writing
existing files with files with the same name from the archive.
The --keep-old-files (-k) option is meaningless with --list (-t).
Prevents tar from overwriting files in the file system during
extraction.
Some people argue that GNU tar should not hesitate to overwrite
files with other files when extracting. When extracting a tar
archive, they expect to see a faithful copy of the state of the filesystem
when the archive was created. It is debatable that this would always
be a proper behaviour. For example, suppose one has an archive in
which usr/local is a link to usr/local2. Since the
archive was created,
maybe the site removed the link and renamed the whole hierarchy from
/usr/local2 to /usr/local. Such things happen all the time.
I guess it would not be welcome at all that GNU tar removes the
whole hierarchy just to make room for the link to be reinstated (unless it
also simultaneously restores the full /usr/local2, of course!
GNU tar is indeed able to remove a whole hierarchy to reestablish a
symbolic link, for example, but only if --recursive-unlink
is specified to allow this behaviour. In any case, single files are
silently removed.
Normally, tar sets the modification times of extracted files to
the modification times recorded for the files in the archive, but
limits the permissions of extracted files by the current umask
setting.
To set the modification times of extracted files to the time when the files were extracted, use the --touch (-m) option in conjunction with --extract (--get, -x).
To set the modes (access permissions) of extracted files to those
recorded for those files in the archive, use --same-persmissions
in conjunction with the --extract (--get, -x) subcommand.
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. You might need to use this option to
browse one or more files with a pager such as less, or if you
want the extracted file to have another name on 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 message will disappear, once this node revised.)
If a previous attempt to extract files failed due to lack of disk
space, you can use --starting-file=name (-K name) to start extracting only
after member name of the archive. This assumes, of course, that
there is now free space, or that you are now extracting into a
different file system. (You could also choose to suspend tar,
remove unnecessary files from the file system, and then restart the
same tar subcommand. In this case, --starting-file=name (-K name) is
not necessary. See Inc Dumps, See interactive,
and exclude.)
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.
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. (This is true only if we have a good scheme for truncated backup names, which I'm not sure at all: I suspect work is needed in this area. The MS-DOS/MS-Windows version works with numbered backups, even when file names are truncated.) When any existing file is backed up before being overwritten by extraction, then clashing files are automatically 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. We recommend that you always use numbered backups when unpacking archives on MS-DOS and on other systems that have file name limitations.
At the detail level, some decisions are still experimental, and may change in the future, we are waiting comments from our users. So, please do not learn to depend blindly on the details of the backup features. For example, currently, directories themselves are never renamed through using these options, so, extracting a file over a directory still has good chances to fail. Also, backup options apply to created archives, not only to extracted members. For created archives, backups will not be attempted when the archive is a block or character device, or when it refers to a remote file.
For the sake of simplicity and efficiency, backups are made by renaming old files prior to creation or extraction, and not by copying. The original name is restored if the file creation fails. If a failure occurs after a partial extraction of a file, both the backup and the partially extracted file are kept.
--backup
--suffix=suffix
-b. If this
option is not specified, the value of the SIMPLE_BACKUP_SUFFIX
environment variable is used. And if SIMPLE_BACKUP_SUFFIX is not
set, the default is ~, just as in Emacs.
--version-control=method
VERSION_CONTROL
environment variable is used. And if VERSION_CONTROL is not set,
the default backup type is existing.
This option corresponds to the Emacs variable version-control;
the same values for method are accepted as in Emacs. The values
that this option accepts have also more descriptive name. Here are the
valid methods (unique abbreviations are accepted):
t
numbered
nil
existing
never
simple
Some people express the desire to always use the --backup option, by defining some kind of alias or script. This is not as easy as one may thing, due to the fact that old style options should appear first and consume arguments a bit unpredictably for an alias or script. But, if you are ready to give up using old style options, you may resort to using something like a (Bourne shell) function here:
tar () { /usr/local/bin/tar --backup $*; }
tar Usages(This message will disappear, once this node revised.)
You can easily use archive files to transport a group of files from
one system to another: put all relevant files into an archive on one
computer system, transfer the archive to another system, and extract
the contents there. The basic transfer medium might be magnetic tape,
Internet FTP, or even electronic mail (though you must encode the
archive with uuencode in order to transport it properly by
mail). Both machines do not have to use the same operating system, as
long as they both support the tar program.
For example, here is how you might copy a directory's contents from one disk to another, while preserving the dates, modes, owners and link-structure of all the files therein. In this case, the transfer medium is a pipe, which is one a Unix redirection mechanism:
$ cd sourcedir; tar --create --file=- . | (cd targetdir; tar --extract --file=-)
The command also works using short option forms:
$ cd sourcedir; tar -cf - . | (cd targetdir; tar -xf -)
If you are really confident you are using GNU tar and that standard
input and output were not overriden by something else as the default archive
at your installation, you might write the above examples more simply:
$ cd sourcedir; tar --create . | (cd targetdir; tar --extract) $ cd sourcedir; tar -c . | (cd targetdir; tar -x)
This is one of the easiest methods to transfer a tar archive.
You have now seen how to use all eight of the subcommands available to
tar, and a number of the possible options. The next chapter
explains how to choose and change file and archive names, how to use
files to store names of other files which you can then call as
arguments to tar (this can help you save time if you expect to
archive the same list of files a number of times), and how to
If there are too many files to conveniently list on the command line,
you can list the names in a file, and tar will read that file.
See files.
There are various ways of causing tar to skip over some files,
and not archive them. See Choosing.
(This message will disappear, once this node revised.)
GNU tar is distributed along with the scripts which the Free
Software Foundation uses for performing backups. There is no corresponding
scripts available yet for doing restoration of files. Even if there is
a good chance those scripts may be satisfying to you, they are not the
only scripts or methods available for doing backups and restore. You may
well create your own, or use more sophisticated packages dedicated to
that purpose.
Some users are enthusiastic about Amanda (The Advanced Maryland
Automatic Network Disk Archiver), a backup system developed by James
da Silva jds@cs.umd.edu and available on many Unix systems.
This is free software, and it is available at these places:
http://www.cs.umd.edu/projects/amanda/amanda.html ftp://ftp.cs.umd.edu/pub/amanda
Here is a possible plan for a future documentation about the backuping
scripts which are provided within the GNU tar distribution.
.* dumps
. + what are dumps
. + different levels of dumps
. - full dump = dump everything
. - level 1, level 2 dumps etc, -
A level n dump dumps everything changed since the last level
n-1 dump (?)
. + how to use scripts for dumps (ie, the concept)
. - scripts to run after editing backup specs (details)
. + Backup Specs, what is it.
. - how to customize
. - actual text of script [/sp/dump/backup-specs]
. + Problems
. - rsh doesn't work
. - rtape isn't installed
. - (others?)
. + the --incremental option of tar
. + tapes
. - write protection
. - types of media
. : different sizes and types, useful for different things
. - files and tape marks
one tape mark between files, two at end.
. - positioning the tape
MT writes two at end of write,
backspaces over one when writing again.
This chapter documents both the provided FSF scripts and tar
options which are more specific to usage as a backup tool.
To back up a file system means to create archives that contain all the files in that file system. Those archives can then be used to restore any or all of those files (for instance if a disk crashes or a file is accidently deleted). File system backups are also called dumps.
tar to Perform Full Dumps
tar to Perform Incremental Dumps
tar to Perform Full Dumps(This message will disappear, once this node revised.)
Full dumps should only be made when no other people or programs
are modifying files in the filesystem. If files are modified while
tar is making the backup, they may not be stored properly in
the archive, in which case you won't be able to restore them if you
have to. (Files being modified are written with no trouble, and do
not corrupt the entire archive.)
You will want to use the --label=archive-label (-V archive-label) option to give the archive a volume label, so you can tell what this archive is even if the label falls off the tape, or anything like that.
Unless the filesystem you are dumping is guaranteed to fit on one volume, you will need to use the --multi-volume (-M) option. Make sure you have enough tapes on hand to complete the backup.
If you want to dump each filesystem separately you will need to use
the --one-file-system (-l) option to prevent tar from crossing
filesystem boundaries when storing (sub)directories.
The --incremental (-G) option is not needed, since this is a complete copy of everything in the filesystem, and a full restore from this backup would only be done onto a completely empty disk.
Unless you are in a hurry, and trust the tar program (and your
tapes), it is a good idea to use the --verify (-W) option, to make
sure your files really made it onto the dump properly. This will
also detect cases where the file was modified while (or just after)
it was being archived. Not all media (notably cartridge tapes) are
capable of being verified, unfortunately.
--listed-incremental=snapshot-file (-g snapshot-file) takes a file name argument always. If the
file doesn't exist, tar runs a level zero dump, creating the
file. If the file exists, tar uses that file to see what has
changed.
--incremental (-G) handle old GNU-format incremental backup.
The --incremental (-G) option means the archive is an incremental backup. Its meaning depends on the command that it modifies.
When this option is used for creating an incremental backup of
a filesystem, tar
writes, at the beginning of the archive, an entry for each of the
directories that will be operated on. The entry for a directory
includes a list of all the files in the directory at the time the
dump was done, and a flag for each file indicating whether the file
is going to be put in the archive. This information is used when
doing a complete incremental restore.
Note that this option causes tar to create a non-standard
archive that may not be readable by non-GNU versions of the tar
program.
If the --incremental (-G) option is used with --list (-t), tar
will list, for each directory in the archive, the list of files in
that directory at the time the archive was created. This information
is put out in a format that is not easy for humans to read, but which
is unambiguous for a program: each file name is preceded by either a
Y if the file is present in the archive, an N if the
file is not included in the archive, or a D if the file is
a directory (and is included in the archive). Each file name is
terminated by a null character. The last file is followed by an
additional null and a newline to indicate the end of the data.
If the --incremental (-G) option is used with --extract (--get, -x), then when the entry for a directory is found, all files that currently exist in that directory but are not listed in the archive are deleted from the directory.
This behavior is convenient when you are restoring a damaged file
system from a succession of incremental backups: it restores the
entire state of the file system to that which obtained when the backup
was made. If you don't use --incremental (-G), the file system will
probably fill up with files that shouldn't exist any more.
--listed-incremental=snapshot-file (-g snapshot-file) handles new GNU-format incremental backup.
--listed-incremental=snapshot-file (-g snapshot-file) acts like --incremental (-G), but when
used in conjunction with --create (-c) will also cause tar to
use the file file, which contains information about the state
of the filesystem at the time of the last backup, to decide which
files to include in the archive being created. That file will then
be updated by tar. If the file file does not exist when
this option is specified, tar will create it, and include all
appropriate files in the archive.
The file, which is archive independent, contains the date it was last
modified and a list of devices, inode numbers and directory names.
tar will archive files with newer mod dates or inode change
times, and directories with an unchanged inode number and device but
a changed directory name. The file is updated after the files to
be archived are determined, but before the new archive is actually
created.
When restoring, only files newer than the saved time are restored, and the directory list is used to speed up subcommands.
GNU tar actually writes the file twice: once before the data
and written, and once after.
tar to Perform Incremental Dumps(This message will disappear, once this node revised.)
Performing incremental dumps is similar to performing full dumps, although a few more options will usually be needed.
You will need to use the -N date option to tell tar
to only store files that have been modified since date.
date should be the date and time of the last full/incremental
dump. See after.
A standard scheme is to do a monthly (full) dump once a month, a weekly dump once a week of everything since the last monthly and a daily dump every day of everything since the last (weekly or monthly) dump.
Here is a copy of the script used to dump the filesystems of the
machines here at the Free Software Foundation. This script is run via
cron late at night when people are least likely to be using the
machines. This script dumps several filesystems from several machines
at once (via NFS). The operator is responsible for ensuring that all
the machines will be up at the time the dump happens. If a machine is
not running, its files will not be dumped, and the next day's
incremental dump will not store files that would have gone onto
that dump.
#!/bin/csh # Dump thingie set now = `date` set then = `cat date.nfs.dump` /u/hack/bin/tar -c -G -v\ -f /dev/rtu20\ -b 126\ -N "$then"\ -V "Dump from $then to $now"\ /alpha-bits/gp\ /gnu/hack\ /hobbes/u\ /spiff/u\ /sugar-bombs/u echo $now > date.nfs.dump mt -f /dev/rtu20 rew
Output from this script is stored in a file, for the operator to read later.
This script uses the file date.nfs.dump to store the date/time
of the last dump.
Since this is a streaming tape drive, no attempt to verify the archive
is done. This is also why the high blocking factor (126) is used.
The tape drive must also be rewound by the mt command after
the dump is made.
(This message will disappear, once this node revised.)--incremental (-G) is used in conjunction with --create (-c), --extract (--get, -x) or --list (-t) when backing up and restoring file systems. An archive cannot be extracted or listed with the --incremental (-G) option specified unless it was created with this option specified. This option should only be used by a script, not by the user, and is usually disregarded in favor of --listed-incremental=snapshot-file (-g snapshot-file), which is described below. --incremental (-G) in conjunction with --create (-c) causes
tar to write, at the beginning of the archive, an entry for
each of the directories that will be archived. The entry for a
directory includes a list of all the files in the directory at the
time the archive was created and a flag for each file indicating
whether or not the file is going to be put in the archive.
Note that this option causes tar to create a non-standard
archive that may not be readable by non-GNU versions of the tar
program.
--incremental (-G) in conjunction with --extract (--get, -x) causes
tar to read the lists of directory contents previously stored
in the archive, delete files in the file system that did not
exist in their directories when the archive was created, and then
extract the files in the archive.
This behavior is convenient when restoring a damaged file system from
a succession of incremental backups: it restores the entire state of
the file system to that which obtained when the backup was made. If
--incremental (-G) isn't specified, the file system will probably
fill up with files that shouldn't exist any more.
--incremental (-G) in conjunction with --list (-t), causes
tar to print, for each directory in the archive, the list of
files in that directory at the time the archive was created. This
information is put out in a format that is not easy for humans to
read, but which is unambiguous for a program: each file name is
preceded by either a Y if the file is present in the archive,
an N if the file is not included in the archive, or a D
if the file is a directory (and is included in the archive). Each
file name is terminated by a null character. The last file is followed
by an additional null and a newline to indicate the end of the data.
--listed-incremental=snapshot-file (-g snapshot-file) acts like --incremental (-G), but when
used in conjunction with --create (-c) will also cause tar
to use the file snapshot-file, which contains information about
the state of the file system at the time of the last backup, to decide
which files to include in the archive being created. That file will
then be updated by tar. If the file file does not exist
when this option is specified, tar will create it, and include
all appropriate files in the archive.
The file file, which is archive independent, contains the date
it was last modified and a list of devices, inode numbers and
directory names. tar will archive files with newer mod dates
or inode change times, and directories with an unchanged inode number
and device but a changed directory name. The file is updated after
the files to be archived are determined, but before the new archive is
actually created.
Althought it should be obvious that a device has a non-volatile value, NFS
devices have non-dependable values when an automounter gets in the picture.
This leads to a great deal of spurious redumping in incremental dumps,
so it is somewhat useless to compare two NFS devices numbers over time.
So tar now considers all NFS devices as being equal when it comes
to comparing directories; this is fairly gross, but there does not seem
to be a better way to go.
(This message will disappear, once this node revised.)
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 do an incremental dump. A level one dump archives all the files that have changed since the last full dump.
A typical dump strategy would be to perform a full dump once a week, and a level one dump once a day. This means some versions of files will in fact be archived more than once, but this dump strategy makes it possible to restore a file system to within one day of accuracy by only extracting two archives--the last weekly (full) dump and the last daily (level one) dump. The only information lost would be in files changed or created since the last daily backup. (Doing dumps more than once a day is usually not worth the trouble).
GNU tar comes with scripts you can use to do full and level-one
dumps. Using scripts (shell programs) to perform backups and
restoration is a convenient and reliable alternative to typing out
file name lists and tar commands by hand.
Before you use these scripts, you need to edit the file
backup-specs, which specifies parameters used by the backup
scripts and by the restore script. See Script Syntax. Once the backup parameters
are set, you can perform backups or restoration by running the
appropriate script.
The name of the restore script is restore. The names of the level one and full backup
scripts are, respectively, level-1 and level-0.
The level-0 script also exists under the name weekly, and
the level-1 under the name daily--these additional names
can be changed according to your backup schedule. See Scripted Restoration, for more information on running the restoration script.
See Scripted Backups, for more information on running the
backup scripts.
Please Note: The backup scripts and the restoration scripts are designed to be used together. While it is possible to restore files by hand from an archive which was created using a backup script, and to create an archive by hand which could then be extracted using the restore script, it is easier to use the scripts. See Inc Dumps, and See Inc Dumps, before making such an attempt.
(This message will disappear, once this node revised.)
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.
See Script Syntax, for an explanation of this syntax.
backup-specs specifies the following parameters:
ADMINISTRATOR
BACKUP_HOUR
now.
TAPE_FILE
tar writes the archive to. This device should be
attached to the host on which the dump scripts are run.
TAPE_STATUS
BLOCKING
tar will use when writing the dump archive.
See Blocking Factor.
BACKUP_DIRS
HOST_NAME
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.
BACKUP_FILES
Backup-specs
Backup-specs
Backup-specs(This message will disappear, once this node revised.)
The following is the text of backup-specs as it appears at FSF:
# site-specific parameters for file system backup. ADMINISTRATOR=friedman BACKUP_HOUR=1 TAPE_FILE=/dev/nrsmt0 TAPE_STATUS="mts -t $TAPE_FILE" BLOCKING=124 BACKUP_DIRS=" albert:/fs/fsf apple-gunkies:/gd albert:/fs/gd2 albert:/fs/gp geech:/usr/jla churchy:/usr/roland albert:/ albert:/usr apple-gunkies:/ apple-gunkies:/usr gnu:/hack gnu:/u apple-gunkies:/com/mailer/gnu apple-gunkies:/com/archive/gnu" BACKUP_FILES="/com/mailer/aliases /com/mailer/league*[a-z]"
Backup-specs(This message will disappear, once this node revised.)
backup-specs is in shell script syntax. The following
conventions should be considered when editing the script:
A quoted string is considered to be contiguous, even if it is on more than one line. Therefore, you cannot include commented-out lines within a multi-line quoted string. BACKUP_FILES and BACKUP_DIRS are the two most likely parameters to be multi-line.
A quoted string typically cannot contain wildcards. In
backup-specs, however, the parameters BACKUP_DIRS and
BACKUP_FILES can contain wildcards.
(This message will disappear, once this node revised.)
The syntax for running a backup script is:
script-name [time-to-be-run]
where time-to-be-run can be a specific system time, or can be
now. If you do not specify a time, the script runs at the time
specified in backup-specs (see Script Syntax).
You should start a script with a tape or disk mounted. Once you
start a script, it prompts you for new tapes or disks as it
needs them. Media volumes don't have to correspond to archive
files--a multi-volume archive can be started in the middle of a
tape that already contains the end of another multi-volume archive.
The restore script prompts for media by its archive volume,
so to avoid an error message you should keep track of which tape
(or disk) contains which volume of the archive. See Scripted Restoration.
The backup scripts write two files on the file system. The first is a
record file in /etc/tar-backup/, which is used by the scripts
to store and retrieve information about which files were dumped. This
file is not meant to be read by humans, and should not be deleted by
them. See incremental and listed-incremental, for a more
detailed explanation of this file.
The second file is a log file containing the names of the file systems
and files dumped, the time when the backup was made, and any error
messages that were generated, as well as how much space was left in
the media volume after the last volume of the archive was written.
You should check this log file after every backup. The file name is
log-mmm-ddd-yyyy-level-1 or
log-mmm-ddd-yyyy-full.
The script also prints the name of each system being dumped to the standard output.
(This message will disappear, once this node revised.)
Warning: The GNUtardistribution does not provide any suchrestorescript yet. This section is only listed here for documentation maintenance purposes. In any case, all contents is subject to change as things develop.
To restore files that were archived using a scripted backup, use the
restore script. The syntax for the script is:
where ***** are the file systems to restore from, and ***** is a regular expression which specifies which files to restore. If you specify -all, the script restores all the files in the file system.
You should start the restore script with the media containing the first volume of the archive mounted. The script will prompt for other volumes as they are needed. If the archive is on tape, you don't need to rewind the tape to to its beginning--if the tape head is positioned past the beginning of the archive, the script will rewind the tape as needed. See Media, for a discussion of tape positioning.
If you specify --all as the files argument, the
restore script extracts all the files in the archived file
system into the active file system.
Warning: The script will delete files from the active file system if they were not in the file system when the archive was made.See Inc Dumps, and Inc Dumps, for an explanation of how the script makes that determination.
tar(This message will disappear, once this node revised.)
Certain options to tar enable you to specify a name for your
archive. Other options let you decide which files to include or exclude
from the archive, based on when or whether files were modified, whether
the file names do or don't match specified patterns, or whether files
are in specified directories.
(This message will disappear, once this node revised.)
By default, tar uses an archive file name that was compiled into
it when tar was built on the system; usually this name refers to
some physical tape drive on the machine. Later versions of GNU
tar use standard output as the default archive file name (wich
you can redirect to another file).
However, the person who installed tar
on the system may not set the default to a meaningful value as far as
most users are concerned. As a result, you will usually want to tell
tar where to find (or create) the archive. The --file=archive-name (-f archive-name)
option allows you to either specify or name a file to use as the archive
instead of the default archive file location.
For example, in this tar command,
$ tar -cvf collection.tar blues folk jazz
collection.tar is the name of the archive. It must directly
follow the -f option, since whatever directly follows -f
will end up naming the archive. If you neglect to specify an
archive name, you may end up overwriting a file in the working directory
with the archive you create since tar will use this file's name
for the archive name.
An archive can be saved as a file in the file system, sent through a pipe or over a network, or written to an I/O device such as a tape, floppy disk, or CD write drive.
If you do not name the archive, tar uses the value of the
environment variable TAPE as the file name for the archive. If
that is not available, tar uses a default, compiled-in archive
name, usually that for tape unit zero (ie. /dev/tu00), or the
standard output.
tar always needs an archive name.
If you use - as an archive-name, tar reads the
archive from standard input (when listing or extracting files), or
writes it to standard output (when creating an archive). If you use
- as an archive-name when modifying an archive,
tar reads the original archive from its standard input and
writes the entire new archive to its standard output.
To specify an archive file on a device attached to a remote machine, use the following:
--file=hostname:/dev/file name
tar will initiate the remote connection, if possible, and prompt
you for a username and password. If you use
--file=user@hostname:/dev/file name,
tar will initiate the remote connection, if possible, using your
username as the username on the remote machine.
If the archive file name includes a colon (:), then it is assumed
to be a file on another machine. If the archive file is
user@host:file, then file is used on the
host host. The remote host is accessed using the rsh
program, with a username of user. If the username is omitted
(along with the @ sign), then your user name will be used.
(This is the normal rsh behavior.) It is necessary for the
remote machine, in addition to permitting your rsh access, to
have the /usr/ucb/rmt program installed. If you need to use a
file whose name includes a colon, then the remote tape drive behavior
can be inhibited by using the --force-local option.
The MS-DOS/MS-Windows version of tar makes an exception to the
above: it does not treat archive names with drive letters, such
as d:/backups/dayly.tar, as remote. This is so you would not be
forced to use --force-local with the usual DOS-style absolute
file names. tar only makes this exception when the colon is
preceded by a single character in the range [A-z], which are valid
MS-DOS/MS-Windows drive letters. (Yes, DOS also allows the drive letter
to be one of the 6 characters between uppercase Z and lowercase
a.) This means that remote machines with single-letter names are
effectively disallowed by the MS-DOS version of tar. However, we
find it hard to believe that anybody will use a one-letter name for a
machine. (And as of now, tar doesn't support remote access on
MS-DOS anyway.)
When the archive is being created to /dev/null, GNU tar
tries to minimize input and output subcommands. The Amanda backup
system, when used with GNU tar, has an initial sizing pass which
uses this feature.
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. .
To specify file names, you can include them as the last arguments on the command line, as follows:
tar subcommand [option1 option2 ...] [file name-1 file name-2 ...]
If you specify a directory name as a file name argument, all the files
in that directory are operated on by tar.
If you do not specify files when tar is invoked with
--create (-c), tar operates on all the non-directory files in
the working directory. If you specify either --list (-t) or
--extract (--get, -x), tar operates on all the archive members in the
archive. If you specify any subcommand other than one of these three,
tar does nothing.
By default, tar takes file names from the command line. However,
there are other ways to specify file or member names, or to modify the
manner in which tar selects the files or members upon which to
operate; See files; see exclude. In general, these methods
work both for specifying the names of files and archive members.
(This message will disappear, once this node revised.)
Instead of giving the names of files or archive members on the command
line, you can put the names into a file, and then use the
--files-from=file-of-names (-T file-of-names) option to tar. Give the name of the file
which contains the list of files to include as the argument to
--files-from. In the list, the file names should be separated by
newlines. You will frequently use this option when you have generated
the list of files to archive with the find utility.
If you give a single dash as a file name for --files-from, (i.e.,
you specify either --files-from=- or -T -), then the file
names are read from standard input.
Unless you are running tar with --create, you can not use
both --files-from=- and --file=- (-f -) in the same
command.
The following example shows how to use find to generate a list of
files smaller than 400K in length and put that list into a file
called small-files. You can then use the -T option to
tar to specify the files from that file, small-files, to
create the archive little.tgz. (The -z option to
tar compresses the archive with gzip; see gzip for
more information.)
$ find . ! -type d -size -400 -print > small-files $ tar -c -v -z -T small-files -f little.tgz
The ! -type d predicate is needed so that find won't print
the name . directory itself, if it happens to be small in size.
If find were to print the name of ., all of the files in
the current directory would end up in the archive, in addition to the
files that are smaller than 400KB!
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.
The --null option is just like the one in GNU xargs and
cpio, and is useful with the -print0 predicate of GNU
find. In tar, --null also causes
--directory=directory (-C directory) options to be treated as file names to archive, in
case there are any files out there called -C.
This example shows how to use find to generate a list of files
larger than 800K in length and put that list into a file called
long-files. The -print0 option to find works just
like -print, except that it separates files with a NUL
rather than with a newline. You can then run tar with both the
--null and -T options to specify that tar get the
files from that file, long-files, to create the archive
big.tgz. The --null option to tar will cause
tar to recognize the NUL separator between files.
$ find . ! -type d -size +800 -print0 > long-files $ tar -c -v --null --files-from=long-files --file=big.tar
Again, you need the ! -type d predicate to prevent find
from printing the . directory name, which would have caused
tar to include the entire directory in the archive.
(This message will disappear, once this node revised.)
To avoid operating on files whose names match a particular pattern, use the --exclude=pattern or --exclude-from=file-of-patterns (-X file-of-patterns) options.
tar to ignore files that match the pattern.
The --exclude=pattern option will prevent any file or member which
matches the shell wildcards (pattern) from being operated on
(pattern can be a single file name or a more complex expression).
For example, if you want to create an archive with all the contents of
/tmp except the file /tmp/foo, you can use the command
tar --create --file=arch.tar --exclude=foo. You may give
multiple --exclude options.
tar to ignore files that match the patterns listed in
file.
Use the --exclude-from=file-of-patterns option to read a
list of shell wildcards, one per line, from file; tar will
ignore files matching those wildcards. Thus if tar is
called as tar -c -X foo . and the file foo contains a
single line *.o, no files whose names end in .o will be
added to the archive.
exclude OptionsSome users find exclude options confusing. Here are some common
pitfalls:
tar will always act on file names
listed on the command line, no matter whether or not there is an
exclusion which would otherwise affect them. In the example above, if
you create an archive and exclude files that end with *.o, but
explicitly name the file catc.o after all the options have been
listed, catc.o will be included in the archive.
--exclude-from=file-of-patterns to introduce the name of a
file which contains a list of patterns, one per line; each of these
patterns can exclude zero, one, or many files.
tar sees wildcard characters like *.
If you do not do this, the shell might expand the * itself
using files at hand, so tar might receive a list of files
instead of one pattern, or none at all, making the command somewhat
illegal. This might not correspond to what you want.
For example, write:
$ tar -c -f archive.tar -X '*/tmp/*' directory
rather than:
$ tar -c -f archive.tar -X */tmp/* directory
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. See Wildcards.
tar, what is now the
--exclude-from=file-of-patterns option was called
--exclude-pattern instead. Now,
--exclude=pattern applies to patterns listed on the command
line and --exclude-from=file-of-patterns applies to
patterns listed in a file.
Globbing is the operation by which wildcard characters,
* or ? for example, are replaced and expanded into all
existing files matching the given pattern. However, tar often
uses wildcard patterns for matching (or globbing) archive members instead
of actual files in the filesystem. Wildcard patterns are also used for
verifying volume labels of tar archives. This section has the
purpose of explaining wildcard syntax for tar.
A pattern should be written according to shell syntax, using wildcard
characters to effect globbing. Most characters in the pattern stand
for themselves in the matched string, and case is significant: a
will match only a, and not A. The character ? in the
pattern matches any single character in the matched string. The character
* in the pattern matches zero, one, or more single characters in
the matched string. The character \ says to take the following
character of the pattern literally; it is useful when one needs to
match the ?, *, [ or \ characters, themselves.
The character [, up to the matching ], introduces a
character class. A character class is a list of acceptable characters
for the next single character of the matched string. For example,
[abcde] would match any of the first five letters of the alphabet.
Note that within a character class, all of the "special characters"
listed above other than \ lose their special meaning; for example,
[-\\[*?]] would match any of the characters, -, \,
[, *, ?, or ]. (Due to parsing constraints,
the characters - and ] must either come first or
last in a character class.)
If the first character of the class after the opening [
is ! or ^, then the meaning of the class is reversed.
Rather than listing character to match, it lists those characters which
are forbidden as the next single character of the matched string.
Other characters of the class stand for themselves. The special
construction [a-e], using an hyphen between two
letters, is meant to represent all characters between a and
e, inclusive.
Periods (.) or forward slashes (/) are not considered
special for wildcard matches. However, if a pattern completely matches
a directory prefix of a matched string, then it matches the full matched
string: excluding a directory also excludes all the files beneath it.
There are some discussions floating in the air and asking for modifications
in the way GNU tar accomplishes wildcard matches. We perceive
any change of semantics in this area as a delicate thing to impose on
GNU tar users. On the other hand, the GNU project should be
progressive enough to correct any ill design: compatibility at all price
is not always a good attitude. In conclusion, it is possible
that slight amendments be later brought to the previous description.
Your opinions on the matter are welcome.
(This message will disappear, once this node revised.)
The --after-date=date (--newer=date, -N date) option causes tar to only work on files
whose modification or inode-changed times are newer than the date
given. If you use this option when creating or appending to an archive,
the archive will only include new files. If you use --after-date
when extracting an archive, tar will only extract files newer
than the date you specify.
If you only want tar to make the date comparison based on
modification of the actual contents of the file (rather than inode
changes), then use the --newer-mtime=date option.
You may use these options with any subcommand. Note that these options
differ from the --update (-u) subcommand 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 modification or inode-changed times are
later than date. Use in conjunction with any subcommand.
These options limit tar to only operating on files which have
been modified after the date specified. A file is considered to have
changed if the contents have been modified, or if the owner,
permissions, and so forth, have been changed. (For more information on
how to specify a date, see Date input formats; remember that the
entire date argument must be quoted if it contains any spaces.)
Gurus would say that --after-date=date (--newer=date, -N date) tests both the mtime
(time the contents of the file were last modified) and ctime
(time the file's status was last changed: owner, permissions, etc)
fields, while --newer-mtime=date tests only mtime field.
To be precise, --after-date=date (--newer=date, -N date) checks both mtime and
ctime and processes the file if either one is more recent than
date, while --newer-mtime=date only checks mtime and
disregards ctime. Neither uses atime (the last time the
contents of the file were looked at).
The MS-DOS/MS-Windows file systems record only one time stamp per file (Windows 9X and NT file systems usually do have all three time fields). On those file systems, --after-date=date (--newer=date, -N date) and --newer-mtime=date will have the same effect. MS-DOS stores file time with 2-second granularity; you need to consider this when setting the value of date for these two options.
Date specifiers can have embedded spaces. Because of this, you may need to quote date arguments to keep the shell from parsing them as separate arguments.
Please Note: --after-date=date (--newer=date, -N date) and --newer-mtime=date should not be used for incremental backups. Some files (such as those in renamed directories) are not selected properly by these options. See incremental and listed-incremental.
To select files newer than the modification time of a file that already
exists, you can use the --reference (-r) option of GNU
date, available in GNU shell utilities 1.13 or later. For example,
you could say,
$ tar -cf archive.tar --newer="`date -r file`" /home
It returns the timestamp of that already existing file; this timestamp
expands to become the referent date which --newer uses to determine
which files to archive
(This message will disappear, once this node revised.)
Usually, tar will recursively explore all directories (either
those given on the command line or through the --files-from=file-of-names (-T file-of-names)
option) for the various files they contain. However, you may not always
want tar to act this way.
The --no-recursion option inhibits tar's recursive descent
into specified directories. If you specify --no-recursion, you can
use the find utility for hunting through levels of directories to
construct a list of file names which you could then pass to tar.
find allows you to be more selective when choosing which files to
archive; see files for more information on using find with
tar.
tar from recursively descending directories.
When you use --no-recursion, GNU tar grabs directory entries
themselves, but does not descend on them recursively. Many people use
find for locating files they want to back up, and since
tar usually recursively descends on directories, they have
to use the ! -d option to find as they usually do not want
all the files in a directory. They then use the {No value for `op-file-from'}
option to archive the files located via find.
The problem when restoring files archived in this manner is that the
directories themselves are not in the archive; so the
--same-permissions (--preserve-permissions, -p) option does not affect them--while users
might really like it to. Specifying --no-recursion is a way to
tell tar to grab only the directory entries given to it, adding
no new files on its own.
(This message will disappear, once this node revised.)
tar will normally automatically cross file system boundaries in
order to archive files which are part of a directory tree. You can
change this behavior by running tar and specifying
--one-file-system (-l). This option only affects files that are
archived because they are in a directory that is being archived;
tar will still archive files explicitly named on the command line
or through --files-from=file-of-names (-T file-of-names), regardless of where they reside.
tar from crossing file system boundaries when
archiving. Use in conjunction with any write subcommand.
The --one-file-system option causes tar to modify its
normal behavior in archiving the contents of directories. If a file in
a directory is not on the same filesystem as the directory itself, then
tar will not archive that file. If the file is a directory
itself, tar will not archive anything beneath it; in other words,
tar will not cross mount points.
It is reported that using this option, the mount point is archived, but nothing under it is.
This option is useful for making full or incremental archival backups of a single file system. If this option is used in conjunction with --verbose (-v), files that are excluded are mentioned by name on the standard error.
(This message will disappear, once this node revised.)
To change the working directory in the middle of a list of file names, either on the command line or in a file specified using --files-from=file-of-names (-T file-of-names), use --directory=directory (-C directory). This will change the working directory to the directory directory after that point in the list.
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 as if they were in the same directory.
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 food/cherry
which records the third file in the archive under the name
food/cherry so that, if the archive is extracted using
tar --extract, the third file will be written in a subdirectory
named food.
You can use the --directory=directory (-C 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=directory (-C directory) options are interpreted consecutively. If
--directory=directory (-C directory) specifies a relative file name, it is interpreted
relative to the then current directory, which might not be the same as
the original current working directory of tar, due to a previous
--directory option.
When using --files-from (see files), you can put -C
options in the file list. Unfortunately, you cannot put
--directory options in the file list. (This interpretation can
be disabled by using the --null option.)
(This message will disappear, once this node revised.)
By default, GNU tar drops a leading / on input or output.
This option turns off this behavior; it is equivalent to changing to the
root directory before running tar (except it also turns off the
usual warning message).
When tar extracts archive members from an archive, it strips any
leading slashes (/) from the member name. This causes absolute
member names in the archive to be treated as relative file names. This
allows you to have such members extracted wherever you want, instead of
being restricted to extracting the member in the exact directory named
in the archive. For example, if the archive member has the name
/etc/passwd, tar will extract it as if the name were
really etc/passwd.
On MS-DOS and MS-Windows, tar also strips the drive letter and
the colon that follows it, if the original file names include them.
Other tar programs do not do this. As a result, if you create an
archive whose member names start with a slash, they will be difficult
for other people with a non-GNU tar program to use. Therefore,
GNU tar also strips leading slashes from member names when
putting members into the archive. For example, if you ask tar to
add the file /bin/ls to an archive, it will do so, but the member
name will be bin/ls.
If you use the --absolute-names (-P) option, tar will do
neither of these transformations.
To archive or extract files relative to the root directory, specify the --absolute-names (-P) option.
Normally, tar acts on files relative to the working
directory--ignoring superior directory names when archiving, and
ignoring leading slashes when extracting. (On MS-DOS/MS-Windows, it
also ignores any drive specifications, a letter and a colon, so
d:/foo/bar is archived as foo/bar.)
When you specify --absolute-names (-P), tar stores file names
including all superior directory names, and preserves leading slashes
(and drive specs on MS-DOS/MS-Windows).
If you only invoked tar from the root directory you would never
need the --absolute-names (-P) option, but using this option may be
more convenient than switching to root.
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. (On MS-DOS/MS-Windows, tar
also reports the removing of drive spec.)
Some people, nevertheless, do not want to see this message. Wanting to
play really dangerously, one may of course redirect tar standard
error to the sink. For example, under sh:
$ tar -c -f archive.tar /home 2> /dev/null
Another solution, both nicer and simpler, would be to change to
the / directory first, and then avoid absolute notation.
For example:
$ (cd / && tar -c -f archive.tar home) $ tar -c -f archive.tar -C / home
Our units of temporal measurement, from seconds on up to months, are so complicated, asymmetrical and disjunctive so as to make coherent mental reckoning in time all but impossible. Indeed, had some tyrannical god contrived to enslave our minds to time, to make it all but impossible for us to escape subjection to sodden routines and unpleasant surprises, he could hardly have done better than handing down our present system. It is like a set of trapezoidal building blocks, with no vertical or horizontal surfaces, like a language in which the simplest thought demands ornate constructions, useless particles and lengthy circumlocutions. Unlike the more successful patterns of language and science, which enable us to face experience boldly or at least level-headedly, our system of temporal calculation silently and persistently encourages our terror of time. ... It is as though architects had to measure length in feet, width in meters and height in ells; as though basic instruction manuals demanded a knowledge of five different languages. It is no wonder then that we often look into our own immediate past or future, last Tuesday or a week from Sunday, with feelings of helpless confusion. ...-- Robert Grudin, Time and the Art of Living.
This section describes the textual date representations that GNU
programs accept. These are the strings you, as a user, can supply as
arguments to the various programs. The C interface (via the
getdate function) is not described here.
Although the date syntax here can represent any possible time since zero A.D., computer integers are not big enough for such a (comparatively) long time. The earliest date semantically allowed on Unix systems is midnight, 1 January 1970 UCT.
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 numbers may be written out in words in most contexts. This is
most useful for specifying day of the week items or relative items (see
below). Here is the list: first for 1, next for 2,
third for 3, fourth for 4, fifth for 5,
sixth for 6, seventh for 7, eighth for 8,
ninth for 9, tenth for 10, eleventh for 11 and
twelfth for 12. Also, last means exactly -1.
When a month is written this way, it is still considered to be written numerically, instead of being "spelled in full"; this changes the allowed strings.
Alphabetic case is completely ignored in dates. Comments may be introduced between round parentheses, as long as included parentheses are properly nested. Hyphens not followed by a digit are currently ignored. Leading zeros on numbers are ignored.
A calendar date item specifies a day of the year. It is specified differently, depending on whether the month is specified numerically or literally. All these strings specify the same calendar date:
1970-09-17 # ISO 8601. 70-9-17 # This century assumed by default. 70-09-17 # Leading zeros are ignored. 9/17/72 # Common U.S. writing. 24 September 1972 24 Sept 72 # September has a special abbreviation. 24 Sep 72 # Three-letter abbreviations always allowed. Sep 24, 1972 24-sep-72 24sep72
The year can also be omitted. In this case, the last specified year is used, or the current year if none. For example:
9/17 sep 17
Here are the rules.
For numeric months, the ISO 8601 format
year-month-day is allowed, where year is
any positive number, month is a number between 01 and 12, and
day is a number between 01 and 31. A leading zero must be present
if a number is less than ten. If year is less than 100, then 1900
is added to it to force a date in this century. The construct
month/day/year, popular in the United States,
is accepted. Also month/day, omitting the year.
Literal months may be spelled out in full: January,
February, March, April, May, June,
July, August, September, October,
November or December. Literal months may be abbreviated
to their first three letters, possibly followed by an abbreviating dot.
It is also permitted to write Sept instead of September.
When months are written literally, the calendar date may be given as any of the following:
day month year day month month day year day-month-year
Or, omitting the year:
month day
A time of day item in date strings specifies the time on a given day. Here are some examples, all of which represent the same time:
20:02:0 20:02 8:02pm 20:02-0500 # In EST (Eastern U.S. Standard Time).
More generally, the time of the day may be given as
hour:minute:second, where hour is
a number between 0 and 23, minute is a number between 0 and
59, and second is a number between 0 and 59. Alternatively,
:second can be omitted, in which case it is taken to
be zero.
If the time is followed by am or pm (or a.m.
or p.m.), hour is restricted to run from 1 to 12, and
:minute may be omitted (taken to be zero). am
indicates the first half of the day, pm indicates the second
half of the day. In this notation, 12 is the predecessor of 1:
midnight is 12am while noon is 12pm.
The time may alternatively be followed by a timezone correction,
expressed as shhmm, where s is +
or -, hh is a number of zone hours and mm is a number
of zone minutes. When a timezone correction is given this way, it
forces interpretation of the time in UTC, overriding any previous
specification for the timezone or the local timezone. The minute
part of the time of the day may not be elided when a timezone correction
is used. This is the only way to specify a timezone correction by
fractional parts of an hour.
Either am/pm or a timezone correction may be specified,
but not both.
A timezone item specifies an international timezone, indicated by a small set of letters. Any included period is ignored. Military timezone designations use a single letter. Currently, only integral zone hours may be represented in a timezone item. See the previous section for a finer control over the timezone correction.
Here are many non-daylight-savings-time timezones, indexed by the zone hour value.
+000
GMT for Greenwich Mean, UT or UTC for Universal
(Coordinated), WET for Western European and Z for
militaries.
+100
WAT for West Africa and
A for militaries.
+200
AT for Azores and B for militaries.
+300
C for militaries.
+400
AST for Atlantic Standard and D for militaries.
+500
E for militaries and EST for Eastern Standard.
+600
CST for Central Standard and F for militaries.
+700
G for militaries and MST for Mountain Standard.
+800
H for militaries and PST for Pacific Standard.
+900
I for militaries and YST for Yukon Standard.
+1000
AHST for Alaska-Hawaii Standard, CAT for Central Alaska,
HST for Hawaii Standard and K for militaries.
+1100
L for militaries and NT for Nome.
+1200
IDLW for International Date Line West and M for
militaries.
-100
CET for Central European, FWT for French Winter,
MET for Middle European, MEWT for Middle European
Winter, N for militaries and SWT for Swedish Winter.
-200
EET for Eastern European, USSR Zone 1 and O for militaries.
-300
BT for Baghdad, USSR Zone 2 and P for militaries.
-400
Q for militaries and ZP4 for USSR Zone 3.
-500
R for militaries and ZP5 for USSR Zone 4.
-600
S for militaries and ZP6 for USSR Zone 5.
-700
T for militaries and WAST for West Australian Standard.
-800
CCT for China Coast, USSR Zone 7 and U for militaries.
-900
JST for Japan Standard, USSR Zone 8 and V for militaries.
-1000
EAST for East Australian Standard, GST for Guam
Standard, USSR Zone 9 and W for militaries.
-1100
X for militaries.
-1200
IDLE for International Date Line East, NZST for
New Zealand Standard, NZT for New Zealand and Y for
militaries.
Here are many DST timezones, indexed by the zone hour value. Also, by
following a non-DST timezone by the string DST in a separate word
(that is, separated by some whitespace), the corresponding DST timezone
may be specified.
0
BST for British Summer.
+400
ADT for Atlantic Daylight.
+500
EDT for Eastern Daylight.
+600
CDT for Central Daylight.
+700
MDT for Mountain Daylight.
+800
PDT for Pacific Daylight.
+900
YDT for Yukon Daylight.
+1000
HDT for Hawaii Daylight.
-100
MEST for Middle European Summer, MESZ for Middle European
Summer, SST for Swedish Summer and FST for French Summer.
-700
WADT for West Australian Daylight.
-1000
EADT for Eastern Australian Daylight.
-1200
NZDT for New Zealand Daylight.
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.
Relative items adjust a date (or the current date if none) forward or backward. The effect of relative items accumulate. Here are some examples:
1 year 1 year ago 3 years 2 days
The unit of time displacement may be selected by the string year
or month for moving by whole years or months. These are fuzzy
units, as years and months are not all of equal duration. More precise
units are fortnight which is worth 14 days, week worth 7
days, day worth 24 hours, hour worth 60 minutes,
minute or min worth 60 seconds, and second or
sec worth one second. An s suffix on these units is
accepted and ignored.
The unit of time may be preceded by a multiplier, given as an optionally
signed number. Unsigned numbers are taken as positively signed. No
number at all implies 1 for a multiplier. Following a relative item by
the string ago is equivalent to preceding the unit by a
multiplicator with value -1.
The string tomorrow is worth one day in the future (equivalent
to day), the string yesterday is worth
one day in the past (equivalent to day ago).
The strings now or today are relative items corresponding
to zero-valued time displacement, these strings come from the fact
a zero-valued time displacement represents the current time when not
otherwise change by previous items. They may be used to stress other
items, like in 12:00 today. The string this also has
the meaning of a zero-valued time displacement, but is preferred in
date strings like this thursday.
When a relative item makes the resulting date to cross the boundary between DST and non-DST (or vice-versa), the hour is adjusted according to the local time.
The precise intepretation of a pure decimal number is dependent of the context in the date string.
If the decimal number is of the form yyyymmdd and no other calendar date item (see Calendar date item) appears before it in the date string, then yyyy is read as the year, mm as the month number and dd as the day of the month, for the specified calendar date.
If the decimal number is of the form hhmm and no other time of day item appears before it in the date string, then hh is read as the hour of the day and mm as the minute of the hour, for the specified time of the day. mm can also be omitted.
If both a calendar date and a time of day appear to the left of a number in the date string, but no relative item, then the number overrides the year.
getdategetdate was originally implemented by Steven M. Bellovin
(smb@research.att.com) while at the University of North Carolina
at Chapel Hill. The code was later tweaked by a couple of people on
Usenet, then completely overhauled by Rich $alz (rsalz@bbn.com)
and Jim Berets (jberets@bbn.com) in August, 1990. Various
revisions for the GNU system were made by David MacKenzie, Jim Meyering,
and others.
François Pinard (pinard@iro.umontreal.ca) wrote the
above description out of the getdate.y source code, K. Berry
(kb@cs.umb.edu) later edited it.
tar Archives More Portable
tar and cpio
tar Archives More PortableCreating a tar archive on a particular system that is meant to be
useful later on many other machines and with other versions of tar
is more challenging than you might think. tar archive formats
have been evolving since the first versions of Unix. Many such formats
are around, and are not always comptible with each other. This section
discusses a few problems, and gives some advice about making tar
archives more portable.
One golden rule is simplicity. For example, limit your tar
archives to contain only regular files and directories, avoiding
other kind of special files. Do not attempt to save sparse files or
contiguous files as such. Let's discuss a few more problems, in turn.
Use straight file and directory names, made up of printable ASCII characters, avoiding colons, slashes, backslashes, spaces, and other dangerous characters. Avoid deep directory nesting. Accounting for oldish System V machines, limit your file and directory names to 14 characters or less.
If you intend to have your tar archives to be read under MS-DOS,
you should not rely on case distinction for file names, and you might
use the GNU doschk program for helping you further diagnosing
illegal MS-DOS names, which are even more limited than System V's.
tar compiled for MS-DOS does try to cope with filenames
which are illegal on MS-DOS file systems (see Basic tar), but the
best portability strategy is to avoid such problems in the first place.
Even newer versions of MS-Windows, such as Windows 9X and Windows/NT,
still have some restrictions on characters which can appear in a file
name. Typically, characters which are special to the shell, like
?, * and | are not allowed and should be avoided.
Normally, when tar archives a symbolic link, it writes a
block to the archive naming the target of the link. In that way, the
tar archive is a faithful record of the filesystem contents.
--dereference (-h) is used with --create (-c), and causes tar
to archive the files symbolic links point to, instead of the links
themselves. When this option is used, when tar encounters a
symbolic link, it will archive the linked-to file, instead of simply
recording the presence of a symbolic link.
The name under which the file is stored in the file system is not
recorded in the archive. To record both the symbolic link name and
the file name in the system, archive the file under both names. If
all links were recorded automatically by tar, an extracted file
might be linked to a file name that no longer exists in the file
system.
If a linked-to file is encountered again by tar while creating
the same archive, an entire second copy of it will be stored. (This
might be considered a bug.)
So, for portable archives, do not archive symbolic links as such, and use --dereference (-h): many systems do not support symbolic links, and moreover, your distribution might be unusable if it contains unresolved symbolic links.
Certain old versions of tar cannot handle additional
information recorded by newer tar programs. To create an
archive in V7 format (not ANSI), which can be read by these old
versions, specify the --old-archive (-o) option in
conjunction with the --create (-c). tar also
accepts --portability for this option. When you specify it,
tar leaves out information about directories, pipes, fifos,
contiguous files, and device files, and specifies file ownership by
group and user IDs instead of group and user names.
When updating an archive, do not use --old-archive (-o) unless the archive was created using this option.
In most cases, a new format archive can be read by an old
tar program without serious trouble, so this option should
seldom be needed. On the other hand, most modern tars are
able to read old format archives, so it might be safer for you to
always use --old-archive (-o) for your distributions.
tar and POSIX tarGNU tar was based on an early draft of the POSIX 1003.1
ustar standard. GNU extensions to tar, such as the
support for file names longer than 100 characters, use portions of the
tar header record which were specified in that POSIX draft as
unused. Subsequent changes in POSIX have allocated the same parts of
the header record for other purposes. As a result, GNU tar is
incompatible with the current POSIX spec, and with tar programs
that follow it.
We plan to reimplement these GNU extensions in a new way which is
upward compatible with the latest POSIX tar format, but we
don't know when this will be done.
In the mean time, there is simply no way of telling what might happen if you
read a GNU tar archive, which uses the GNU extensions, using
some other tar program. So if you want to read the archive
with another tar program, be sure to write it using the
--old-archive option (-o).
Traditionally, old tars have a limit of 100 characters. GNU
tar attempted two different approaches to overcome this limit,
using and extending a format specified by a draft of some P1003.1.
The first way was not that successful, and involved @MaNgLeD@
file names, or such; while a second approach used ././@LongLink
and other tricks, yielding better success. In theory, GNU tar
should be able to handle file names of practically unlimited length.
So, if GNU tar fails to dump and retrieve files having more
than 100 characters, then there is a bug in GNU tar, indeed.
But, being strictly POSIX, the limit was still 100 characters.
For various other purposes, GNU tar used areas left unassigned
in the POSIX draft. POSIX later revised P1003.1 ustar format by
assigning previously unused header fields, in such a way that the upper
limit for file name length was raised to 256 characters. However, the
actual POSIX limit oscillates between 100 and 256, depending on the
precise location of slashes in full file name (this is rather ugly).
Since GNU tar use the same fields for quite other purposes,
it became incompatible with the latest POSIX standards.
For longer or non-fitting file names, we plan to use yet another set
of GNU extensions, but this time, complying with the provisions POSIX
offers for extending the format, rather than conflicting with it.
Whenever an archive uses old GNU tar extension format or POSIX
extensions, would it be for very long file names or other specialities,
this archive becomes non-portable to other tar implementations.
In fact, anything can happen. The most forgiving tars will
merely unpack the file using a wrong name, and maybe create another
file named something like @LongName, with the true file name
in it. tars not protecting themselves may segment violate!
Compatibility concerns make all this thing more difficult, as we
will have to support all these things together, for a while.
GNU tar should be able to produce and read true POSIX format
files, while being able to detect old GNU tar formats, besides
old V7 format, and process them conveniently. It would take years
before this whole area stabilizes...
There are plans to raise this 100 limit to 256, and yet produce POSIX
conformant archives. Past 256, I do not know yet if GNU tar
will go non-POSIX again, or merely refuse to archive the file.
There are plans so GNU tar support more fully the latest POSIX
format, while being able to read old V7 format, GNU (semi-POSIX plus
extension), as well as full POSIX. One may ask if there is part of
the POSIX format that we still cannot support. This simple question
has a complex answer. Maybe that, on intimate look, some strong
limitations will pop up, but until now, nothing sounds too difficult
(but see below). I only have these few pages of POSIX telling about
`Extended tar Format' (P1003.1-1990 - section 10.1.1), and there are
references to other parts of the standard I do not have, which should
normally enforce limitations on stored file names (I suspect things
like fixing what / and <NUL> means). There are also
some points which the standard does not make clear. Existing practice
will then drive what I should do.
POSIX mandates that, when a file name cannot fit within 100 to
256 characters (the variance comes from the fact a / is
ideally needed as the 156'th character), or a link name cannot
fit within 100 characters, a warning should be issued and the file
not be stored. Unless some --posix option is given
(or POSIXLY_CORRECT is set), I suspect that GNU tar
should disobey this specification, and automatically switch to using
GNU extensions to overcome file name or link name length limitations.
There is a problem, however, which I have not intimately studied yet.
Given a truly POSIX archive with names having more than 100 characters,
I guess that GNU tar up to 1.11.8 will process it as if it were an
old V7 archive, and be fooled by some fields which are coded differently.
So, the question is to decide if the next generation of GNU tar
should produce POSIX format by default, whenever possible, producing
archives older versions of GNU tar might not be able to read
correctly. I fear that we will have to suffer such a choice one of these
days, if we want GNU tar to go closer to POSIX. We can rush it.
Another possibility is to produce the current GNU tar format
by default for a few years, but have GNU tar versions from some
1.POSIX and up able to recognize all three formats, and let older
GNU tar fade out slowly. Then, we could switch to producing POSIX
format by default, with not much harm to those still having (very old at
that time) GNU tar versions prior to 1.POSIX.
POSIX format cannot represent very long names, volume headers,
splitting of files in multi-volumes, sparse files, and incremental
dumps; these would be all disallowed if --posix or
POSIXLY_CORRECT. Otherwise, if tar is given long
names, or -[VMSgG], then it should automatically go non-POSIX.
I think this is easily granted without much discussion.
Another point is that only mtime is stored in POSIX
archives, while GNU tar currently also store atime
and ctime. If we want GNU tar to go closer to POSIX,
my choice would be to drop atime and ctime support on
average. On the other hand, I perceive that full dumps or incremental
dumps need atime and ctime support, so for those special
applications, POSIX has to be avoided altogether.
A few users requested that --sparse (-S) be always active by
default, I think that before replying to them, we have to decide
if we want GNU tar to go closer to POSIX on average, while
producing files. My choice would be to go closer to POSIX in the
long run. Besides possible double reading, I do not see any point
of not trying to save files as sparse when creating archives which
are neither POSIX nor old-V7, so the actual --sparse (-S) would
become selected by default when producing such archives, whatever
the reason is. So, --sparse (-S) alone might be redefined to force
GNU-format archives, and recover its previous meaning from this fact.
GNU-format as it exists now can easily fool other POSIX tar,
as it uses fields which POSIX considers to be part of the file name
prefix. I wonder if it would not be a good idea, in the long run,
to try changing GNU-format so any added field (like ctime,
atime, file offset in subsequent volumes, or sparse file
descriptions) be wholly and always pushed into an extension block,
instead of using space in the POSIX header block. I could manage
to do that portably between future GNU tars. So other POSIX
tars might be at least able to provide kind of correct listings
for the archives produced by GNU tar, if not able to process
them otherwise.
Using these projected extensions might induce older tars to fail.
We would use the same approach as for POSIX. I'll put out a tar
capable of reading POSIXier, yet extended archives, but will not produce
this format by default, in GNU mode. In a few years, when newer GNU
tars will have flooded out tar 1.11.X and previous, we
could switch to producing POSIXier extended archives, with no real harm
to users, as almost all existing GNU tars will be ready to read
POSIXier format. In fact, I'll do both changes at the same time, in a
few years, and just prepare tar for both changes, without effecting
them, from 1.POSIX. (Both changes: 1--using POSIX convention for
getting over 100 characters; 2--avoiding mangling POSIX headers for GNU
extensions, using only POSIX mandated extension techniques).
So, a future tar will have a --posix
flag forcing the usage of truly POSIX headers, and so, producing
archives previous GNU tar will not be able to read.
So, once pretest will announce that feature, it would be
particularly useful that users test how exchangeable will be archives
between GNU tar with --posix and other POSIX tar.
In a few years, when GNU tar will produce POSIX headers by
default, --posix will have a strong meaning and will disallow
GNU extensions. But in the meantime, for a long while, --posix
in GNU tar will not disallow GNU extensions like --label=archive-label (-V archive-label),
--multi-volume (-M), --sparse (-S), or very long file or link names.
However, --posix with GNU extensions will use POSIX
headers with reserved-for-users extensions to headers, and I will be
curious to know how well or bad POSIX tars will react to these.
GNU tar prior to 1.POSIX, and after 1.POSIX without
--posix, generates and checks ustar , with two
suffixed spaces. This is sufficient for older GNU tar not to
recognize POSIX archives, and consequently, wrongly decide those archives
are in old V7 format. It is a useful bug for me, because GNU tar
has other POSIX incompatibilities, and I need to segregate GNU tar
semi-POSIX archives from truly POSIX archives, for GNU tar should
be somewhat compatible with itself, while migrating closer to latest
POSIX standards. So, I'll be very careful about how and when I will do
the correction.
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 when creating
archives, as per POSIX standards. On reading, GNU tar computes
both checksums and accept any. It is somewhat worrying that a lot of
people may go around doing backup of their files using faulty (or at
least non-standard) software, not learning about it until it's time
to restore their missing files with an incompatible file extractor,
or vice versa.
GNU tar 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
tars. 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 chars 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.
(This message will disappear, once this node revised.)
You can have archives be compressed by using the --gzip (--gunzip, --ungzip, -z) option.
This will arrange for tar to use the gzip program to be
used to compress or uncompress the archive wren writing or reading it.
To use the older, obsolete, compress program, use the
--compress (--uncompress, -Z) option. The GNU Project recommends you not use
compress, because there is a patent covering the algorithm it
uses. You could be sued for patent infringment merely by running
compress.
gzip.
To perform compression and uncompression on the archive, tar runs
the gzip utility. tar uses the default compression
parameters; if you need to override them, avoid the --gzip (--gunzip, --ungzip, -z)
option and run the gzip utility explicitly. (Or set the
GZIP environment variable.) It is useful to be able to call the
gzip utility from within tar because the gzip
utility by itself cannot access remote tape drives.
The --gzip (--gunzip, --ungzip, -z) option will not work in conjunction with the --multi-volume (-M) option or the --append (-r), --update (-u), --concatenate (--catenate, -A) and --delete subcommands. See Subcommands, for more information on these subcommands.
You can use --gzip and --gunzip on physical devices
(tape drives, etc.) and remote files as well as on normal files; data
to or from such devices or remote files is reblocked by another copy
of the tar program to enforce the specified (or default) record
size.
compress. Otherwise like --gzip (--gunzip, --ungzip, -z).
-d).
It is not exact to say that GNU tar is to work in concert
with gzip in a way similar to zip, say. Surely, it is
possible that tar and gzip be done with a single call,
like in:
$ tar cfz archive.tar.gz subdir
to save all of subdir into a gzip'ed archive. Later you
can do:
$ tar xfz archive.tar.gz
to explode and unpack.
The difference is that the whole archive is compressed. With
zip, archive members are archived individually. tar's
method yields better compression. On the other hand, one can view the
contents of a zip archive without having to decompress it. As
for the tar and gzip tandem, you need to decompress the
archive to see its contents. However, this may be done without needing
disk space, by using pipes internally:
$ tar tfz archive.tar.gz
(tar on MS-DOS and MS-Windows also supports this on-the-fly
compression with gzip, but since pipes are simulated with disk
files on MS-DOS, you do need disk space to store the uncompressed
copy, while tar runs. In particular, make sure the disk with the
directory which is the value of the environment variable TMPDIR
has enough free space, as many DOS users tend to point it to a RAM
disk.)
If there is no compress utility available, tar will report an error.
Please note that the compress program may be covered by
a patent, and therefore we recommend you stop using it.
tar will compress (when writing
an archive), or uncompress (when reading an archive). Used in
conjunction with the --create (-c), --extract (--get, -x), --list (-t) and
--compare (--diff, -d) subcommands.
You can have archives be compressed by using the --gzip (--gunzip, --ungzip, -z) option.
This will arrange for tar to use the gzip program to be
used to compress or uncompress the archive wren writing or reading it.
To use the older, obsolete, compress program, use the
--compress (--uncompress, -Z) option. The GNU Project recommends you not use
compress, because there is a patent covering the algorithm it
uses. You could be sued for patent infringment merely by running
compress.
About corrupted compressed archives: gzip'ed files have no
redundancy, for maximum compression. The adaptive nature of the
compression scheme means that the compression tables are implicitly
spread all over the archive. If you lose a few blocks, the dynamic
construction of the compression tables becomes unsychronized, and there
is little chance that you could recover later in the archive.
There are pending suggestions for having a per-volume or per-file
compression in GNU tar. This would allow for viewing the
contents without decompression, and for resynchronizing decompression at
every volume or file, in case of corrupted archives. Doing so, we might
loose some compressibility. But this would have make recovering easier.
So, there are pros and cons. We'll see!
-d).
tar to compress when writing the archive, or to uncompress when
reading the archive.
To perform compression and uncompression on the archive, tar
runs the compress utility. tar uses the default
compression parameters; if you need to override them, avoid the
--compress (--uncompress, -Z) option and run the compress utility
explicitly. It is useful to be able to call the compress
utility from within tar because the compress utility by
itself cannot access remote tape drives.
The --compress (--uncompress, -Z) option will not work in conjunction with the --multi-volume (-M) option or the --append (-r), --update (-u), --append (-r) and --delete subcommands. , for more information on these subcommands.
If there is no compress utility available, tar will report an error.
Please note that the compress program may be covered by
a patent, and therefore we recommend you stop using it.
tar will compress (when writing
an archive), or uncompress (when reading an archive). Used in
conjunction with the --create (-c), --extract (--get, -x), --list (-t) and
--compare (--diff, -d) subcommands.
You can have archives be compressed by using the --gzip (--gunzip, --ungzip, -z) option.
This will arrange for tar to use the gzip program to be
used to compress or uncompress the archive wren writing or reading it.
To use the older, obsolete, compress program, use the
--compress (--uncompress, -Z) option. The GNU Project recommends you not use
compress, because there is a patent covering the algorithm it
uses. You could be sued for patent infringment merely by running
compress.
I have one question, or maybe it's a suggestion if there isn't a way
to do it now. I would like to use --gzip (--gunzip, --ungzip, -z), but I'd also like the
output to be fed through a program like GNU ecc (actually, right
now that's exactly what I'd like to use :-)), basically adding
ECC protection on top of compression. It seems as if this should be
quite easy to do, but I can't work out exactly how to go about it.
Of course, I can pipe the standard output of tar through
ecc, but then I lose (though I haven't started using it yet,
I confess) the ability to have tar use rmt for it's I/O
(I think).
I think the most straightforward thing would be to let me specify a general set of filters outboard of compression (preferably ordered, so the order can be automatically reversed on input subcommands, and with the options they require specifiable), but beggars shouldn't be choosers and anything you decide on would be fine with me.
By the way, I like ecc but if (as the comments say) it can't
deal with loss of block sync, I'm tempted to throw some time at adding
that capability. Supposing I were to actually do such a thing and
get it (apparantly) working, do you accept contributed changes to
utilities like that? (Leigh Clayton loc@soliton.com, May 1995).
Isn't that exactly the role of the --use-compress-prog=program option?
I never tried it myself, but I suspect you may want to write a
prog script or program able to filter stdin to stdout to
way you want. It should recognize the -d option, for when
extraction is needed rather than creation.
It has been reported that if one writes compressed data (through the --gzip (--gunzip, --ungzip, -z) or --compress (--uncompress, -Z) options) to a DLT and tries to use the DLT compression mode, the data will actually get bigger and one will end up with less space on the tape.
(This message will disappear, once this node revised.)
This option causes all files to be put in the archive to be tested for
sparseness, and handled specially if they are. The --sparse (-S)
option is useful when many dbm files, for example, are being
backed up. Using this option dramatically decreases the amount of
space needed to store such a file.
However, users should be well aware that at archive creation time, GNU
tar still has to read whole disk file to locate the holes,
and so, even if sparse files use little space on disk and in the archive,
they may sometimes require inordinate amount of time for analysis.
This reading is required in all cases and is not related to the fact
the --sparse (-S) option is used or not, so by merely not
using the option, you are not saving time3.
In later versions, this option may be removed, and the testing and treatment of sparse files may be done automatically with any special GNU options. For now, it is an option needing to be specified on the command line with the creation or updating of an archive.
Files in the filesystem occasionally have "holes." A hole in a file
is a section of the file's contents which was never written. The
contents of a hole read as all zeroes. On many operating systems,
actual disk storage is not allocated for holes, but they are counted
in the length of the file. If you archive such a file, tar
could create an archive longer than the original. To have tar
attempt to recognize the holes in a file, use --sparse (-S). When
you use the --sparse (-S) option, then, for any file using less
disk space than would be expected from its length, tar searches
the file for consecutive stretches of zeroes. It then records in the
archive for the file where the consecutive stretches of zeroes are, and
only archives the "real contents" of the file. On extraction (using
--sparse (-S) is not needed on extraction) any such files have
holes created wherever the continuous stretches of zeroes were found.
Thus, if you use --sparse (-S), tar archives won't take
more space than the original.
A file is sparse if it contains blocks of zeroes whose existence is
recorded, but that have no space allocated on disk. When you specify
the --sparse (-S) option in conjunction with the --create (-c)
subcommand, tar tests all files for sparseness while archiving.
If tar finds a file to be sparse, it uses a sparse representation of
the file in the archive. See create, for more information
about creating archives.
--sparse (-S) is useful when archiving files, such as dbm files,
that are likely to contain many nulls. This option dramatically
decreases the amount of space needed to store such an archive.
Please Note: Always use --sparse (-S) when performing file system backups, to avoid archiving the expanded forms of files stored sparsely in the system.Even if your system has no sparse files currently, some may be created in the future. If you use --sparse (-S) while making file system backups as a matter of course, you can be assured the archive will never take more space on the media than the files take on disk (otherwise, archiving a disk filled with sparse files might take hundreds of tapes).
tar ignores the --sparse (-S) option when reading an archive.
However, users should be well aware that at archive creation time, GNU
tar still has to read the whole disk file to locate the holes, and
so, even if sparse files use little space on disk and in the archive, they
may sometimes require inordinate amount of time for reading and examining
all-zero blocks of a file. Although it works, it's painfully slow for a
large (sparse) file, even though the resulting tar archive may be small.
(One user reports that dumping a core file of over 400 megabytes,
but with only about 3 megabytes of actual data, took about 9 minutes on
a Sun Sparstation ELC, with full CPU utilisation.)
This reading is required in all cases and is not related to the fact the --sparse (-S) option is used or not, so by merely not using the option, you are not saving time4.
Programs like dump do not have to read the entire file; by examining
the file system directly, they can determine in advance exactly where the
holes are and thus avoid reading through them. The only data it need read
are the actual allocated data blocks. GNU tar uses a more portable
and straightforward archiving approach, it would be fairly difficult that
it does otherwise. Elizabeth Zwicky writes to comp.unix.internals,
on 1990-12-10:
What I did say is that you cannot tell the difference between a hole and an equivalent number of nulls without reading raw blocks.st_blocksat best tells you how many holes there are; it doesn't tell you where. Just as programs may, conceivably, care whatst_blocksis (care to name one that does?), they may also care where the holes are (I have no examples of this one either, but it's equally imaginable).I conclude from this that good archivers are not portable. One can arguably conclude that if you want a portable program, you can in good conscience restore files with as many holes as possible, since you can't get it right.
(This message will disappear, once this node revised.)
When tar reads files, this causes them to have the access times
updated. To have tar attempt to set the access times back to
what they were before they were read, use the --atime-preserve
option. This doesn't work for files that you don't own, unless
you're root, and it doesn't interact with incremental dumps nicely
(see Backups), but it is good enough for some purposes.
Handling of file attributes
atime is not supported by the DOS FAT
file system. (It does work on MS-Windows 9X.)
When this option is used, tar leaves the modification times
of the files it extracts as the time when the files were extracted,
instead of setting it to the time recorded in the archive.
This option is meaningless with --list (-t).
When using super-user at extraction time, ownership is always restored.
So, this option is meaningful only for non-root users, when tar
is executed on those systems able to give files away. This is
considered as a security flaw by many people, at least because it
makes quite difficult to correctly account users for the disk space
they occupy. Also, the suid or sgid attributes of
files are easily and silently lost when files are given away.
When writing an archive, tar writes the user id and user name
separately. If it can't find a user name (because the user id is not
in /etc/passwd), then it does not write one. When restoring,
and doing a chmod like when you use --same-permissions (--preserve-permissions, -p),
() it tries to look the name (if one was written)
up in /etc/passwd. If it fails, then it uses the user id
stored in the archive instead.
This is useful in certain circumstances, when restoring a backup from an emergency floppy with different passwd/group files for example. It is otherwise impossible to extract files with the right ownerships if the password file in use during the extraction does not match the one belonging to the filesystem(s) being extracted. This occurs, for example, if you are restoring your files after a major crash and had booted from an emergency floppy with no password file or put your disk into another machine to do the restore.
The numeric ids are always saved into tar archives.
The identifying names are added at create time when provided by the
system, unless --old-archive (-o) is used. Numeric ids could be
used when moving archives between a collection of machines using
a centralized management for attribution of numeric ids to users
and groups. This is often made through using the NIS capabilities.
When making a tar file for distribution to other sites, it
is sometimes cleaner to use a single owner for all files in the
distribution, and nicer to specify the write permission bits of the
files as stored in the archive independently of their actual value on
the file system. The way to prepare a clean distribution is usually
to have some Makefile rule creating a directory, copying all needed
files in that directory, then setting ownership and permissions as
wanted (there are a lot of possible schemes), and only then making a
tar archive out of this directory, before cleaning everything
out. Of course, we could add a lot of options to GNU tar for
fine tuning permissions and ownership. This is not the good way,
I think. GNU tar is already crowded with options and moreover,
the approach just explained gives you a great deal of control already.
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. tar on MS-DOS/MS-Windows allows
group to be any string. These systems don't support group
id's, so tar allows them to give away files to anybody. If
group consists only of digits, it is treated as a numeric GID;
otherwise it is treated as a group name.
tar should use user as the owner of members
when creating archives, instead of the user associated with the source
file. user is first decoded as a user symbolic name, but if
this interpretation fails, it has to be a decimal numeric user ID.
There is no value indicating a missing number, and 0 usually means
root. Some people like to force 0 as the value to offer in
their distributions for the owner of files, because the root user is
anonymous anyway, so that might as well be the owner of anonymous archives.
tar on MS-DOS/MS-Windows allows user to be any
string. These systems don't support file ownership, so tar allows
them to give away files to anybody. If user includes only digits,
it is treated as a numeric UID; otherwise, it is treated as a user name.
This option causes tar to set the modes (access permissions) of
extracted files exactly as recorded in the archive. If this option
is not used, the current umask setting limits the permissions
on extracted files.
This option is meaningless with --list (-t).
The --preserve option has no equivalent short option name. It is equivalent to --same-permissions (--preserve-permissions, -p) plus --same-order (--preserve-order, -s).
(This message will disappear, once this node revised.)
While an archive may contain many files, the archive itself is a
single ordinary file. Like any other file, an archive file can be
written to a storage device such as a tape or disk, sent through a
pipe or over a network, saved on the active file system, or even
stored in another archive. An archive file is not easy to read or
manipulate without using the tar utility or Tar mode in GNU
Emacs.
Physically, an archive consists of a series of file entries terminated
by an end-of-archive entry, which consists of 512 zero bytes. A file
entry usually describes one of the files in the archive (an
archive member), and consists of a file header and the contents
of the file. File headers contain file names and statistics, checksum
information which tar uses to detect file corruption, and
information about file types.
Archives are permitted to have more than one member with the same member name. One way this situation can occur is if more than one version of a file has been stored in the archive. For information about adding new versions of a file to an archive, see update, and to learn more about having more than one archive member with the same name, see .
In addition to entries describing archive members, an archive may
contain entries which tar itself uses to store information.
See label, for an example of such an archive entry.
A tar archive file contains a series of blocks. Each block
contains BLOCKSIZE bytes. Although this format may be thought
of as being on magnetic tape, other media are often used.
Each file archived is represented by a header block which describes the file, followed by zero or more blocks which give the contents of the file. At the end of the archive file there may be a block filled with binary zeros as an end-of-file marker. A reasonable system should write a block of zeros at the end, but must not assume that such a block exists when reading an archive.
The blocks may be blocked for physical I/O subcommands.
Each record of n blocks (where n is set by the
--blocking-factor=blocks (-b blocks) option to tar) is written with a single
write () subcommand. On magnetic tapes, the result of
such a write is a single record. When writing an archive,
the last record of blocks should be written at the full size, with
blocks after the zero block containing all zeros. When reading
an archive, a reasonable system should properly handle an archive
whose last record is shorter than the rest, or which contains garbage
records after a zero block.
The header block is defined in C as follows. In the GNU tar
distribution, this is part of file src/tar.h:
/* GNU tar Archive Format description. */
/* If OLDGNU_COMPATIBILITY is set, tar produces archives which, by default,
are readable by older versions of GNU tar. This can be overriden by
using --posix; in this case, POSIXLY_CORRECT in environment may be set
for enforcing stricter conformance. If OLDGNU_COMPATIBILITY is zero or
undefined, tar will eventually produces archives which, by default, POSIX
compatible; then either using --posix or defining POSIXLY_CORRECT
enforces stricter conformance.
This #define will disappear in a few years. FP, June 1995. */
#define OLDGNU_COMPATIBILITY 1
/*---------------------------------------------.
| `tar' Header Block, from POSIX 1003.1-1990. |
`---------------------------------------------*/
/* POSIX header. */
struct posix_header
{ /* byte offset */
char name[100]; /* 0 */
char mode[8]; /* 100 */
char uid[8]; /* 108 */
char gid[8]; /* 116 */
char size[12]; /* 124 */
char mtime[12]; /* 136 */
char chksum[8]; /* 148 */
char typeflag; /* 156 */
char linkname[100]; /* 157 */
char magic[6]; /* 257 */
char version[2]; /* 263 */
char uname[32]; /* 265 */
char gname[32]; /* 297 */
char devmajor[8]; /* 329 */
char devminor[8]; /* 337 */
char prefix[155]; /* 345 */
/* 500 */
};
#define TMAGIC "ustar" /* ustar and a null */
#define TMAGLEN 6
#define TVERSION "00" /* 00 and no null */
#define TVERSLEN 2
/* Values used in typeflag field. */
#define REGTYPE '0' /* regular file */
#define AREGTYPE '\0' /* regular file */
#define LNKTYPE '1' /* link */
#define SYMTYPE '2' /* reserved */
#define CHRTYPE '3' /* character special */
#define BLKTYPE '4' /* block special */
#define DIRTYPE '5' /* directory */
#define FIFOTYPE '6' /* FIFO special */
#define CONTTYPE '7' /* reserved */
/* Bits used in the mode field, values in octal. */
#define TSUID 04000 /* set UID on execution */
#define TSGID 02000 /* set GID on execution */
#define TSVTX 01000 /* reserved */
/* file permissions */
#define TUREAD 00400 /* read by owner */
#define TUWRITE 00200 /* write by owner */
#define TUEXEC 00100 /* execute/search by owner */
#define TGREAD 00040 /* read by group */
#define TGWRITE 00020 /* write by group */
#define TGEXEC 00010 /* execute/search by group */
#define TOREAD 00004 /* read by other */
#define TOWRITE 00002 /* write by other */
#define TOEXEC 00001 /* execute/search by other */
/*-------------------------------------.
| `tar' Header Block, GNU extensions. |
`-------------------------------------*/
/* In GNU tar, SYMTYPE is for to symbolic links, and CONTTYPE is for
contiguous files, so maybe disobeying the `reserved' comment in POSIX
header description. I suspect these were meant to be used this way, and
should not have really been `reserved' in the published standards. */
/* *BEWARE* *BEWARE* *BEWARE* that the following information is still
boiling, and may change. Even if the OLDGNU format description should be
accurate, the so-called GNU format is not yet fully decided. It is
surely meant to use only extensions allowed by POSIX, but the sketch
below repeats some ugliness from the OLDGNU format, which should rather
go away. Sparse files should be saved in such a way that they do *not*
require two passes at archive creation time. Huge files get some POSIX
fields to overflow, alternate solutions have to be sought for this. */
/* Descriptor for a single file hole. */
struct sparse
{ /* byte offset */
char offset[12]; /* 0 */
char numbytes[12]; /* 12 */
/* 24 */
};
/* Sparse files are not supported in POSIX ustar format. For sparse files
with a POSIX header, a GNU extra header is provided which holds overall
sparse information and a few sparse descriptors. When an old GNU header
replaces both the POSIX header and the GNU extra header, it holds some
sparse descriptors too. Whether POSIX or not, if more sparse descriptors
are still needed, they are put into as many successive sparse headers as
necessary. The following constants tell how many sparse descriptors fit
in each kind of header able to hold them. */
#define SPARSES_IN_EXTRA_HEADER 16
#define SPARSES_IN_OLDGNU_HEADER 4
#define SPARSES_IN_SPARSE_HEADER 21
/* The GNU extra header contains some information GNU tar needs, but not
foreseen in POSIX header format. It is only used after a POSIX header
(and never with old GNU headers), and immediately follows this POSIX
header, when typeflag is a letter rather than a digit, so signaling a GNU
extension. */
struct extra_header
{ /* byte offset */
char atime[12]; /* 0 */
char ctime[12]; /* 12 */
char offset[12]; /* 24 */
char realsize[12]; /* 36 */
char longnames[4]; /* 48 */
char unused_pad1[68]; /* 52 */
struct sparse sp[SPARSES_IN_EXTRA_HEADER];
/* 120 */
char isextended; /* 504 */
/* 505 */
};
/* Extension header for sparse files, used immediately after the GNU extra
header, and used only if all sparse information cannot fit into that
extra header. There might even be many such extension headers, one after
the other, until all sparse information has been recorded. */
struct sparse_header
{ /* byte offset */
struct sparse sp[SPARSES_IN_SPARSE_HEADER];
/* 0 */
char isextended; /* 504 */
/* 505 */
};
/* The old GNU format header conflicts with POSIX format in such a way that
POSIX archives may fool old GNU tar's, and POSIX tar's might well be
fooled by old GNU tar archives. An old GNU format header uses the space
used by the prefix field in a POSIX header, and cumulates information
normally found in a GNU extra header. With an old GNU tar header, we
never see any POSIX header nor GNU extra header. Supplementary sparse
headers are allowed, however. */
struct oldgnu_header
{ /* byte offset */
char unused_pad1[345]; /* 0 */
char atime[12]; /* 345 */
char ctime[12]; /* 357 */
char offset[12]; /* 369 */
char longnames[4]; /* 381 */
char unused_pad2; /* 385 */
struct sparse sp[SPARSES_IN_OLDGNU_HEADER];
/* 386 */
char isextended; /* 482 */
char realsize[12]; /* 483 */
/* 495 */
};
/* OLDGNU_MAGIC uses both magic and version fields, which are contiguous.
Found in an archive, it indicates an old GNU header format, which will be
hopefully become obsolescent. With OLDGNU_MAGIC, uname and gname are
valid, though the header is not truly POSIX conforming. */
#define OLDGNU_MAGIC "ustar " /* 7 chars and a null */
/* The standards committee allows only capital A through capital Z for
user-defined expansion. */
/* This is a dir entry that contains the names of files that were in the
dir at the time the dump was made. */
#define GNUTYPE_DUMPDIR 'D'
/* Identifies the *next* file on the tape as having a long linkname. */
#define GNUTYPE_LONGLINK 'K'
/* Identifies the *next* file on the tape as having a long name. */
#define GNUTYPE_LONGNAME 'L'
/* This is the continuation of a file that began on another volume. */
#define GNUTYPE_MULTIVOL 'M'
/* For storing filenames that do not fit into the main header. */
#define GNUTYPE_NAMES 'N'
/* This is for sparse files. */
#define GNUTYPE_SPARSE 'S'
/* This file is a tape/volume header. Ignore it on extraction. */
#define GNUTYPE_VOLHDR 'V'
/*--------------------------------------.
| tar Header Block, overall structure. |
`--------------------------------------*/
/* tar files are made in basic blocks of this size. */
#define BLOCKSIZE ((size_t) 512)
enum archive_format
{
DEFAULT_FORMAT, /* format to be decided later */
V7_FORMAT, /* old V7 tar format */
OLDGNU_FORMAT, /* GNU format as per before tar 2.0 */
POSIX_FORMAT, /* restricted, pure POSIX format */
GNU_FORMAT /* POSIX format with GNU extensions */
};
union block
{
char buffer[BLOCKSIZE];
struct posix_header header;
struct extra_header extra_header;
struct oldgnu_header oldgnu_header;
struct sparse_header sparse_header;
};
/* End of Format description. */
All characters in header blocks are represented by using 8-bit characters in the local variant of ASCII. Each field within the structure is contiguous; that is, there is no padding used within the structure. Each character on the archive medium is stored contiguously.
Bytes representing the contents of files (after the header block
of each file) are not translated in any way and are not constrained
to represent characters in any character set. The tar format
does not distinguish text files from binary files, and no translation
of file contents is performed.
The name, linkname, magic, uname, and
gname are null-terminated character strings. All other fileds
are zero-filled octal numbers in ASCII. Each numeric field of width
w contains w minus 2 digits, a space, and a null, except
size, and mtime, which do not contain the trailing null.
The name field is the file name of the file, with directory names
(if any) preceding the file name, separated by slashes.
The mode field provides nine bits specifying file permissions
and three bits to specify the Set UID, Set GID, and Save Text
(sticky) modes. Values for these bits are defined above.
When special permissions are required to create a file with a given
mode, and the user restoring files from the archive does not hold such
permissions, the mode bit(s) specifying those special permissions
are ignored. Modes which are not supported by the operating system
restoring files from the archive will be ignored. Unsupported modes
should be faked up when creating or updating an archive; e.g. the
group permission could be copied from the other permission.
The uid and gid fields are the numeric user and group
ID of the file owners, respectively. If the operating system does
not support numeric user or group IDs, these fields should be ignored.
The size field is the size of the file in bytes; linked files
are archived with this field specified as zero. See Backups, in
particular the --incremental (-G) option.
The mtime field is the modification time of the file at the time
it was archived. It is the ASCII representation of the octal value of
the last time the file was modified, represented as an integer number of
seconds since January 1, 1970, 00:00 Coordinated Universal Time.
The chksum field is the ASCII representation of the octal value
of the simple sum of all bytes in the header block. Each 8-bit
byte in the header is added to an unsigned integer, initialized to
zero, the precision of which shall be no less than seventeen bits.
When calculating the checksum, the chksum field is treated as
if it were all blanks.
The typeflag field specifies the type of file archived. If a
particular implementation does not recognize or permit the specified
type, the file will be extracted as if it were a regular file. As this
action occurs, tar issues a warning to the standard error.
The atime and ctime fields are used in making incremental
backups; they store, respectively, the particular file's access time
and last inode-change time.
The offset is used by the --multi-volume (-M) option, when
making a multi-volume archive. The offset is number of bytes into
the file that we need to restart at to continue the file on the next
tape, i.e., where we store the location that a continued file is
continued at.
The following fields were added to deal with sparse files. A file
is sparse if it takes in unallocated blocks which end up being
represented as zeros, i.e., no useful data. A test to see if a file
is sparse is to look at the number blocks allocated for it versus the
number of characters in the file; if there are fewer blocks allocated
for the file than would normally be allocated for a file of that
size, then the file is sparse. This is the method tar uses to
detect a sparse file, and once such a file is detected, it is treated
differently from non-sparse files.
Sparse files are often dbm files, or other database-type files
which have data at some points and emptiness in the greater part of
the file. Such files can appear to be very large when an ls
-l is done on them, when in truth, there may be a very small amount
of important data contained in the file. It is thus undesirable
to have tar think that it must back up this entire file, as
great quantities of room are wasted on empty blocks, which can lead
to running out of room on a tape far earlier than is necessary.
Thus, sparse files are dealt with so that these empty blocks are
not written to the tape. Instead, what is written to the tape is a
description, of sorts, of the sparse file: where the holes are, how
big the holes are, and how much data is found at the end of the hole.
This way, the file takes up potentially far less room on the tape,
and when the file is extracted later on, it will look exactly the way
it looked beforehand. The following is a description of the fields
used to handle a sparse file:
The sp is an array of struct sparse. Each struct
sparse contains two 12-character strings which represent an offset
into the file and a number of bytes to be written at that offset.
The offset is absolute, and not relative to the offset in preceding
array element.
The header can hold four of these struct sparse at the moment;
if more are needed, they are not stored in the header.
The isextended flag is set when an extended_header
is needed to deal with a file. Note that this means that this flag
can only be set when dealing with a sparse file, and it is only set
in the event that the description of the file will not fit in the
alloted room for sparse structures in the header. In other words,
an extended_header is needed.
The extended_header structure is used for sparse files which
need more sparse structures than can fit in the header. The header can
fit 4 such structures; if more are needed, the flag isextended
gets set and the next block is an extended_header.
Each extended_header structure contains an array of 21
sparse structures, along with a similar isextended flag
that the header had. There can be an indeterminate number of such
extended_headers 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.
(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.
tar and cpio(This message will disappear, once this node revised.)
The cpio archive formats, like tar, do have maximum
pathname lengths. The binary and old ASCII formats have a max path
length of 256, and the new ASCII and CRC ASCII formats have a max
path length of 1024. GNU cpio can read and write archives
with arbitrary pathname lengths, but other cpio implementations
may crash unexplainedly trying to read them.
tar handles symbolic links in the form in which it comes in BSD;
cpio doesn't handle symbolic links in the form in which it comes
in System V prior to SVR4, and some vendors may have added symlinks
to their system without enhancing cpio to know about them.
Others may have enhanced it in a way other than the way I did it
at Sun, and which was adopted by AT&T (and which is, I think, also
present in the cpio that Berkeley picked up from AT&T and put
into a later BSD release--I think I gave them my changes).
(SVR4 does some funny stuff with tar; basically, its cpio
can handle tar format input, and write it on output, and it
probably handles symbolic links. They may not have bothered doing
anything to enhance tar as a result.)
cpio handles special files; traditional tar doesn't.
tar comes with V7, System III, System V, and BSD source;
cpio comes only with System III, System V, and later BSD
(4.3-tahoe and later).
tar's way of handling multiple hard links to a file can handle
file systems that support 32-bit inumbers (e.g., the BSD file system);
cpios way requires you to play some games (in its "binary"
format, i-numbers are only 16 bits, and in its "portable ASCII" format,
they're 18 bits--it would have to play games with the "file system ID"
field of the header to make sure that the file system ID/i-number pairs
of different files were always different), and I don't know which
cpios, if any, play those games. Those that don't might get
confused and think two files are the same file when they're not, and
make hard links between them.
tars way of handling multiple hard links to a file places only
one copy of the link on the tape, but the name attached to that copy
is the only one you can use to retrieve the file; cpios
way puts one copy for every link, but you can retrieve it using any
of the names.
What type of check sum (if any) is used, and how is this calculated?
See the attached manual pages for tar and cpio format.
tar uses a checksum which is the sum of all the bytes in the
tar header for a file; cpio uses no checksum.
Does anyone know whycpiowas made whentarwas present at the unix scene?
It wasn't. cpio first showed up in PWB/UNIX 1.0; no
generally-available version of UNIX had tar at the time. I don't
know whether any version that was generally available within AT&T
had tar, or, if so, whether the people within AT&T who did
cpio knew about it.
On restore, if there is a corruption on a tape, tar will stop at
that point, while cpio will skip over it and try to restore the
rest of the files.
The main difference is just in the command syntax and header format.
tar is a little more tape-oriented in that everything is blocked
to start on a record boundary.
Is there any differences between the ability to recover crashed archives between the two of them? (Is there any chance of recovering crashed archives at all?)
Theoretically, it should be easier under tar since the blocking
lets you find a header with some variation of dd skip=nn.
However, modern cpio's and variations have an option to just
search for the next file header after an error with a reasonable chance
of re-syncing. Note that 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 whycpiowas made whentarwas present at the unix scene, please tell me about this too.
Probably because it is more media efficient (by not blocking everything
and using only the space needed for the headers where tar
always uses 512 bytes per file header) and it knows how to archive
special files.
You might want to look at the freely available alternatives. The major
ones are afio, GNU tar, and pax, each of which
have their own extensions with some backwards compatibility.
Sparse files were tarred as sparse files (which you can easily
test, because the resulting archive gets smaller, and GNU cpio
can no longer read it).
(This message will disappear, once this node revised.)
A few special cases about tape handling warrant more detailed description. These special cases are discussed below.
Many complexities surround the use of tar on tape drives. Since
the creation and manipulation of archives located on magnetic tape was
the original purpose of tar, it contains many features making
such manipulation easier.
Archives are usually written on dismountable media--tape cartridges, mag tapes, or floppy disks.
The amount of data a tape or disk holds depends not only on its size, but also on how it is formatted. A 2400 foot long reel of mag tape holds 40 megabytes of data when formated at 1600 bits per inch. The physically smaller EXABYTE tape cartridge holds 2.3 gigabytes.
Magnetic media are re-usable--once the archive on a tape is no longer needed, the archive can be erased and the tape or disk used over. Media quality does deteriorate with use, however. Most tapes or disks should be 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.
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This option is used to specify the file name of the archive tar
works on.
If the file name is -, tar reads the archive from standard
input (when listing or extracting), or writes it to standard output
(when creating). If the - file name is given when updating an
archive, tar will read the original archive from its standard
input, and will write the entire new archive to its standard output.
If the file name contains a :, it is interpreted as
hostname:file name. If the hostname contains an at
sign (@), it is treated as user@hostname:file name. In
either case, tar will invoke the command rsh (or
remsh) to start up an /etc/rmt on the remote machine. If
you give an alternate login name, it will be given to the rsh.
Naturally, the remote machine must have an executable /etc/rmt.
This program is free software from the University of California, and a
copy of the source code can be found with the sources for tar;
it's compiled and installed by default.
Since absolute file names on MS-DOS and MS-Windows include a drive
letter, like in d:/dir/subdir/file, tar compiled for these
platforms does not treat file names with : as remote files if
there is a single letter before the colon. tar also does not
currently support archives on remote machines when it runs on
MS-DOS/MS-Windows.
If --file=archive-name (-f archive-name) 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 so many cases that it was hopeless to repair. This is now
completely left to the installer to override standard input and standard
output for default device, if this seems preferrable to him/her.
Further, I think most actual usages of tar are done with
pipes or disks, not really tapes, cartridges or diskettes.
Some users think that using standard input and output is running
after trouble. This could lead to a nasty surprise on your screen if
you forget to specify an output file name--especially if you are going
through a network or terminal server capable of buffering large amounts
of output. We had so many bug reports in that area of configuring
default tapes automatically, and so many contradicting requests, that
we finally consider the problem to be portably intractable. We could
of course use something like /dev/tape as a default, but this
is also running after various kind of trouble, going from hung
processes to accidental destruction of real tapes. After having seen
all this mess, using standard input and output as a default really
sounds like the only clean choice left, and a very useful one too.
GNU tar reads and writes archive in records, I suspect this is the
main reason why block devices are preferred over character devices.
Most probably, block devices are more efficient too. The installer
could also check for DEFTAPE in <sys/mtio.h>.
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 option is not used, the shell command found when
the tar program was installed is used instead. This is
the first found of /usr/ucb/rsh, /usr/bin/remsh,
/usr/bin/rsh, /usr/bsd/rsh or /usr/bin/nsh.
The installer may have overriden this by defining the environment
variable RSH at installation time.
This option causes tar to write a multi-volume archive--one
that may be larger than will fit on the medium used to hold it.
See Multi-Volume Archives.
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.
file at end of each tape. This implies
--multi-volume (-M).
In order to access the tape drive on a remote machine, tar
uses the remote tape server written at the University of California at
Berkeley. The remote tape server must be installed as /etc/rmt
on any machine whose tape drive you want to use. tar calls
/etc/rmt by running an rsh or remsh to the remote
machine, optionally using a different login name if one is supplied.
A copy of the source for the remote tape server is provided. It is
Copyright © 1983 by the Regents of the University of
California, but can be freely distributed. Instructions for compiling
and installing it are included in the Makefile.
Unless you use the --absolute-names (-P) option, GNU tar will
not allow you to create an archive that contains absolute file names
(a file name beginning with /.) If you try, tar will
automatically remove the leading / from the file names it
stores in the archive. (On MS-DOS/MS-Windows, tar also removes
the drive specification d:/.) It will also type a warning
message telling you that it is doing so.
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.
For example, if the archive contained a file /usr/bin/computoy,
GNU tar would extract the file to usr/bin/computoy,
relative to the current directory. If you want to extract the files in
an archive to the same absolute names that they had when the archive
was created, you should do a cd / before extracting the files
from the archive, or you should either use the --absolute-names (-P)
option, or use the command tar -C / .... (On MS-DOS, include
the drive specification in the -C option, or change the current
drive before the cd command.)
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.
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 (-r), --update (-u), --concatenate (--catenate, -A), and --delete commands will not work on any other kind of file. Some media simply cannot be backspaced, which means these commands and options will never be able to work on them. These non-backspacing media include pipes and cartridge tape drives.
Some other media can be backspaced, and tar will work on them
once tar is modified to do so.
Archives created with the --multi-volume (-M), --label=archive-label (-V archive-label), and
--incremental (-G) options may not be readable by other version
of tar. In particular, restoring a file that was split over
a volume boundary will require some careful work with dd, if
it can be done at all. Other versions of tar may also create
an empty file whose name is that of the volume header. Some versions
of tar may create normal files instead of directories archived
with the --incremental (-G) option.
errors from system:
permission denied
no such file or directory
not owner
errors from tar:
directory checksum error
header format error
errors from media/system:
i/o error
device busy
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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, includingF(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. TheJCLDD RECFORM=parameter specified this to the operating system.The Unix man page on
tarwas totally confused about this. When I wrotePD TAR, I used the historically correct terminology (tarwrites data records, which are grouped into blocks). It appears that the bogus terminology made it into POSIX (no surprise here), and now François 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 ignores 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 means 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 of speaking about 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=blocks (-b blocks) option. Each record will
then be composed of 512-size blocks. (Each tar block is
512 bytes. See Standard.) Each file written to the archive uses
at least one full record. As a result, using a larger record size
can result in more wasted space for small files. On the other hand, a
larger record size can often be read and written much more efficiently.
Further complicating the problem is that some tape drives ignore the blocking entirely. For these, a larger record size can still improve performance (because the software layers above the tape drive still honor the blocking), but not as dramatically as on tape drives that honor blocking.
When reading an archive, tar can usually figure out the record
size on itself. When this is the case, and a non-standard record size
was used when the archive was created, tar will print a message
about a non-standard blocking factor, and then operate normally. On
some tape devices, however, tar cannot figure out the record size
itself. On most of those, you can specify a blocking factor (with
--blocking-factor=blocks (-b blocks)) larger than the actual blocking factor, and then use
the --read-full-records (-B) option. (If you specify a blocking factor
with --blocking-factor=blocks (-b blocks) and don't use the --read-full-records (-B)
option, then tar will not attempt to figure out the recording size
itself.) On some devices, you must always specify the record size
exactly with --blocking-factor=blocks (-b blocks) 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=blocks (-b blocks) 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.
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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=blocks (-b blocks) option
specified (see Blocking Factor), you must specify that
blocking-factor when operating on the archive. See Formats, for other
examples of format parameter considerations.
(This message will disappear, once this node revised.)
The data in an archive is grouped into blocks, which are 512 bytes.
Blocks are read and written in whole number multiples called
records. The number of blocks in a record (ie. the size of a
record in units of 512 bytes) is called the blocking factor.
The --blocking-factor=blocks (-b blocks) option specifies the blocking factor of
an archive. The default blocking factor is typically 20 (ie.
10240 bytes), but can be specified at installation. To find out
the blocking factor of an existing archive, use 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 usefull when archiving small files, to avoid archiving lots
of nulls as tar fills out the archive to the end of the record.
In general, the ideal record size depends on the size of the
inter-record gaps on the tape you are using, and the average size of the
files you are archiving. See create, for information on
writing archives.
Archives with blocking factors larger than 20 cannot be read
by very old versions of tar, or by some newer versions
of tar running on old machines with small address spaces.
With GNU tar, the blocking factor of an archive is limited
only by the maximum record size of the device containing the archive,
or by the amount of available virtual memory.
Also, on some systems, not using adequate blocking factors, as sometimes imposed by the device drivers, may yield unexpected diagnostics. For example, this has been reported:
Cannot write to /dev/dlt: Invalid argument
In such cases, it sometimes happen that the tar bundled by the
system is aware of block size idiosyncrasies, while GNU tar requires
an explicit specification for the block size, which it cannot guess.
This yields some people to consider GNU tar is misbehaving, because
by comparison, the bundle tar works OK. Adding -b 256, for example, might resolve the problem.
If you use a non-default blocking factor when you create an archive, you
must specify the same blocking factor when you modify that archive. Some
archive devices will also require you to specify the blocking factor when
reading that archive, however this is not typically the case. Usually, you
can use --list (-t) without specifying a blocking factor--tar
reports a non-default record size and then lists the archive members as
it would normally. To extract files from an archive with a non-standard
blocking factor (particularly if you're not sure what the blocking factor
is), you can usually use the --read-full-records (-B) option while
specifying a blocking factor larger then the blocking factor of the archive
(ie. tar --extract --read-full-records --blocking-factor=300).
See list, for more information on the --list (-t)
subcommand. See Reading, 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 subcommands 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 dissapper. 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.
In previous versions of GNU tar, the --compress-block
option (or even older: --block-compress) was necessary to
reblock compressed archives. It is now a dummy option just asking
not to be used, and otherwise ignored. If the output goes directly
to a local disk, and not through stdout, then the last write is
not extended to a full record size. Otherwise, reblocking occurs.
Here are a few other remarks on this topic:
gzip will complain about trailing garbage if asked to
uncompress a compressed archive on tape, there is an option to turn
the message off, but it breaks the regularity of simply having to use
prog -d for decompression. It would be nice if gzip was
silently ignoring any number of trailing zeros. I'll ask Jean-loup
Gailly, by sending a copy of this message to him.
compress does not show this problem, but as Jean-loup pointed
out to Michael, compress -d silently adds garbage after
the result of decompression, which tar ignores because it already
recognized its end-of-file indicator. So this bug may be safely
ignored.
gzip -d -q will be silent about the trailing zeros indeed,
but will still return an exit status of 2 which tar reports in turn.
tar might ignore the exit status returned, but I hate doing
that, as it weakens the protection tar offers users against
other possible problems at decompression time. If gzip was
silently skipping trailing zeros and also avoiding setting the
exit status in this innocuous case, that would solve this situation.
tar should become more solid at not stopping to read a pipe at
the first null block encountered. This inelegantly breaks the pipe.
tar should rather drain the pipe out before exiting itself.
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 cat-ing 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 (-B) 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 accomodate 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 depend of the buffering techniques used inside modern
tape controllers. Some impose 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.
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 subcommand 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
positionning 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 the non-rewinding tape device:
$ 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 files 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 non-rewinding tape by issuing the command:
$ tar cf /dev/nrtape directory-2
and so on for all the archives you want to put on the same tape.
Another usual case is that you do not write all the archives the same day, and you need to remove and store the tape between two archive sessions. In general, you must remember how many files are already saved on your tape. Suppose your tape already has 16 files on it, and that you are ready to write the 17th. You have to take care of skipping the first 16 tape marks before saving directory-17, say, by using these commands:
$ mt -f /dev/nrtape rewind $ mt -f /dev/nrtape fsf 16 $ tar cf /dev/nrtape directory-17
In all the previous examples, we put aside blocking considerations, but you should do the proper things for that as well. See Blocking.
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
head--a physical part of the device which can only access one
point on the tape at a time. When you use tar to read or
write archive data from a tape device, the device will begin reading
or writing from wherever on the tape the tape head happens to be,
regardless of which archive or what part of the archive the tape
head is on. Before writing an archive, you should make sure that no
data on the tape will be overwritten (unless it is no longer needed).
Before reading an archive, you should make sure the tape head is at
the beginning of the archive you want to read. (The restore
script will find the archive automatically. .) See mt, for
an explanation of the tape moving utility.
If you want to add new archive file entries to a tape, you should advance the tape to the end of the existing file entries, backspace over the last tape mark, and write the new archive file. If you were to add two archives to the example above, the tape might look like the following:
rrrr*rrrrrr*rrrrr*rr*rrrrr*rrr*rrrr**----------------
mt Utility(This message will disappear, once this node revised.)See Blocking Factor.
You can use the mt utility to advance or rewind a tape past a
specified number of archive files on the tape. This will allow you
to move to the beginning of an archive before extracting or reading
it, or to the end of all the archives before writing a new one.
The syntax of the mt command is:
mt [-f tapename] operation [number]
where tapename is the name of the tape device, number is the number of times a subcommand 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 defined, mt uses the device
/dev/rmt12.
mt returns a 0 exit status when the operation(s) were
successful, 1 if the command was unrecognized, and 2 if a subcommand
failed.
If you use --extract (--get, -x) with the --label=archive-label (-V archive-label) option
specified, tar will read an archive label (the tape head has to
be positioned on it) and print an error if the archive label doesn't
match the archive-name specified. archive-name can be
any regular expression. If the labels match, tar extracts the
archive. See label, .
tar --list --label will cause tar to print the label.
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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 filesystems.
Therefore, tar supports multiple tapes automatically.
Use --multi-volume (-M) on the command line, and then tar will,
when it reaches the end of the tape, prompt for another tape, and
continue the archive. Each tape will have an independent archive, and
can be read without needing the other. (As an exception to this, the
file that tar was archiving when it ran out of tape will usually
be split between the two archives; in this case you need to extract from
the first archive, using --multi-volume (-M), and then put in the
second tape when prompted, so tar can restore both halves of the
file.)
GNU tar multi-volume archives do not use a truly portable format.
You need GNU tar at both end to process them properly.
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.
tar to begin writing the next volume.
(You should type y only after you have changed the tape;
otherwise tar will write over the volume it just finished.)
If you want more elaborate behavior than this, give tar the
--info-script=script-name (--new-volume-script=script-name, -F script-name) option. The file script-name is expected
to be a program (or shell script) to be run instead of the normal
prompting procedure. When the program finishes, tar will
immediately begin writing the next volume. The behavior of the
n response to the normal tape-change prompt is not available
if you use --info-script=script-name (--new-volume-script=script-name, -F script-name).
The method tar uses to detect end of tape is not perfect, and
fails on some operating systems or on some devices. You can use the
--tape-length=KBytes (-L KBytes) option if tar can't detect the end of the
tape itself. This option selects --multi-volume (-M) automatically.
The size argument should then be the usable size of the tape.
But for many devices, and floppy disks in particular, this option is
never required for real, as far as we know.
The volume number used by tar in its tape-change prompt
can be changed; if you give the --volno-file=file-of-number option, then
file-of-number should be an unexisting file to be created, or else,
a file already containing a decimal number. That number will be used
as the volume number of the first volume written. When tar is
finished, it will rewrite the file with the now-current volume number.
(This does not change the volume number written on a tape label, as
per label, it only affects the number used in
the prompt.)
If you want tar to cycle through a series of tape drives, then
you can use the n response to the tape-change prompt. This is
error prone, however, and doesn't work at all with --info-script=script-name (--new-volume-script=script-name, -F script-name).
Therefore, if you give tar multiple --file=archive-name (-f archive-name) options, then
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).
Multi-volume archives
With --multi-volume (-M), tar will not abort when it cannot
read or write any more data. Instead, it will ask you to prepare a new
volume. If the archive is on a magnetic tape, you should change tapes
now; if the archive is on a floppy disk, you should change disks, etc.
Each volume of a multi-volume archive is an independent tar
archive, complete in itself. For example, you can list or extract any
volume alone; just don't specify --multi-volume (-M). However, if one
file in the archive is split across volumes, the only way to extract
it successfully is with a multi-volume extract command --extract
--multi-volume (-xM) starting on or before the volume where
the file begins.
For example, let's presume 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
(This message will disappear, once this node revised.)
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 (-c) option (see create). A multi-volume archive can be manipulated like any other archive (provided the --multi-volume (-M) option is specified), but is stored on more than one tape or disk.
When you specify --multi-volume (-M), tar does not report an
error when it comes to the end of an archive volume (when reading), or
the end of the media (when writing). Instead, it prompts you to load
a new storage volume. If the archive is on a magnetic tape, you
should change tapes when you see the prompt; if the archive is on a
floppy disk, you should change disks; etc.
You can read each individual volume of a multi-volume archive as if it were an archive by itself. For example, to list the contents of one volume, use --list (-t), without --multi-volume (-M) specified. To extract an archive member from one volume (assuming it is on that volume), use --extract (--get, -x), again without --multi-volume (-M).
If an archive member is split across volumes (ie. its entry begins on
one volume of the media and ends on another), you need to specify
--multi-volume (-M) to extract it successfully. In this case, you
should load the volume where the archive member starts, and use
tar --extract --multi-volume--tar will prompt for later
volumes as it needs them. See extracting archives, for more
information about extracting archives.
--info-script=script-name (--new-volume-script=script-name, -F script-name) is like --multi-volume (-M), except that
tar does not prompt you directly to change media volumes when
a volume is full--instead, tar runs commands you have stored
in script-name. For example, this option can be used to eject
cassettes, or to broadcast messages such as Someone please come
change my tape when performing unattended backups. When script-name
is done, tar will assume that the media has been changed.
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 subcommands, you need to use the entire archive.
If a multi-volume archive was labeled using --label=archive-label (-V archive-label)
(see label) when it was created, tar will not
automatically label volumes which are added later. To label subsequent
volumes, specify --label=archive-label (-V archive-label) again in conjunction with the
--append (-r), --update (-u) or --concatenate (--catenate, -A) subcommand.
Beware that there is no real standard about the proper way, for a
tar archive, to span volume boundaries. If you have a multi-volume
created by some vendor's tar, there is almost no chance you could
read all the volumes with GNU tar. The converse is also true:
you may not expect multi-volume archives created by GNU tar to
be fully recovered by vendor's tar. Since there is little chance
that, in mixed system configurations, some vendor's tar will work on
another vendor's machine, and there is a great chance that GNU tar
will work on most of them, your best bet is to install GNU tar
on all machines between which you know exchange of files is possible.
(This message will disappear, once this node revised.)
To give the archive a name which will be recorded in it, use the
--label=archive-label (-V archive-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 (-t).
If you are creating a multi-volume archive with --multi-volume (-M)
(), 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=archive-label (-V archive-label)
option when reading an archive, it checks to make sure the label on the
tape matches the one you give. See label.
When tar writes an archive to tape, it creates a single
tape file. If multiple archives are written to the same tape, one
after the other, they each get written as separate tape files. When
extracting, it is necessary to position the tape at the right place
before running tar. To do this, use the mt command.
For more information on the mt command and on the organization
of tapes into a sequence of tape files, see mt.
People seem to often do:
tar --label="some-prefix `date +some-format`"
or such, for pushing a common date in all volumes or an archive set.
(This message will disappear, once this node revised.)
This option causes tar to write out a volume header at
the beginning of the archive. If --multi-volume (-M) is used, each
volume of the archive will have a volume header of name
Volume n, where n is 1 for the first volume, 2 for the
next, and so on.
To avoid problems caused by misplaced paper labels on the archive media, you can include a label entry--an archive member which contains the name of the archive--in the archive itself. Use the --label=archive-label (-V archive-label) option in conjunction with the --create (-c) subcommand 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 Multi-Volume Archives, for information on creating multiple
volume archives.
If you list or extract an archive using --label=archive-label (-V archive-label), tar will
print an error if the archive label doesn't match the archive-label
specified, and will then not list nor extract the archive. In those cases,
archive-label argument is interpreted as a globbing-style pattern
which must match the actual magnetic volume label. See exclude, for
a precise description of how match is attempted5.
If the switch --multi-volume (-M) is being used, the volume label
matcher will also suffix archive-label by Volume [1-9]*
if the initial match fails, before giving up. Since the volume numbering
is automatically added in labels at creation time, it sounded logical to
equally help the user taking care of it when the archive is being read.
The --label=archive-label (-V archive-label) was once called --volume, but is not available
under that name anymore.
To find out an archive's label entry (or to find out if an archive has
a label at all), use tar --list --verbose. tar will print the
label first, and then print archive member information, as in the
example below:
$ tar --verbose --list --file=iamanarchive V--------- 0 0 0 1992-03-07 12:01 iamalabel--Volume Header-- -rw-rw-rw- ringo user 40 1990-05-21 13:30 iamafilename
To get a common information on all tapes of a series, use the --label=archive-label (-V archive-label) option. 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`"
Also note that each label has its own date and time, which corresponds
to when GNU tar initially attempted to write it, often soon
after the operator launches tar or types the carriage return
telling that the next tape is ready. Comparing date labels does give
an idea of tape throughput only if the delays for rewinding tapes
and the operator switching them were negligible, which is ususally
not the case.
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 (-c) subcommand. When this option is
specified, tar checks archive members against their counterparts
in the file system, and reports discrepancies on the standard error. In
multi-volume archives, each volume is verified after it is written,
before the next volume is written.
To verify an archive, you must be able to read it from before the end of the last written entry. This option is useful for detecting data errors on some tapes. Archives written to pipes, some cartridge tape drives, and some other devices cannot be verified.
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 (-W) option. See compare.
Note that these two options have a slightly different intent. The
--compare (--diff, -d) option checks how identical are the logical contents of some
archive with what is on your disks, while the --verify (-W) option is
really for checking if the physical contents agree and if the recording
media itself is of dependable quality. So, for the --verify (-W)
option, tar tries to defeat all in-memory cache pertaining to
the archive, while it lets the speed optimization undisturbed for the
--compare (--diff, -d) option. If you nevertheless use --compare (--diff, -d) 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 (-W) 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.
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 accidently overwritten or deleted. (This will protect the archive from being changed with a tape or floppy drive--it will not protect it from magnet fields or other physical hazards).
The write protection device itself is usually an integral part of the physical media, and can be a two position (write enabled/write disabled) switch, a notch which can be popped out or covered, a ring which can be removed from the center of a tape reel, or some other changeable feature.
tar(This message will disappear, once this node revised.)
This chapter is about how one invokes the GNU tar command, from
the command synopsis (see Synopsis). There are numerous options,
and many styles for writing them. One mandatory option specifies
the subcommand tar should perform (see Subcommand Summary),
other options are meant to detail how this subcommand should be performed
(see Option Summary). Non-option arguments are not always interpreted
the same way, depending on what the subcommand is.
You will find in this chapter everything about option styles and rules for
writing them (see Styles). On the other hand, subcommands and options
are fully described elsewhere, in other chapters. Here, you will find
only synthetic descriptions for subcommands 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 (), --verbose (-v) (see verbose)
and --interactive (-w) options (see interactive).
tarThe 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 subcommand, specifies
which action to take. The other arguments to tar are either
options, which change the way tar performs a subcommand,
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 subcommand mode
(the tar main command) is usually given first.
Each name in the synopsis above is interpreted as an archive member
name when the main command is one of --compare (--diff, -d), --delete,
--extract (--get, -x), --list (-t) or --update (-u). When naming
archive members, you must give the exact name of the member in the
archive, as it is printed by --list (-t). For --append (-r)
and --create (-c), these name arguments specify the names
of either files or directory hierarchies to place in the archive.
These files or hierarchies should already exist in the file system,
prior to the execution of the tar command.
tar interprets relative file names as being relative to the
working directory. tar will make all file names relative
(by removing leading slashes when archiving or restoring files),
unless you specify otherwise (using the --absolute-names (-P)
option). See absolute, for more information about
--absolute-names (-P).
If you give the name of a directory as either a file name or a member
name, then tar acts recursively on all the files and directories
beneath that directory. For example, the name / identifies all
the files in the filesystem to tar.
The distinction between file names and archive member names is especially
important when shell globbing is used, and sometimes a source of confusion
for newcomers. See Wildcards, for more information about globbing.
The problem is that shells may only glob using existing files in the
file system. Only tar itself may glob on archive members, so when
needed, you must ensure that wildcard characters reach tar without
being interpreted by the shell first. Using a backslash before *
or ?, or putting the whole argument between quotes, is usually
sufficient for this. (On MS-DOS/MS-Windows, you must use
quotes, since the backslash is a directory separator, and cannot be used
to escape-protect wildcard characters.)
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 subcommands of tar (--list (-t),
--extract (--get, -x), --compare (--diff, -d), and --update (-u)) will act
on the entire contents of the archive.
Besides successful exits, GNU tar may fail for many reasons.
Some reasons correspond to bad usage, that is, when the tar
command is improperly written.
Errors may be encountered later, while
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.
GNU tar returns only a few exit statuses. I'm really
aiming simplicity in that area, for now. If you are not using the
--compare (--diff, -d) option, zero means that everything went well, besides
maybe innocuous warnings. Nonzero means that something went wrong.
As of today, "nonzero" is almost always 2, except for
remote subcommands, where it may be 128.
tar OptionsGNU 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 subcommand as an argument to the tar command (two lists
of four subcommands each may be found at frequent subcommands and
). 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 how tar works on archives, such as archive
format, archive name, or level of user interaction. Some options make
sense with all operating modes, while others are meaningful only with
particular modes. You will likely use some options frequently, while
you will only use others infrequently, or not at all. (A full list of
options is available in see All Options.)
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 subcommands, 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 subcommands can be written in any of these three
styles.
There are three styles for writing subcommands and options to the command
line invoking tar. The different styles were developed at
different times during the history of tar. These styles will be
presented below, from the most recent to the oldest.
Some options must take an argument. (For example, --file=archive-name (-f archive-name) takes
the name of an archive file as an argument. If you do not supply an
archive file name, tar will use a default, but this can be
confusing; thus, we recommend that you always supply a specific archive
file name.) Where you place the arguments generally depends on
which style of options you choose. We will detail specific information
relevant to each option style in the sections on the different option
styles, below. The differences are subtle, yet can often be very
important; incorrect option placement can cause you to overwrite a
number of important files. We urge you to note these differences, and
only use the option style(s) which makes the most sense to you until you
feel comfortable with the others.
Each option has at least one long (or mnemonic) name starting with two
dashes in a row, e.g. --list. The long names are more clear than
their corresponding short or old names. It sometimes happens that a
single mnemonic option has many different names which are
synonymous, such as --compare and --diff. In addition,
long option names can be given unique abbreviations. For example,
--cre can be used in place of --create because there is no
other mnemonic option which begins with cre. (One way to find
this out is by trying it and seeing what happens; if a particular
abbreviation could represent more than one option, tar will tell
you that that abbreviation is ambiguous and you'll know that that
abbreviation won't work. You may also choose to run tar --help
to see a list of options. Be aware that if you run tar with a
unique abbreviation for the long name of an option you didn't want to
use, you are stuck; tar will perform the command as ordered.)
Mnemonic options are meant to be obvious and easy to remember, and their meanings are generally easier to discern than those of their corresponding short options (see below). For example:
$ tar --create --verbose --blocking-factor=20 --file=/dev/rmt0
gives a fairly good set of hints about what the command does, even
for those not fully acquainted with tar.
Mnemonic options which require arguments take those arguments
immediately following the option name; they are introduced by an equal
sign. For example, the --file option (which tells the name
of the tar archive) is given a file such as archive.tar
as argument by using the notation --file=archive.tar for the
mnemonic option.
Most options also have a short option name. Short options start with
a single dash, and are followed by a single character, e.g. -t
(which is equivalent to --list). The forms are absolutely
identical in function; they are interchangeable.
The short option names are faster to type than long option names.
Short options which require arguments take their arguments immediately
following the option, usually separated by white space. It is also
possible to stick the argument right after the short option name, using
no intervening space. For example, you might write -f archive.tar or -farchive.tar instead of using
--file=archive.tar. Both --file=archive-name and
-f archive-name denote the option which indicates a
specific archive, here named archive.tar.
Short options' letters may be clumped together, but you are not
required to do this (as compared to old options; see below). When short
options are clumped as a set, use one (single) dash for them all, e.g.
tar -cvf
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.
(This message will disappear, once this node revised.)
Like short options, old options are single letters. However, old options
must be written together as a single clumped set, without spaces separating
them or dashes preceding them7. This set
of letters must be the first to appear on the command line, after the
tar program name and some whitespace; old options cannot appear
anywhere else. The letter of an old option is exactly the same letter as
the corresponding short option. For example, the old option t is
the same as the short option -t, and consequently, the same as the
mnemonic option --list. So for example, the command tar cv specifies the option -v in addition to the subcommand -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.
All three styles may be intermixed in a single tar command, so
long as the rules for each style are fully respected8. Old style options and either of the
modern styles of options may be mixed within a single tar command.
However, old style options must be introduced as the first arguments only,
following the rule for old options (old options must appear directly
after the tar command and some whitespace). Modern options may
be given only after all arguments to the old options have been collected.
If this rule is not respected, a modern option might be falsely interpreted
as the value of the argument to one of the old style options.
For example, all the following commands are wholly equivalent, and illustrate the many combinations and orderings of option styles.
tar --create --file=archive.tar tar --create -f archive.tar tar --create -farchive.tar tar --file=archive.tar --create tar --file=archive.tar -c tar -c --file=archive.tar tar -c -f archive.tar tar -c -farchive.tar tar -cf archive.tar tar -cfarchive.tar tar -f archive.tar --create tar -f archive.tar -c tar -farchive.tar --create tar -farchive.tar -c tar c --file=archive.tar tar c -f archive.tar tar c -farchive.tar tar cf archive.tar tar f archive.tar --create tar f archive.tar -c tar fc archive.tar
On the other hand, the following commands are not equivalent to the previous set:
tar -f -c archive.tar tar -fc archive.tar tar -fcarchive.tar tar -farchive.tarc tar cfarchive.tar
These last examples mean something completely different from what the
user intended (judging by 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.
tar OptionsThe coming manual sections contain an alphabetical listing of all
tar subcommands 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.
--concatenate. See concatenate.
tar archives to the end of the archive.
See concatenate.
tar archive. See create.
--compare. See compare.
--extract. See extract.
Adds files to the end of the archive, but only if they are newer than their counterparts already in the archive, or if they do not already exist in the archive. See update.
tar Optionstar converts absolute file
names to relative, by stripping an initial / from member names
(on MS-DOS and MS-Windows, tar also strips the drive letter and
the colon that follows it). This option disables that behavior.
See absolute.
--newer; see after.)
tar to preserve the access time field in a file's inode when
dumping it. See Attributes.
tar will back them up
using simple or numbered backups, depending upon backup-type.
See backup.
tar prints error messages for read errors
with the block number in the archive file. See verbose.
tar uses to blocking x 512 bytes per
record. See Blocking Factor.
tar to print periodic checkpoint messages as it
reads through the archive. It's intended for when you want a visual
indication that tar is still running, but don't want to see
--verbose output. See verbose.
tar will use the compress program when reading or writing the
archive. This allows you to directly act on archives while saving
space. See gzip.
--interactive and see verbose.)
tar archive, tar will archive the file that a symbolic
link points to, rather than archiving the symlink. See dereference.
tar will change its current directory
to dir before performing any subcommands. When this option is used
during archive creation, it is order sensitive. See directory.
tar will skip files that match
pattern. See exclude.
--exclude, except tar will use the list of patterns
in the file file. See exclude.
tar will use the file archive as the tar archive it
performs subcommands on, rather than tar's compilation-dependent
default. See file and See Device.
tar will use the contents of file as a list of archive members
or files to operate on, in addition to those specified on the
command-line. See files.
tar to interpret the filename given to --file as a local
file, even if it looks like a remote tape drive name. See Device.
tar archive will have a group id of group,
rather than the group from the source file. See Attributes.
Also see the comments for the --owner=user option.
--gzip; see gzip.)
tar to read or write archives through gzip,
allowing tar to directly operate on several kinds of compressed
archives transparently. See gzip.
tar will print out a short message summarizing the subcommands and
options to tar and exit. See help tutorial.
tar to exit successfully if it encounters an
unreadable file. See Ignore Failed Read.
(See --preserve-permissions; see Writing.)
tar will ignore zeroed blocks in the archive, which
normally signals EOF. See Ignore Zeros and See Blocking Factor.
tar that it is working with an old GNU-format
incremental backup archive. It is intended primarily for backwards
compatibility. See incremental and listed-incremental.
tar is performing multi-tape backups, script-file is run
at the end of each tape. See Device and See Using Multiple Tapes.
tar should ask the user for confirmation before
performing potentially destructive options, such as overwriting files.
See verbose.
tar will not overwrite existing
files if this option is present. See Keep Old Files.
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 mt and See label.
--create subcommand, specifies that the archive that
tar creates is a new GNU-format incremental backup, using
snapshot-file to determine which files to backup.
With other subcommands, informs tar that the archive is in incremental
format. See incremental and listed-incremental.
tar will use permissions
for the archive members, rather than the permissions from the files.
The program chmod and this tar option share the same syntax
for what permissions might be. See File permissions. This reference also
has useful information for those not being overly familiar with the Unix
permission system.
Of course, permissions might be plainly specified as an octal number.
However, by using generic symbolic modifications to mode bits, this allows
more flexibility. For example, the value a+rw adds read and write
permissions for everybody, while retaining executable bits on directories
or on any other file already marked as executable.
tar that it should create or otherwise operate on a
multi-volume tar archive. See Multi-Volume Archives and
See Using Multiple Tapes.
tar
to prepend the given prefix to each archive file name as it is gathered
into the archive. This prefix won't show in any --verbose (-v) progress
message, but it will if you list the archive.
tar will only add files that have changed
since date. See after.
--newer, tar will only add files whose
contents have changed (as opposed to just --newer, which will
also back up files for which any status information has changed). On
MS-DOS, where there is only one time stamp for a file, --newer
and --newer-mtime have identical effect. See after.
tar will not recurse into directories unless a
directory is explicitly named as an argument to tar.
See recurse.
tar is using the --files-from option, this option
instructs tar to expect filenames terminated with NUL, so
tar can correctly work with file names that contain newlines.
See nul.
tar that it should use numeric user and group
IDs when creating a tar file, rather than names. See Attributes.
--portability; see old.)
tar from recursing into
directories that are on different file systems from the current
directory. See one.
tar should use user as the owner of members
when creating archives, instead of the user associated with the source
file. See Attributes.
tar to create an archive that is compatible with Unix V7
tar. See old.
tar to create a POSIX compliant tar archive.
See posix.
--preserve-permissions and
--same-order. See Attributes.
--same-order; see Same Order.)
tar is extracting an archive, it normally subtracts the users'
umask from the permissions specified in the archive and uses that
number as the permissions to create the destination file. Specifying
this option instructs tar that it should use the permissions directly
from the archive. See Setting Access Permissions and See Attributes.
tar should reblock its input, for reading from pipes on
systems with buggy implementations. See Reading and See Blocking.
tar to use size bytes per record when accessing the
archive. See Blocking.
--unlink-first option, removing existing
directory hierarchies before extracting directories of the same name
from the archive. See Recursive Unlink.
tar to remove the source file from the file system after
appending it to an archive. See remove files.
tar that is should use cmd to communicate with remote
devices. See Device.
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 Same Order.
tar will attempt to preserve the owner
specified in the tar archive with this option present.
See Attributes.
--preserve-permissions; see Writing.)
tar to mention directories it's skipping over when operating
on a tar archive. See verbose.
tar will skip extracting
files in the archive until it finds one that matches name.
See Starting File.
tar uses when backing up files from the default
~. See backup.
tar is writing as being
num x 1024 bytes long. See Device.
tar will extract files to stdout rather than to the
file system. See Writing to Standard Output.
--compress; see gzip.)
--gzip; see gzip.)
tar to remove the corresponding file from the file system
before extracting it from the archive. See Prevention Overwriting.
tar to access the archive through prog, which is
presumed to be a compression program of some sort. See gzip.
tar should be more verbose about the subcommands its
performing. This option can be specified multiple times for some
subcommands to increase the amount of information displayed. See verbose.
tar will print an informational message about what version it is and a
copyright message, some credits, and then exit. See help tutorial.
--multi-volume. tar will keep track
of which volume of a multi-volume archive its working in file.
See Using Multiple Tapes.
Here is an alphabetized list of all of the short option forms, matching them with the equivalent long option.
--concatenate
--read-full-records
--directory
--info-script
--incremental
--starting-file
--tape-length
--multi-volume
--newer
--to-stdout
--absolute-names
--block-number
--sparse
--files-from
--unlink-first
--label
--verify
--exclude-from
--compress
--blocking-factor
--create
--compare
--file
--listed-incremental
--dereference
--ignore-zeros
--keep-old-files
--one-file-system
--touch
--portability
--preserve-permissions
--append
--same-order
--list
--update
--verbose
--interactive
--extract
--gzip
Being careful, the first thing is really checking that you are using GNU
tar, indeed. The --version option will generate a message
giving confirmation that you are using GNU tar, with the precise
version of GNU tar you are using. tar identifies itself
and prints the version number to the standard output, then immediately
exits successfully, without doing anything else, ignoring all other
options. For example, tar --version might return:
tar (GNU tar) 1.12a
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 contains9.
Another thing you might want to do is checking the spelling or meaning
of some particular tar option, without resorting to this manual,
for once you have carefully read it. GNU tar has a short help
feature, triggerable through the --help option. By using this
option, tar will print a usage message listing all available
options on standard output, then exit successfully, without doing
anything else and ignoring all other options. Even if this is only a
brief summary, it may be several screens long. So, if you are not
using some kind of scrollable window, you might prefer to use something
like:
$ tar --help | less
presuming, here, that you like using less for a pager. Other
popular pagers are more and pg. If you know about some
keyword which interests you and do not want to read all the
--help output, another common idiom is doing:
tar --help | grep keyword
for getting only the pertinent lines.
The perceptive reader would have noticed some contradiction in the previous paragraphs. It is written that both --version and --help print something, and have all other options ignored. In fact, they cannot ignore each other, and one of them has to win. We do not specify which is stronger, here; experiment if you really wonder!
The short help output is quite succint, and you might have to get back
to the full documentation for precise points. If you are reading this
paragraph, you already have the tar manual in some form. This
manual is available in printed form, as a kind of small book. It may 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, and choose tar from the main Info menu (with m tar
<RET>).
A short reference to tar in the form of a man page is also
provided. Currently, it doesn't tell much more that tar --help
does.
tar progressTypically, tar performs most subcommands 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 subcommands, the --verbose (-v) option causes
tar to print the name of each file or archive member as it
is processed. This and the other options which make tar print
status information can be useful in monitoring tar.
With --create (-c) or --extract (--get, -x), --verbose (-v) used once
just prints the names of the files or members as they are processed.
Using it twice causes tar to print a longer listing (reminiscent
of ls -l) for each member. Since --list (-t) already prints
the names of the members, --verbose (-v) used once with --list (-t)
causes tar to print an ls -l type listing of the files
in the archive. The following examples both extract members with
long list output:
$ tar --extract --file=archive.tar --verbose --verbose $ tar xvv archive.tar
Verbose output appears on the standard output except when an archive is
being written to the standard output, as with tar --create
--file=- --verbose (tar cfv -, or even tar cv--if the
installer let standard output be the default archive). In that case
tar writes verbose output to the standard error stream.
The --totals option--which is only meaningful when used with
--create (-c)--causes tar to print the total
amount written to the archive, after it has been fully created.
The --checkpoint option prints an occasional message
as tar reads or writes the archive. It is designed for
those who don't need the more detailed (and voluminous) output of
--block-number (-R), but do want visual confirmation that tar
is actually making forward progress.
The --show-omitted-dirs option, when reading an archive--with --list (-t) or --extract (--get, -x), for example--causes a message to be printed for each directory in the archive which is skipped. This happens regardless of the reason for skipping: the directory might not have been named on the command line (implicitly or explicitly), it might be excluded by the use of the --exclude=pattern option, or some other reason.
If --block-number (-R) is used, tar prints, along with every
message it would normally produce, the block number within the archive
where the message was triggered. Also, supplementary messages are
triggered when reading blocks full of NULs, or when hitting end of file on
the archive. As of now, if the archive 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).
Typically, tar carries out a command without stopping for
further instructions. In some situations however, you may want to
exclude some files and archive members from the operation (for instance
if disk or storage space is tight). You can do this by excluding
certain files automatically (see Choosing), or by performing
a subcommand 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 action 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.
--list with file name arguments: list
ago in date stringsam in date stringsgetdate: Authors of getdate
cat vs concatenate: concatenate
concatenate vs cat: concatenate
day in date stringsexclude: exclude
exclude-from: exclude
tar: verbose
--list with: list
first in date stringsfortnight in date stringsgetdate: Date input formats
hour in date stringslast day: Day of week item
last in date stringsmidnight in date stringsminute in date stringsmonth in date stringsnext day: Day of week item
next in date stringsnoon in date stringsnow in date stringsntape devicepm in date stringsrmt: Remote Tape Server
SIMPLE_BACKUP_SUFFIX: backup
--delete and: delete
this in date stringstoday in date stringstomorrow in date stringsuuencode: Applications
tar program: verbose
version-control Emacs variableVERSION_CONTROL: backup
week in date stringsyear in date stringsyesterday in date stringstar
tar Subcommands
tar Operations
--append
--concatenate
--delete
--extract
tar Usages
tar
tar
tar Options
In addition, archives record access permissions, user and group, size in bytes, and last modification time. Some archives also record the file names in each archived directory, as well as other file and directory information.
This is well described in Unix-haters Handbook, by Simson Garfinkel, Daniel Weise & Steven Strassmann, IDG Books, ISBN 1-56884-203-1.
Well! We should say
the whole truth, here. When --sparse (-S) is selected while creating
an archive, the current tar algorithm requires sparse files to be
read twice, not once. We hope to develop a new archive format for saving
sparse files in which one pass will be sufficient.
Well! We should say
the whole truth, here. When --sparse (-S) is selected while creating
an archive, the current tar algorithm requires sparse files to be
read twice, not once. We hope to develop a new archive format for saving
sparse files in which one pass will be sufficient.
Previous versions
of tar used full regular expression matching, or before that, only
exact string matching, instead of wildcard matchers. We decided for the
sake of simplicity to use a uniform matching device through tar.
Clustering many options, the last of which
has an argument, is a rather opaque way to write options. Some wonder if
GNU getopt should not even be made helpful enough for considering
such usages as invalid.
Beware that if you precede options with a dash, you are announcing the short option style instead of the old option style; short options are decoded differently.
Before GNU
tar version 1.11.6, a bug prevented intermixing old style options
with mnemonic options in some cases.
There
are plans to merge the cpio and tar packages into a single one
which would be called paxutils. So, who knows if, one of this days,
the --version would not yield tar (GNU paxutils) 3.2