tar
tar
tar
Operations
tar
Operations
tar
Operations
--extract
tar
Usages
tar
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.
The first part of this chapter introduces you to various terms that
will recur throughout the book. It also tells you who has worked on GNU tar
and its documentation, and where you should send bug reports or
comments.
The second chapter is a tutorial (see section Tutorial
Introduction to tar) which provides a gentle
introduction for people who are new to using tar. It is
meant to be self contained, not requiring any reading from subsequent
chapters to make sense. It moves from topic to topic in a logical,
progressive order, building on information already explained.
Although the tutorial is paced and structured to allow beginners to
learn how to use tar, it is not intended solely for
beginners. The tutorial explains how to use the three most frequently
used operations (`create', `list', and `extract')
as well as two frequently used options (`file' and `verbose').
The other chapters do not refer to the tutorial frequently; however, if
a section discusses something which is a complex variant of a basic
concept, there may be a cross reference to that basic concept. (The
entire book, including the tutorial, assumes that the reader
understands some basic concepts of using a Unix-type operating system;
see section Tutorial Introduction to tar.)
The third chapter presents the remaining five operations, and
information about using tar options and option syntax.
@FIXME{this sounds more like a GNU Project Manuals Concept [tm] more than the reality. should think about whether this makes sense to say here, or not.} The other chapters are meant to be used as a reference. Each chapter presents everything that needs to be said about a specific topic.
One of the chapters (see section Date input
formats) exists in its entirety in other GNU manuals, and is mostly
self-contained. In addition, one section of this manual (see section The Standard Format) 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 forth. (In addition,
archives record access permissions, user and group, size in bytes, and
last modification time. Some archives also record the file names in
each archived directory, as well as other file and directory
information.) You can use tar to create a new
archive in a specified directory.
The files inside an archive are called members.
Within this manual, we use the term file to refer only to
files accessible in the normal ways (by ls, cat,
and so forth), and the term member to refer only to the
members of an archive. Similarly, a file name is the name of a
file, as it resides in the filesystem, and a member name is
the name of an archive member within the archive.
The term extraction refers to the process
of copying an archive member (or multiple members) into a file in the
filesystem. Extracting all the members of an archive is often called extracting
the archive. The term unpack can also be used to refer to
the extraction of many or all the members of an archive. Extracting an
archive does not destroy the archive's structure, just as creating an
archive does not destroy the copies of the files that exist outside of
the archive. You may also list the members in a given archive
(this is often thought of as "printing" them to the standard
output, or the command line), or append members to a
pre-existing archive. All of these operations can be peformed using tar.
tar Does
The tar program provides the ability to
create tar archives, as well as various other kinds of
manipulation. For example, you can use tar on previously
created archives to extract files, to store additional files, or to
update or list files which were already stored.
Initially, tar archives were used to store files
conveniently on magnetic tape. The name `tar' comes from
this use; it stands for tape archiver.
Despite the utility's name, tar can direct its output to
available devices, files, or other programs (using pipes). tar
may even access remote devices or files (as archives).
@FIXME{the following table entries need a bit of work..}
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
Named
Conventionally, tar archives are given names ending with `.tar'.
This is not necessary for tar to operate properly, but
this manual follows that convention in order to accustom readers to it
and to make examples more clear.
Often, people refer to tar archives as
"tar files," and archive members as
"files" or "entries". For people familiar with the
operation of tar, this causes no difficulty. However, in
this manual, we consistently refer to "archives" and
"archive members" to make learning to use tar
easier for novice users.
@FIXME{must ask franc,ois about this. dan hagerty thinks this might be an issue, but we're not really sure at this time. dan just tried a test case of mixing up options' orders while the variable was set, and there was no problem...}
We make some of our recommendations throughout this book for one reason
in addition to what we think of as "good sense". The main
additional reason for a recommendation is to be compliant with the
POSIX standards. If you set the shell environment variable POSIXLY_CORRECT,
GNU tar will force you to adhere to these standards.
Therefore, if this variable is set and you violate one of the POSIX
standards in the way you phrase a command, for example, GNU tar
will not allow the command and will signal an error message. You would
then have to reorder the options or rephrase the command to comply with
the POSIX standards.
There is a chance in the future that, if you set this environment
variable, your archives will be forced to comply with POSIX standards,
also. No GNU tar extensions will be allowed.
tar Authors
GNU tar was originally written by John Gilmore, and
modified by many people. The GNU enhancements were written by Jay
Fenlason, then Joy Kendall, and the whole package has been further
maintained by Thomas Bushnell, n/BSG, and finally Fran@,{c}ois Pinard,
with the help of numerous and kind users.
We wish to stress that tar is a collective work, and owes
much to all those people who reported problems, offered solutions and
other insights, or shared their thoughts and suggestions. An
impressive, yet partial list of those contributors can be found in the `THANKS'
file from the GNU tar distribution.
@FIXME{i want all of these names mentioned, Absolutely. BUT, i'm not sure i want to spell out the history in this detail, at least not for the printed book. i'm just not sure it needs to be said this way. i'll think about it.}
@FIXME{History is more important, and surely more interesting, than actual names. Quoting names without history would be meaningless. FP}
Jay Fenlason put together a draft of a GNU tar manual,
borrowing notes from the original man page from John Gilmore. This
draft has been distributed in tar versions 1.04 (or even
before?) @FIXME{huh? IMO, either we know or we don't; the parenthetical
is confusing.} through 1.10, then withdrawn in version 1.11. Thomas
Bushnell, n/BSG and Amy Gorin worked on a tutorial and manual for GNU tar.
Fran@,{c}ois Pinard put version 1.11.8 of the manual together by taking
information from all these sources and merging them. Melissa Weisshaus
finally edited and redesigned the book to create version 1.12.
@FIXME{update version number as necessary; i'm being optimistic!}
@FIXME{Someone [maybe karl berry? maybe bob chassell? maybe melissa?
maybe julie sussman?] needs to properly index the thing.}
For version 1.12, Daniel Hagerty contributed a great deal of technical consulting. In particular, he is the primary author of section Performing Backups and Restoring Files.
If you find problems or have suggestions about this program or manual, please report them to `bug-gnu-utils@prep.ai.mit.edu'.
tar
This chapter guides you through some basic examples of three tar
operations: `--create', `--list', and `--extract'.
If you already know how to use some other version of tar,
then you may not need to read this chapter. This chapter omits most
complicated details about how tar works.
This chapter is paced to allow beginners to learn about tar
slowly. At the same time, we will try to cover all the basic aspects
of these three operations. In order to accomplish both of these tasks,
we have made certain assumptions about your knowledge before reading
this manual, and the hardware you will be using:
tar commands in. When we show
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. @FIXME{this is a cop out. need to add some
simple tape drive info.}
In the examples, `$' represents a typical shell prompt. It
precedes lines you should type; to make this more clear, those lines
are shown in this font, as opposed to lines which represent
the computer's response; those lines are shown in this font,
or sometimes `like this'. When we have lines which are too
long to be displayed in any other way, we will show them like this:
This is an example of a line which would otherwise not fit in this space.
@FIXME{how often do we use smallexample?}
tar Operations
and Options
tar can take a wide variety of arguments which specify and
define the actions it will have on the particular set of files or the
archive. The main types of arguments to tar fall into one
of two classes: operations, and options.
Some arguments fall into a class called operations; exactly
one of these is both allowed and required for any instance of using tar;
you may not specify more than one. People sometimes speak of operating
modes. You are in a particular operating mode when you have
specified the operation which specifies it; there are eight operations
in total, and thus there are eight operating modes.
The other arguments fall into the class known as options. You
are not required to specify any options, and you are allowed to specify
more than one at a time (depending on the way you are using tar
at that time). Some options are used so frequently, and are so useful
for helping you type commands more carefully that they are effectively
"required". We will discuss them in this chapter.
You can write most of the tar operations and options in
any of three forms: long (mnemonic) form, short form, and old style.
Some of the operations and options have no short or "old"
forms; however, the operations and options which we will cover in this
tutorial have corresponding abbreviations. @FIXME{make sure this is
still the case, at the end} We will indicate those abbreviations
appropriately to get you used to seeing them. (Note that the "old
style" option forms exist in GNU tar for
compatibility with Unix tar. We present a full discussion
of this way of writing options and operations appears in section Old Option Style, and we discuss the other
two styles of writing options in section Mnemonic
Option Style and section Short Option Style.)
In the examples and in the text of this tutorial, we usually use the
long forms of operations and options; but the "short" forms
produce the same result and can make typing long tar
commands easier. For example, instead of typing
tar --create --verbose --file=afiles.tar apple angst aspic
you can type
tar -c -v -f afiles.tar apple angst aspic
or even
tar -cvf afiles.tar apple angst aspic
For more information on option syntax, see section Advanced GNU tar Operations. In
discussions in the text, when we name an option by its long form, we
also give the corresponding short option in parentheses.
The term, "option", can be confusing at times, since
"operations" are often lumped in with the actual, optional
"options" in certain general class statements. For example,
we just talked about "short and long forms of options and
operations". However, experienced tar users often
refer to these by shorthand terms such as, "short and long
options". This term assumes that the "operations" are
included, also. Context will help you determine which definition of
"options" to use.
Similarly, the term "command" can be confusing, as it is
often used in two different ways. People sometimes refer to tar
"commands". A tar command is the
entire command line of user input which tells tar what to
do -- including the operation, options, and any arguments (file names,
pipes, other commands, etc). However, you will also sometimes hear the
term "the tar command". When the word
"command" is used specifically like this, a person is usually
referring to the tar operation, not the whole
line. Again, use context to figure out which of the meanings the
speaker intends.
Here are the three most frequently used operations (both short and long forms), as well as a brief description of their meanings. The rest of this chapter will cover how to use these operations in detail. We will present the rest of the operations in the next chapter.
tar archive.
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.)
You can specify an argument for the --file=archive-name
(-f archive-name) option whenever you use tar;
this option determines the name of the archive file that tar
will work on.
If you don't specify this argument, then tar will use a
default, usually some physical tape drive attached to your machine. If
there is no tape drive attached, or the default is not meaningful, then tar
will print an error message. The error message might look roughly like
one of the following:
tar: can't open /dev/rmt8 : No such device or address tar: can't open /dev/rsmt0 : I/O error
To avoid confusion, we recommend that you always specfiy an archive
file name by using --file=archive-name (-f archive-name)
when writing your tar commands. For more information on
using the --file=archive-name (-f archive-name)
option, see section Choosing and Naming
Archive Files.
tar is running.
--verbose (-v) shows details about the results of
running tar. This can be especially useful when the
results might not be obvious. For example, if you want to see the
progress of tar as it writes files into the archive, you
can use the `--verbose' option. In the beginning, you may
find it useful to use `--verbose' at all times; when you
are more accustomed to tar, you will likely want to use it
at certain times but not at others. We will use `--verbose'
at times to help make something clear, and we will give many examples
both using and not using `--verbose' to show the
differences.
Sometimes, a single instance of `--verbose' on the command line will show a full, `ls' style listing of an archive or files, giving sizes, owners, and similar information. Other times, `--verbose' will only show files or members that the particular operation is operating on at the time. In the latter case, you can use `--verbose' twice in a command to get a listing such as that in the former case. For example, instead of saying
tar -cvf afiles.tar apple angst aspic
above, you might say
tar -cvvf afiles.tar apple angst aspic
This works equally well using short or long forms of options. Using long forms, you would simply write out the mnemonic form of the option twice, like this:
$ tar --create --verbose --verbose ...
Note that you must double the hyphens properly each time.
Later in the tutorial, we will give examples using `--verbose --verbose'.
--help
Optiontar prints out a
very brief list of all operations and option available for the current
version of tar available on your system.
@UNREVISED
One of the basic operations of tar is --create (-c),
which you use to create a tar archive. We will explain `--create'
first because, in order to learn about the other operations, you will
find it useful to have an archive available to practice on.
To make this easier, in this section you will first create a directory containing three files. Then, we will show you how to create an archive (inside the new directory). Both the directory, and the archive are specifically for you to practice on. The rest of this chapter and the next chapter will show many examples using this directory and the files you will create: some of those files may be other directories and other archives.
The three files you will archive in this example are called `blues', `folk', and `jazz'. The archive is called `collection.tar'.
This section will proceed slowly, detailing how to use `--create'
in verbose mode, and showing examples using both short
and long forms. In the rest of the tutorial, and in the examples in the
next chapter, we will proceed at a slightly quicker pace. This section
moves more slowly to allow beginning users to understand how tar
works.
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 @FIXME{xref to the node for
--backup!}. To add files to an existing archive, you need to use a
different option, such as --append (-r); see
section How to Add Files to Existing Archives: --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 section Short Forms with `create' for more information on this.
In this example, you type the command as shown above: `--create' is the operation which creates the new archive (`collection.tar'), and `--file' is the option which lets you give it the name you chose. The files, `blues', `folk', and `jazz', are now members of the archive, `collection.tar' (they are file name arguments to the `--create' operation) @FIXME{xref here to the discussion of file name args?}. Now that they are are in the archive, they are called archive members, not files @FIXME{xref to definitions?}.
When you create an archive, you must specify which files you
want placed in the archive. If you do not specify any archive members,
GNU tar will complain.
If you now list the contents of the working directory (ls), you will find the archive file listed as well as the files you saw previously:
blues folk jazz collection.tar
Creating the archive `collection.tar' did not destroy the copies of the files in the directory.
Keep in mind that if you don't indicate an operation, tar
will not run and will prompt you for one. If you don't name any files, tar
will complain. You must have write access to the working directory, or
else you will not be able to create an archive in that directory.
Caution: Do not attempt to use --create (-c)
to add files to an existing archive; it will delete the archive and
write a new one. Use --append (-r) instead. See
section How to Add Files to Existing Archives: --append.
If you include the --verbose (-v) option on the
command line, tar will list the files it is acting on as
it is working. In verbose mode, the create example above
would appear as:
$ tar --create --verbose --file=collection.tar blues folk jazz blues folk jazz
This example is just like the example we showed which did not use `--verbose',
except that tar generated the remaining lines (note the
different font styles).
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.
As we said before, the --create (-c) operation is
one of the most basic uses of tar, and you will use it
countless times. Eventually, you will probably want to use abbreviated
(or "short") forms of options. A full discussion of the three
different forms that options can take appears in section The Three Option Styles; for now, here is
what the previous example (including the --verbose (-v)
option) looks like using short option forms:
$ tar -cvf collection.tar blues folk jazz blues folk jazz
As you can see, the system responds the same no matter whether you use long or short option forms.
@FIXME{i don't like how this is worded:} One difference between using short and long option forms is that, although the exact placement of arguments following options is no more specific when using short forms, it is easier to become confused and make a mistake when using short forms. For example, suppose you attempted the above example in the following way:
$ tar -cfv collection.tar blues folk jazz
In this case, tar will make an archive file called `v',
containing the files `blues', `folk', and `jazz',
because the `v' is the closest "file name" to
the `-f' option, and is thus taken to be the chosen
archive file name. tar will try to add a file called `collection.tar'
to the `v' archive file; if the file `collection.tar'
did not already exist, tar will report an error
indicating that this file does not exist. If the file `collection.tar'
does already exist (e.g., from a previous command you may have run),
then tar will add this file to the archive. Because the `-v'
option did not get registered, tar will not run under `verbose'
mode, and will not report its progress.
The end result is that you may be quite confused about what happened, and possibly overwrite a file. To illustrate this further, we will show you how an example we showed previously would look using short forms.
This example,
$ tar blues --create folk --file=collection.tar jazz
is confusing as it is. When shown using short forms, however, it becomes much more so:
$ tar blues -c folk -f collection.tar jazz
It would be very easy to put the wrong string of characters immediately following the `-f', but doing that could sacrifice valuable data.
For this reason, we recommend that you pay very careful attention to
the order of options and placement of file and archive names,
especially when using short option forms. Not having the option name
written out mnemonically can affect how well you remember which option
does what, and therefore where different names have to be placed.
(Placing options in an unusual order can also cause tar to
report an error if you have set the shell environment variable, POSIXLY_CORRECT;
see section 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
section Absolute File Names. (Note also
that `collection.tar', the original archive file, has itself
been archived. tar will accept any file as a file to be
archived, regardless of its content. When `music.tar' is
extracted, the archive file `collection.tar' will be
re-written into the file system).
If you give tar a command such as
$ tar --create --file=foo.tar .
tar will report `tar: foo.tar is the archive; not
dumped'. This happens because tar creates the
archive `foo.tar' in the current directory before putting any
files into it. Then, when tar attempts to add all the
files in the directory `.' to the archive, it notices that the
file `foo.tar' is the same as the archive, and skips it. (It
makes no sense to put an archive into itself.) GNU tar
will continue in this case, and create the archive normally, except
for the exclusion of that one file. (Please note: Other
versions of tar are not so clever; they will enter an
infinite loop when this happens, so you should not depend on this
behavior unless you are certain you are running GNU tar.
@FIXME{bob doesn't like this sentence, since he does it all the time,
and we've been doing it in the editing passes for this manual: In
general, make sure that the archive is not inside a directory being
dumped.})
Frequently, you will find yourself wanting to determine exactly what a particular archive contains. You can use the --list (-t) operation to get the member names as they currently appear in the archive, as well as various attributes of the files at the time they were archived. For example, you can examine the archive `collection.tar' that you created in the last section with the command,
$ tar --list --file=collection.tar
The output of tar would then be:
blues folk jazz
@FIXME{we hope this will change. if it doesn't, need to show the creation of bfiles somewhere above!!! : }
The archive `bfiles.tar' would list as follows:
./birds baboon ./box
Be sure to use a --file=archive-name (-f archive-name) option just as with --create (-c) to specify the name of the archive.
If you use the --verbose (-v) option with `--list',
then tar will print out a listing reminiscent of `ls
-l', showing owner, file size, and so forth.
If you had used --verbose (-v) mode, the example above would look like:
$ tar --list --verbose --file=collection.tar folk -rw-rw-rw- myself user 62 1990-05-23 10:55 folk
You can specify one or more individual member names as
arguments when using `list'. In this case, tar
will only list the names of members you identify. For example, tar
--list --file=afiles.tar apple would only print `apple'.
@FIXME{we hope the relevant aspects of this will change:}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
section Absolute File Names.
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.
@UNREVISED
@FIXME{i changed the order of these nodes around and haven't had a chance to play around with this node's example, yet. i have to play with it and see what it actually does for my own satisfaction, even if what it says *is* correct..}
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.
@UNREVISED
Creating an archive is only half the job--there is no point in storing files in an archive if you can't retrieve them. The act of retrieving members from an archive so they can be used and manipulated as unarchived files again is called extraction. To extract files from an archive, use the --extract (--get, -x) operation. As with --create (-c), specify the name of the archive with --file=archive-name (-f archive-name). Extracting an archive does not modify the archive in any way; you can extract it multiple times if you want or need to.
Using `--extract', you can extract an entire archive, or specific files. The files can be directories containing other files, or not. As with --create (-c) and --list (-t), you may use the short or the long form of the operation without affecting the performance.
To extract an entire archive, specify the archive file name only, with no individual file names as arguments. For example,
$ tar -xvf collection.tar
produces this:
-rw-rw-rw- me user 28 1996-10-18 16:31 jazz -rw-rw-rw- me user 21 1996-09-23 16:44 blues -rw-rw-rw- me user 20 1996-09-23 16:44 folk
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. @FIXME{check this; will the times, permissions, owner, etc be the same, also?}
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. @FIXME{This is only accidentally true, but
not in general. In most cases, one has to be root for restoring the
owner, and use a special option for restoring permissions. Here, it
just happens that the restoring user is also the owner of the archived
members, and that the current umask is compatible with
original permissions.} (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).
@FIXME{we hope this will change:}Remember that as with other operations, specifying the exact member name is important. tar --extract --file=bfiles.tar birds will fail, because there is no member named `birds'. To extract the member named `./birds', you must specify tar --extract --file=bfiles.tar ./birds. To find the exact member names of the members of an archive, use --list (-t) (see section How to List Archives).
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.
However, if a file was stored with a directory name as part of its file
name, and that directory does not exist under the working directory
when the file is extracted, tar will create the directory.
We can demonstrate how to use `--extract' to extract a directory file with an example. Change to the `practice' directory if you weren't there, and remove the files `folk' and `jazz'. Then, go back to the parent directory and extract the archive `music.tar'. You may either extract the entire archive, or you may extract only the files you just deleted. To extract the entire archive, don't give any file names as arguments after the archive name `music.tar'. To extract only the files you deleted, use the following command:
$ tar -xvf music.tar practice/folk practice/jazz
@FIXME{need to show tar's response; used verbose above. also, here's a good place to demonstrate the -v -v thing. have to write that up (should be trivial, but i'm too tired!).}
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.
@FIXME{IMPORTANT! show the final structure, here. figure out what it will be.}
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
@FIXME{make sure the above works when going through the examples in order...}
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.
@FIXME{more examples, here? hag thinks it's a good idea.}
@FIXME{need to write up a node here about the things that are going to be in the rest of the manual.}
tar@UNREVISED
This chapter is about how one invokes the GNU tar command,
from the command synopsis (see section General
Synopsis of tar). There are numerous options, and many
styles for writing them. One mandatory option specifies the operation tar
should perform (see section Operations),
other options are meant to detail how this operation should be
performed (see section tar Options).
Non-option arguments are not always interpreted the same way, depending
on what the operation is.
You will find in this chapter everything about option styles and rules
for writing them (see section The Three Option
Styles). On the other hand, operations and options are fully
described elsewhere, in other chapters. Here, you will find only
synthetic descriptions for operations and options, together with
pointers to other parts of the tar manual.
Some options are so special they are fully described right in this
chapter. They have the effect of inhibiting the normal operation of tar
or else, they globally alter the amount of feedback the user receives
about what is going on. These are the --help and --version
(see section GNU tar
documentation), --verbose (-v) (see section Checking tar progress) and --interactive
(-w) options (see section Asking
for Confirmation During Operations).
tar
The GNU tar program is invoked as either one of:
tar option... [name]... tar letter... [argument]... [option]... [name]...
The second form is for when old options are being used.
You can use tar to store files in an archive, to extract
them from an archive, and to do other types of archive manipulation.
The primary argument to tar, which is called the operation,
specifies which action to take. The other arguments to tar
are either options, which change the way tar
performs an operation, or file names or archive members, which specify
the files or members tar is to act on.
You can actually type in arguments in any order, even if in this manual
the options always precede the other arguments, to make examples easier
to understand. Further, the option stating the main operation mode (the tar
main command) is usually given first.
Each name in the synopsis above is interpreted as an archive
member name when the main command is one of --compare (--diff, -d), --delete, --extract
(--get, -x), --list (-t)
or --update (-u). When naming archive members,
you must give the exact name of the member in the archive, as it is
printed by --list (-t). For --append (-r)
and --create (-c), these name
arguments specify the names of either files or directory hierarchies
to place in the archive. These files or hierarchies should already
exist in the file system, prior to the execution of the tar
command.
tar interprets relative file names as being relative to
the working directory. tar will make all file names
relative (by removing leading slashes when archiving or restoring
files), unless you specify otherwise (using the --absolute-names
(-P) option). See section Absolute
File Names, 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 section Wildcards
Patterns and Matching, for more information about globbing. The
problem is that shells may only glob using existing files in the file
system. Only tar itself may glob on archive members, so
when needed, you must ensure that wildcard characters reach tar
without being interpreted by the shell first. Using a backslash before `*'
or `?', or putting the whole argument between quotes, is
usually sufficient for this.
Even if names are often specified on the command line, they can also be read from a text file in the file system, using the --files-from=file-of-names (-T file-of-names) option.
If you don't use any file name arguments, --append (-r), --delete
and --concatenate (--catenate, -A)
will do nothing, while --create (-c) will usually
yield a diagnostic and inhibit tar execution. The other
operations of tar (--list (-t), --extract
(--get, -x), --compare (--diff, -d),
and --update (-u)) will act on the entire
contents of the archive.
Besides successful exits, GNU tar may
fail for many reasons. Some reasons correspond to bad usage, that is,
when the tar command is improperly written. Errors may be
encountered later, while encountering an error processing the archive
or the files. Some errors are recoverable, in which case the failure is
delayed until tar has completed all its work. Some errors
are such that it would not meaningful, or at least risky, to continue
processing: tar then aborts processing immediately. All
abnormal exits, whether immediate or delayed, should always be clearly
diagnosed on stderr, after a line stating the nature of
the error.
GNU tar returns only a few exit statuses. I'm really
aiming simplicity in that area, for now. If you are not using the --compare
(--diff, -d) option, zero means that everything
went well, besides maybe innocuous warnings. Nonzero means that
something went wrong. Right now, as of today, "nonzero" is
almost always 2, except for remote operations, where it may be 128.
tar Options
GNU tar has a total of eight operating modes which allow
you to perform a variety of tasks. You are required to choose one
operating mode each time you employ the tar program by
specifying one, and only one operation as an argument to the tar
command (two lists of four operations each may be found at section The Three Most Frequently Used Operations
and section The Five Advanced tar
Operations). Depending on circumstances, you may also wish to
customize how the chosen operating mode behaves. For example, you may
wish to change the way the output looks, or the format of the files
that you wish to archive may require you to do something special in
order to make the archive look right.
You can customize and control tar's performance by running tar
with one or more options (such as --verbose (-v),
which we used in the tutorial). As we said in the tutorial, options
are arguments to tar which are (as their name suggests)
optional. Depending on the operating mode, you may specify one or more
options. Different options will have different effects, but in general
they all change details of the operation, such as archive format,
archive name, or level of user interaction. Some options make sense
with all operating modes, while others are meaningful only with
particular modes. You will likely use some options frequently, while
you will only use others infrequently, or not at all. (A full list of
options is available in see section All tar
Options.)
Note that tar options are case sensitive. For example, the
options `-T' and `-t' are different; the
first requires an argument for stating the name of a file providing a
list of names, while the second does not require an argument
and is another way to write --list (-t).
In addition to the eight operations, there are many options to tar,
and three different styles for writing both: long (mnemonic) form,
short form, and old style. These styles are discussed below. Both the
options and the operations can be written in any of these three styles.
@FIXME{menu at end of this node. need to think of an actual outline for this chapter; probably do that after stuff from chap. 4 is incorporated.}
There are three styles for writing operations and options to the
command line invoking tar. The different styles were
developed at different times during the history of tar.
These styles will be presented below, from the most recent to the
oldest.
Some options must take an argument. (For example, --file=archive-name
(-f archive-name) takes the name of an archive
file as an argument. If you do not supply an archive file name, tar
will use a default, but this can be confusing; thus, we recommend that
you always supply a specific archive file name.) Where you place
the arguments generally depends on which style of options you choose.
We will detail specific information relevant to each option style in
the sections on the different option styles, below. The differences are
subtle, yet can often be very important; incorrect option placement can
cause you to overwrite a number of important files. We urge you to note
these differences, and only use the option style(s) which makes the
most sense to you until you feel comfortable with the others.
@FIXME{hag to write a brief paragraph on the option(s) which can optionally take an argument}
@FIXME{have to decide whether or ot to replace other occurrences of "mnemonic" with "long", or *ugh* vice versa.}
Each option has at least one long (or mnemonic) name starting with two
dashes in a row, e.g. `list'. The long names are more
clear than their corresponding short or old names. It sometimes happens
that a single mnemonic option has many different different names which
are synonymous, such as `--compare' and `--diff'.
In addition, long option names can be given unique abbreviations. For
example, `--cre' can be used in place of `--create'
because there is no other mnemonic option which begins with `cre'.
(One way to find this out is by trying it and seeing what happens; if a
particular abbreviation could represent more than one option, tar
will tell you that that abbreviation is ambiguous and you'll know that
that abbreviation won't work. You may also choose to run `tar
--help' to see a list of options. Be aware that if you run tar
with a unique abbreviation for the long name of an option you didn't
want to use, you are stuck; tar will perform the command
as ordered.)
Mnemonic options are meant to be obvious and easy to remember, and their meanings are generally easier to discern than those of their corresponding short options (see below). For example:
$ tar --create --verbose --blocking-factor=20 --file=/dev/rmt0
gives a fairly good set of hints about what the command does, even for
those not fully acquainted with tar.
Mnemonic options which require arguments take those arguments
immediately following the option name; they are introduced by an equal
sign. For example, the `--file' option (which tells the
name of the tar archive) is given a file such as `archive.tar'
as argument by using the notation `--file=archive.tar'
for the mnemonic option.
Most options also have a short option name. Short options start with a single dash, and are followed by a single character, e.g. `-t' (which is equivalent to `--list'). The forms are absolutely identical in function; they are interchangeable.
The short option names are faster to type than long option names.
Short options which require arguments take their arguments immediately following the option, usually separated by white space. It is also possible to stick the argument right after the short option name, using no intervening space. For example, you might write `-f archive.tar' or `-farchive.tar' instead of using `--file=archive.tar'. Both `--file=archive-name' and `-f archive-name' denote the option which indicates a specific archive, here named `archive.tar'.
Short options' letters may be clumped together, but you are not
required to do this (as compared to old options; see below). When short
options are clumped as a set, use one (single) dash for them all, e.g. `tar
-cvf'. Only the last option in such a set is allowed to have an
argument(1).
When the options are separated, the argument for each option which requires an argument directly follows that option, as is usual for Unix programs. For example:
$ tar -c -v -b 20 -f /dev/rmt0
If you reorder short options' locations, be sure to move any arguments that belong to them. If you do not move the arguments properly, you may end up overwriting files.
@UNREVISED
Like short options, old options are single letters. However, old
options must be written together as a single clumped set, without
spaces separating them or dashes preceding them(2). This set of letters must be the first to appear
on the command line, after the tar program name and some
whitespace; old options cannot appear anywhere else. The letter of an
old option is exactly the same letter as the corresponding short
option. For example, the old option `t' is the same as the
short option `-t', and consequently, the same as the
mnemonic option `--list'. So for example, the command `tar
cv' specifies the option `-v' in addition to the
operation `-c'.
@FIXME{bob suggests having an uglier example. :-) }
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
@FIXME{still could explain this better; it's redundant:}
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 respected(3). Old style options and
either of the modern styles of options may be mixed within a single tar
command. However, old style options must be introduced as the first
arguments only, following the rule for old options (old options must
appear directly after the tar command and some
whitespace). Modern options may be given only after all arguments to
the old options have been collected. If this rule is not respected, a
modern option might be falsely interpreted as the value of the argument
to one of the old style options.
For example, all the following commands are wholly equivalent, and illustrate the many combinations and orderings of option styles.
tar --create --file=archive.tar tar --create -f archive.tar tar --create -farchive.tar tar --file=archive.tar --create tar --file=archive.tar -c tar -c --file=archive.tar tar -c -f archive.tar tar -c -farchive.tar tar -cf archive.tar tar -cfarchive.tar tar -f archive.tar --create tar -f archive.tar -c tar -farchive.tar --create tar -farchive.tar -c tar c --file=archive.tar tar c -f archive.tar tar c -farchive.tar tar cf archive.tar tar f archive.tar --create tar f archive.tar -c tar fc archive.tar
On the other hand, the following commands are not equivalent to the previous set:
tar -f -c archive.tar tar -fc archive.tar tar -fcarchive.tar tar -farchive.tarc tar cfarchive.tar
These last examples mean something completely different from what the
user intended (judging based on the example in the previous set which
uses long options, whose intent is therefore very clear). The first
four specify that the tar archive would be a file named `-c', `c', `carchive.tar'
or `archive.tarc', respectively. The first two examples
also specify a single non-option, name argument having the
value `archive.tar'. The last example contains only old
style option letters (repeating option `c' twice), not all
of which are meaningful (eg., `.', `h', or `i'),
with no argument value. @FIXME{not sure i liked the first sentence of
this paragraph..}
tar Options
The coming manual sections contain an alphabetical listing of all tar
operations and options, with brief descriptions and cross references
to more in-depth explanations in the body of the manual. They also
contain an alphabetically arranged table of the short option forms with
their corresponding long option. You can use this table as a reference
for deciphering tar commands in scripts.
--append.
--concatenate.
tar archives to the end of the archive.
See section Combining Archives with --concatenate.
tar archive. See section How to Create Archives.
tar Optionstar strips an
initial `/' from member names. This option disables
that behavior. @FIXME-xref{}.
tar to preserve the access time field in a
file's inode when dumping it. @FIXME-xref{}.
tar
will back them up using simple or numbered backups, depending upon backup-type.
@FIXME-xref{}.
tar prints error messages
for read errors with the block number in the archive file.
@FIXME-xref{}.
tar uses to blocking
x 512 bytes per record. @FIXME-xref{}.
tar to print periodic checkpoint
messages as it reads through the archive. Its intended for when you
want a visual indication that tar is still running,
but don't want to see `--verbose' output.
@FIXME-xref{}.
tar will use the compress program when
reading or writing the archive. This allows you to directly act on
archives while saving space. @FIXME-xref{}.
tar archive, tar will
archive the file that a symbolic link points to, rather than archiving
the symlink. @FIXME-xref{}.
tar will change its
current directory to dir before performing any
operations. When this option is used during archive creation, it is
order sensitive. @FIXME-xref{}.
tar will skip files that
match pattern. @FIXME-xref{}.
tar will
use the list of patterns in the file file.
@FIXME-xref{}.
tar will use the file archive as the tar
archive it performs operations on, rather than tar's
compilation dependent default. @FIXME-xref{}.
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. @FIXME-xref{}.
tar to interpret the filename given to `--file'
as a local file, even if it looks like a remote tape drive name.
@FIXME-xref{}.
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. @FIXME-xref{}. Also see
the comments for the --owner=user option.
tar to read or write archives
through gzip, allowing tar to directly
operate on several kinds of compressed archives transparently.
@FIXME-xref{}.
tar will print out a short message summarizing the
operations and options to tar and exit. @FIXME-xref{}.
tar to exit successfully if it encounters an
unreadable file. See section Options to
Help Read Archives.
tar
Writes Files.)
tar will ignore zeroed blocks in the
archive, which normally signals EOF. See section Options to Help Read Archives.
tar that it is working with an old
GNU-format incremental backup archive. It is intended primarily for
backwards compatibility only. @FIXME-xref{}.
tar is performing multi-tape backups, script-file
is run at the end of each tape. @FIXME-xref{}.
tar should ask the user for
confirmation before performing potentially destructive options, such as
overwriting files. @FIXME-xref{}.
tar will not
overwrite existing files if this option is present. See section Changing How tar Writes 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. @FIXME-xref{}.
tar creates is a new GNU-format
incremental backup, using snapshot-file to determine
which files to backup. With other operations, informs tar
that the archive is in incremental format. @FIXME-xref{}.
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
section `File permissions' in GNU file utilities. 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. @FIXME-xref{}.
tar will only add files that
have changed since date. @FIXME-xref{}.
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).
tar will not recurse into
directories unless a directory is explicitly named as an argument to tar.
@FIXME-xref{}.
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. @FIXME-xref{}.
tar that it should use numeric
user and group IDs when creating a tar file, rather
than names. @FIXME-xref{}.
tar from
recursing into directories that are on different file systems from the
current directory. @FIXME-xref{}.
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. @FIXME-xref{}. 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 to create an archive that is compatible with
Unix V7 tar. @FIXME-xref{}.
tar to create a POSIX compliant tar
archive. @FIXME-xref{}.
tar is extracting an archive, it normally
subtracts the users' umask from the permissions specified in the
archive and uses that number as the permissions to create the
destination file. Specifying this option instructs tar
that it should use the permissions directly from the archive. See
section Changing How tar
Writes Files.
tar should reblock its input, for
reading from pipes on systems with buggy implementations. See section Options to Help Read Archives.
tar to use size bytes per record
when accessing the archive. @FIXME-xref{}.
tar Writes Files.
tar to remove the source file from the file
system after appending it to an archive. @FIXME-xref{}.
tar that is should use cmd to
communicate with remote devices. @FIXME-xref{}.
tar when running on
machines with small amounts of memory. It informs tar
that the list of file arguments has already been sorted to match the
order of files in the archive. See section Options
to Help Read Archives.
tar will attempt to
preserve the owner specified in the tar archive with
this option present. @FIXME-xref{}.
tar Writes Files.)
tar to mention directories its skipping over
when operating on a tar archive. @FIXME-xref{}.
tar will skip
extracting files in the archive until it finds one that matches name.
See section Coping with Scarce Resources.
tar uses when backing up files from
the default `~'. @FIXME-xref{}.
tar is writing as
being num x 1024 bytes long. @FIXME-xref{}.
tar will extract files to stdout
rather than to the file system. See section Changing How tar Writes Files.
tar Writes Files.
tar to remove the corresponding file from the
file system before extracting it from the archive. See section Changing How tar Writes Files.
tar to access the archive through prog,
which is presumed to be a compression program of some sort.
@FIXME-xref{}.
tar should be more verbose about the
operations its performing. This option can be specified multiple times
for some operations to increase the amount of information displayed.
@FIXME-xref{}.
tar will print an informational message about what
version it is and a copyright message, some credits, and then exit.
@FIXME-xref{}.
tar
will keep track of which volume of a multi-volume archive its working
in file. @FIXME-xref{}.
Here is an alphabetized list of all of the short option forms, matching them with the equivalent long option.
tar documentation
Being careful, the first thing is really checking that you are using
GNU tar, indeed. The --version option will
generate a message giving confirmation that you are using GNU tar,
with the precise version of GNU tar you are using. tar
identifies itself and prints the version number to the standard
output, then immediately exits successfully, without doing anything
else, ignoring all other options. For example, `tar --version'
might return:
tar (GNU tar) 1.12
The first occurrence of `tar' in the result above is the
program name in the package (for example, rmt is another
program), while the second occurrence of `tar' is the name
of the package itself, containing possibly many programs. The package
is currently named `tar', after the name of the main
program it contains(4).
Another thing you might want to do is checking the spelling or meaning
of some particular tar option, without resorting to this
manual, for once you have carefully read it. GNU tar has a
short help feature, triggerable through the --help option.
By using this option, tar will print a usage message
listing all available options on standard output, then exit
successfully, without doing anything else and ignoring all other
options. Even if this is only a brief summary, it may be several
screens long. So, if you are not using some kind of scrollable window,
you might prefer to use something like:
$ tar --help | less
presuming, here, that you like using less for a pager.
Other popular pagers are more and pg. If you
know about some keyword which interests you and do not want
to read all the --help output, another common idiom is
doing:
tar --help | grep keyword
for getting only the pertinent lines.
The perceptive reader would have noticed some contradiction in the previous paragraphs. It is written that both --version and --help print something, and have all other options ignored. In fact, they cannot ignore each other, and one of them has to win. We do not specify which is stronger, here; experiment if you really wonder!
The short help output is quite succint, and you might have to get back
to the full documentation for precise points. If you are reading this
paragraph, you already have the tar manual in some form.
This manual is available in printed form, as a kind of small book. It
may printed out of the GNU tar distribution, provided you
have TeX already installed somewhere, and a laser printer around. Just
configure the distribution, execute the command `make dvi',
then print `doc/tar.dvi' the usual way (contact your local
guru to know how). If GNU tar has been conveniently
installed at your place, this manual is also available in interactive,
hypertextual form as an Info file. Just call `info tar'
or, if you do not have the info program handy, use the
Info reader provided within GNU Emacs, calling `tar' from
the main Info menu.
There is currently no man page for GNU tar.
If you observe such a man page on the system you are
running, either it does not long to GNU tar, or it has not
been produced by GNU. Currently, GNU tar documentation is
provided in Texinfo format only, if we except, of course, the short
result of tar --help.
tar progress
Typically, tar performs most
operations without reporting any information to the user except error
messages. When using tar with many options, particularly
ones with complicated or difficult-to-predict behavior, it is possible
to make serious mistakes. tar provides several options
that make observing tar easier. These options cause tar
to print information as it progresses in its job, and you might want
to use them just for being more careful about what is going on, or
merely for entertaining yourself. If you have encountered a problem
when operating on an archive, however, you may need more information
than just an error message in order to solve the problem. The following
options can be helpful diagnostic tools.
Normally, the --list (-t) command to list an
archive prints just the file names (one per line) and the other
commands are silent. When used with most operations, the --verbose
(-v) option causes tar to print the name of
each file or archive member as it is processed. This and the other
options which make tar print status information can be
useful in monitoring tar.
With --create (-c) or --extract (--get, -x), --verbose
(-v) used once just prints the names of the files or
members as they are processed. Using it twice causes tar
to print a longer listing (reminiscent of `ls -l') for
each member. Since --list (-t) already prints the
names of the members, --verbose (-v) used once
with --list (-t) causes tar to print
an `ls -l' type listing of the files in the archive. The
following examples both extract members with long list output:
$ tar --extract --file=archive.tar --verbose --verbose $ tar xvv archive.tar
Verbose output appears on the standard output except when an archive is
being written to the standard output, as with `tar --create
--file=- --verbose' (`tar cfv -', or even `tar
cv'---if the installer let standard output be the default
archive). In that case tar writes verbose output to the
standard error stream.
The --totals option--which is only meaningful when used with --create
(-c)---causes tar to print the total amount
written to the archive, after it has been fully created.
The --checkpoint option prints an occasional message as tar
reads or writes the archive. In fact, it print directory names while
reading the archive. It is designed for those who don't need the more
detailed (and voluminous) output of --block-number (-R),
but do want visual confirmation that tar is actually
making forward progress.
@FIXME{There is some confusion here. It seems that -R once wrote a message at `every' record read or written.}
The --show-omitted-dirs option, when reading an archive--with --list (-t) or --extract (--get, -x), for example--causes a message to be printed for each directory in the archive which is skipped. This happens regardless of the reason for skipping: the directory might not have been named on the command line (implicitly or explicitly), it might be excluded by the use of the --exclude=pattern option, or some other reason.
If --block-number (-R) is used, tar
prints, along with every message it would normally produce, the block
number within the archive where the message was triggered. Also,
supplementary messages are triggered when reading blocks full of NULs,
or when hitting end of file on the archive. As of now, if the archive
if properly terminated with a NUL block, the reading of the file may
stop before end of file is met, so the position of end of file will not
usually show when --block-number (-R) is used.
Note that GNU tar drains the archive before exiting when
reading the archive from a pipe.
This option is especially useful when reading damaged archives, since it helps pinpoint the damaged sections. It can also be used with --list (-t) when listing a file-system backup tape, allowing you to choose among several backup tapes when retrieving a file later, in favor of the tape where the file appears earliest (closest to the front of the tape). @FIXME-xref{when the node name is set and the backup section written}.
Typically, tar carries out a command
without stopping for further instructions. In some situations however,
you may want to exclude some files and archive members from the
operation (for instance if disk or storage space is tight). You can do
this by excluding certain files automatically (see section Choosing Files and Names for tar),
or by performing an operation interactively, using the --interactive
(-w) option. tar also accepts `--confirmation'
for this option.
When the --interactive (-w) option is specified,
before reading, writing, or deleting files, tar first
prints a message for each such file, telling what operation it intends
to take, then asks for confirmation on the terminal. The actions which
require confirmation include adding a file to the archive, extracting a
file from the archive, deleting a file from the archive, and deleting a
file from disk. To confirm the action, you must type a line of input
beginning with `y'. If your input line begins with
anything other than `y', tar skips that file.
If tar is reading the archive from the standard input, tar
opens the file `/dev/tty' to support the interactive
communications.
Verbose output is normally sent to standard output, separate from other
error messages. However, if the archive is produced directly on
standard output, then verbose output is mixed with errors on stderr.
Producing the archive on standard output may be used as a way to avoid
using disk space, when the archive is soon to be consumed by another
process reading it, say. Some people felt the need of producing an
archive on stdout, still willing to segregate between verbose output
and error output. A possible approach would be using a named pipe to
receive the archive, and having the consumer process to read from that
named pipe. This has the advantage of letting standard output free to
receive verbose output, all separate from errors.
tar Operationstar
Operations
The basic tar operations, --create (-c), --list
(-t) and --extract (--get, -x),
are currently presented and described in the tutorial chapter of this
manual. This section provides some complementary notes for these
operations.
tar
to destroy a magnetic tape with an empty archive(5). 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 :-).
file, when the intent
was to create an archive with a single file in it. This error is likely
because a tired user can easily add the f key to the
cluster of option letters, by the mere force of habit, without
realizing the full consequence of doing so. The usual consequence is
that the single file, which was meant to be saved, is rather destroyed.
So, recognizing the likelihood and the catastrophical nature of these
errors, GNU tar now takes some distance from elegance,
and cowardly refuses to create an archive when --create (-c)
option is given, there are no arguments besides options, and --files-from=file-of-names
(-T file-of-names) option is not
used. To get around the cautiousness of GNU tar and
nevertheless create an archive with nothing in it, one may still use,
as the value for the --files-from=file-of-names
(-T file-of-names) option, a file with no
names in it, as shown in the following commands:
tar --create --file=empty-archive.tar --files-from=/dev/null tar cfT empty-archive.tar /dev/null
tar archive, as a
pipe.
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.
tar
Operations
Now that you have learned the basics of using GNU tar, you
may want to learn about further ways in which tar can help
you.
This chapter presents five, more advanced operations which you probably
won't use on a daily basis, but which serve more specialized functions.
We also explain the different styles of options and why you might want
to use one or another, or a combination of them in your tar
commands. Additionally, this chapter includes options which allow you
to define the output from tar more carefully, and provide
help and error correction in special circumstances.
@FIXME{check this after the chapter is actually revised to make sure it still introduces the info in the chapter correctly : ).}
tar
Operations@UNREVISED
In the last chapter, you learned about the first three operations to tar.
This chapter presents the remaining five operations to tar: `--append', `--update', `--concatenate', `--delete',
and `--compare'.
You are not likely to use these operations as frequently as those covered in the last chapter; however, since they perform specialized functions, they are quite useful when you do need to use them. We will give examples using the same directory and files that you created in the last chapter. As you may recall, the directory is called `practice', the files are `jazz', `blues', `folk', `rock', and the two archive files you created are `collection.tar' and `music.tar'.
We will also use the archive files `afiles.tar' and `bfiles.tar'. `afiles.tar' contains the members `apple', `angst', and `aspic'. `bfiles.tar' contains the members `./birds', `baboon', and `./box'.
Unless we state otherwise, all practicing you do and examples you follow in this chapter will take place in the `practice' directory that you created in the previous chapter; see section Preparing a Practice Directory for Examples. (Below in this section, we will remind you of the state of the examples where the last chapter left them.)
The five operations that we will cover in this chapter are:
Currently, the listing of the directory using ls is as
follows:
The archive file `collection.tar' looks like this:
$ tar -tvf collection.tar
The archive file `music.tar' looks like this:
$ tar -tvf music.tar
@FIXME{need to fill in the above!!!}
--append@UNREVISED
If you want to add files to an existing archive, you don't need to create a new archive; you can use --append (-r). The archive must already exist in order to use `--append'. (A related operation is the `--update' operation; you can use this to add newer versions of archive members to an existing archive. To learn how to do this with `--update', see section Updating an Archive.)
@FIXME{Explain in second paragraph whether you can get to the previous version -- explain whole situation somewhat more clearly.}
If you use --append (-r) to add a file that has
the same name as an archive member to an archive containing that
archive member, then the old member is not deleted. What does happen,
however, is somewhat complex. tar allows you to
have infinite numbers of files with the same name. Some operations
treat these same-named members no differently than any other set of
archive members: for example, if you view an archive with --list
(-t), you will see all of those members listed, with their
modification times, owners, etc.
Other operations don't deal with these members as perfectly as you
might prefer; if you were to use --extract (--get, -x)
to extract the archive, only the most recently added copy of a member
with the same name as four other members would end up in the working
directory. This is because `--extract' extracts an archive
in the order the members appeared in the archive; the most recently
archived members will be extracted last. Additionally, an extracted
member will overwrite a file of the same name which existed in
the directory already, and tar will not prompt you about
this. Thus, only the most recently archived member will end up being
extracted, as it will overwrite the one extracted before it, and so on.
@FIXME{ hag -- you might want to incorporate some of the above into the MMwtSN node; not sure. i didn't know how to make it simpler...}
There are a few ways to get around this. @FIXME-xref{Multiple Members with the Same Name}.
If you want to replace an archive member, use --delete to delete the member you want to remove from the archive, , and then use `--append' to add the member you want to be in the archive. Note that you can not change the order of the archive; the most recently added member will still appear last. In this sense, you cannot truely "replace" one member with another. (Replacing one member with another will not work on certain types of media, such as tapes; see section Removing Archive Members Using `--delete' and section Tapes and Other Archive Media, for more information.)
@UNREVISED
The simplest way to add a file to an already existing archive is the --append (-r) operation, which writes specified files into the archive whether or not they are already among the archived files. When you use `--append', you must specify file name arguments, as there is no default. If you specify a file that already exists in the archive, another copy of the file will be added to the end of the archive. As with other operations, the member names of the newly added files will be exactly the same as their names given on the command line. The --verbose (-v) option will print out the names of the files as they are written into the archive.
`--append' cannot be performed on some tape drives,
unfortunately, due to deficiencies in the formats those tape drives
use. The archive must be a valid tar archive, or else the
results of using this operation will be unpredictable. See section Tapes and Other Archive Media.
To demonstrate using `--append' to add a file to an
archive, create a file called `rock' in the `practice'
directory. Make sure you are in the `practice' directory.
Then, run the following tar command to add `rock'
to `collection.tar':
$ tar --append --file=collection.tar rock
If you now use the --list (-t) operation, you will see that `rock' has been added to the archive:
$ tar --list --file=collection.tar -rw-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
@FIXME{in theory, dan will (soon) try to turn this node into what it's title claims it will become...}
You can use --append (-r) to add copies of files
which have been updated since the archive was created. (However, we do
not recommend doing this since there is another tar option
called `--update'; see section Updating
an Archive for more information. We describe this use of `--append'
here for the sake of completeness.) @FIXME{is this really a good idea,
to give this whole description for something which i believe is
basically a Stupid way of doing something? certain aspects of it show
ways in which tar is more broken than i'd personally like to admit to,
specifically the last sentence. On the other hand, i don't think it's a
good idea to be saying that re explicitly don't recommend using
something, but i can't see any better way to deal with the situation.}
When you extract the archive, the older version will be effectively
lost. This works because files are extracted from an archive in the
order in which they were archived. Thus, when the archive is extracted,
a file archived later in time will overwrite a file of the same name
which was archived earlier, even though the older version of the file
will remain in the archive unless you delete all versions of the file.
Supposing you change the file `blues' and then append the changed version to `collection.tar'. As you saw above, the original `blues' is in the archive `collection.tar'. If you change the file and append the new version of the file to the archive, there will be two copies in the archive. When you extract the archive, the older version of the file will be extracted first, and then overwritten by the newer version when it is extracted.
You can append the new, changed copy of the file `blues' to the archive in this way:
$ tar --append --verbose --file=collection.tar blues blues
Because you specified the `--verbose' option, tar
has printed the name of the file being appended as it was acted on.
Now list the contents of the archive:
$ tar --list --verbose --file=collection.tar -rw-rw-rw- me user 28 1996-10-18 16:31 jazz -rw-rw-rw- me user 21 1996-09-23 16:44 blues -rw-rw-rw- me user 20 1996-09-23 16:44 folk -rw-rw-rw- me user 20 1996-09-23 16:44 rock -rw-rw-rw- me user 58 1996-10-24 18:30 blues
The newest version of `blues' is now at the end of the archive
(note the different creation dates and file sizes). If you extract the
archive, the older version of the file `blues' will be
overwritten by the newer version. You can confirm this by extracting
the archive and running `ls' on the directory. See section Changing How tar Writes Files
for more information. (Please note: This is the case unless
you employ the --backup option; @FIXME-ref{Multiple Members
with the Same Name}.)
@UNREVISED
In the previous section, you learned how to use --append
(-r) to add a file to an existing archive. A related
operation is --update (-u). The `--update'
operation updates a tar archive by comparing the date of
the specified archive members against the date of the file with the
same name. If the file has been modified more recently than the archive
member, then the newer version of the file is added to the archive (as
with --append (-r)).
Unfortunately, you cannot use `--update' with magnetic tape drives. The operation will fail.
@FIXME{other examples of media on which --update will fail? need to ask charles and/or mib/thomas/dave shevett..}
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 (@FIXME-ref{Multiple Members with the Same Name}).
--update
You must use file name arguments with the --update (-u)
operation. If you don't specify any files, tar won't act
on any files and won't tell you that it didn't do anything (which may
end up confusing you).
@FIXME{note: the above parenthetical added because in fact, this behavior just confused the author. :-) }
To see the `--update' option at work, create a new file, `classical',
in your practice directory, and some extra text to the file `blues',
using any text editor. Then invoke tar with the `update'
operation and the --verbose (-v) option
specified, using the names of all the files in the practice directory
as file name arguments:
$ tar --update -v -f collection.tar blues folk rock classical blues classical $
Because we have specified verbose mode, tar prints out the
names of the files it is working on, which in this case are the names
of the files that needed to be updated. If you run `tar --list'
and look at the archive, you will see `blues' and `classical'
at its end. There will be a total of two versions of the member `blues';
the one at the end will be newer and larger, since you added text
before updating it.
(The reason tar does not overwrite the older file when
updating it is because writing to the middle of a section of tape is a
difficult process. Tapes are not designed to go backward. See section Tapes and Other Archive Media for more
information about tapes.
--update (-u) is not suitable for performing
backups for two reasons: it does not change directory content entries,
and it lengthens the archive every time it is used. The GNU tar
options intended specifically for backups are more efficient. If you
need to run backups, please consult section Performing
Backups and Restoring Files.
--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 one or more archives to the end of another archive, you should use the --concatenate (--catenate, -A) operation.
To use `--concatenate', name the archives to be concatenated on the command line. (Nothing happens if you don't list any.) The members, and their member names, will be copied verbatim from those archives. If this causes multiple members to have the same name, it does not delete any members; all the members with the same name coexist. For information on how this affects reading the archive, @FIXME-ref{Multiple Members with the Same Name}.
To demonstrate how `--concatenate' works, create two small archives called `bluesrock.tar' and `folkjazz.tar', using the relevant files from `practice':
$ tar -cvf bluesrock.tar blues rock blues classical $ tar -cvf folkjazz.tar folk jazz folk jazz
If you like, You can run `tar --list' to make sure the archives contain what they are supposed to:
$ tar -tvf bluesrock.tar -rw-rw-rw- melissa user 105 1997-01-21 19:42 blues -rw-rw-rw- melissa user 33 1997-01-20 15:34 rock $ tar -tvf folkjazz.tar -rw-rw-rw- melissa user 20 1996-09-23 16:44 folk -rw-rw-rw- melissa user 65 1997-01-30 14:15 jazz
We can concatenate these two archives with tar:
$ cd .. $ tar --concatenate --file=bluesrock.tar jazzfolk.tar
If you now list the contents of the `bluesclass.tar', you will see that now it also contains the archive members of `jazzfolk.tar':
$ tar --list --file=bluesrock.tar blues rock jazz folk
When you use `--concatenate', the source and target archives must already exist and must have been created using compatable format parameters (@FIXME-pxref{Matching Format Parameters}). The new, concatenated archive will be called by the same name as the first archive listed on the command line. @FIXME{is there a way to specify a new name?}
Like --append (-r), this operation cannot be performed on some tape drives, due to deficiencies in the formats those tape drives use.
It may seem more intuitive to you to want or try to use cat
to concatenate two archives instead of using the `--concatenate'
operation; after all, cat is the utility for combining
files.
However, tar archives incorporate an end-of-file marker
which must be removed if the concatenated archives are to be read
properly as one archive. `--concatenate' removes the
end-of-archive marker from the target archive before each new archive
is appended. If you use cat to combine the archives, the
result will not be a valid tar format archive. If you need
to retrieve files from an archive that was added to using the cat
utility, use the --ignore-zeros (-i) option. See
section Ignoring Blocks of Zeros for
further information on dealing with archives improperly combined using
the cat shell utility.
@FIXME{this shouldn't go here. where should it go?} You must specify
the source archives using --file=archive-name (-f archive-name)
(see section Choosing and Naming Archive Files).
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.
@UNREVISED
You can remove members from an archive by using the --delete
option. Specify the name of the archive with --file=archive-name
(-f archive-name) and then specify the names of
the members to be deleted; if you list no member names, nothing will be
deleted. The --verbose (-v) option will cause tar
to print the names of the members as they are deleted. As with --extract
(--get, -x), you must give the exact member
names when using `tar --delete'. `--delete'
will remove all versions of the named file from the archive. The `--delete'
operation can run very slowly.
Unlike other operations, `--delete' has no short form.
This operation will rewrite the archive. You can only use `--delete' on an archive if the archive device allows you to write to any point on the media, such as a disk; because of this, it does not work on magnetic tapes. Do not try to delete an archive member from a magnetic tape; the action will not succeed, and you will be likely to scramble the archive and damage your tape. There is no safe way (except by completely re-writing the archive) to delete files from most kinds of magnetic tape. See section Tapes and Other Archive Media.
To delete all versions of the file `blues' from the archive `collection.tar' in the `practice' directory, make sure you are in that directory, and then,
$ tar --list --file=collection.tar blues folk jazz rock practice/blues practice/folk practice/jazz practice/rock practice/blues $ tar --delete --file=collection.tar blues $ tar --list --file=collection.tar folk jazz rock $
@FIXME{I changed the order of these nodes around and haven't had a chance to fix the above example's results, yet. I have to play with this and follow it and see what it actually does!}
The --delete option has been reported to work properly when tar
acts as a filter from stdin to stdout.
The `--compare' (`-d'), or `--diff'
operation compares specified archive members against files with the
same names, and then reports differences in file size, mode, owner,
modification date and contents. You should only specify
archive member names, not file names. If you do not name any members,
then tar will compare the entire archive. If a file is
represented in the archive but does not exist in the file system, tar
reports a difference.
You have to specify the record size of the archive when modifying an archive with a non-default record size.
tar ignores files in the file system that do not have
corresponding members in the archive.
The following example compares the archive members `rock', `blues'
and `funk' in the archive `bluesrock.tar' with files
of the same name in the file system. (Note that there is no file, `funk'; tar
will report an error message.)
$ tar --compare --file=bluesrock.tar rock blues funk rock blues tar: funk not found in archive
@FIXME{what does this actually depend on? i'm making a guess,
here.}Depending on the system where you are running tar
and the version you are running, tar may have a different
error message, such as:
funk: does not exist
@FIXME-xref{somewhere, for more information about format parameters. Melissa says: such as "format variations"? But why? Clearly I don't get it yet; I'll deal when I get to that section.}
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 section Verifying Data as It is Stored.
--extract@UNREVISED
@FIXME{i need to get dan to go over these options with me and see if there's a better way of organizing them.}
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 section How to Extract Members
from an Archive for more basic information about the `--extract'
operation.
Normally, tar will request data in full record increments
from an archive storage device. If the device cannot return a full
record, tar will report an error. However, some devices do
not always return full records, or do not require the last record of an
archive to be padded out to the next record boundary. To keep reading
until you obtain a full record, or to accept an incomplete record if it
contains an end-of-archive marker, specify the --read-full-records
(-B) option in conjunction with the --extract (--get, -x)
or --list (-t) operations. See section Blocking.
The --read-full-records (-B) option is turned on
by default when tar reads an archive from standard input,
or from a remote machine. This is because on BSD Unix systems,
attempting to read a pipe returns however much happens to be in the
pipe, even if it is less than was requested. If this option were not
enabled, tar would fail as soon as it read an incomplete
record from the pipe.
If you're not sure of the blocking factor of an archive, you can read the archive by specifying --read-full-records (-B) and --blocking-factor=512-size (-b 512-size), using a blocking factor larger than what the archive uses. This lets you avoid having to determine the blocking factor of an archive. See section The Blocking Factor of an Archive.
@FIXME{need sentence or so of intro here}
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.
@FIXME{Is this in the right place? It doesn't exist anywhere else in the book (except the appendix), and has no further explanation. For that matter, what does it mean?!}
tar
Writes Files@FIXME{need to mention the brand new option, --backup}
Normally, tar writes extracted files into the file system
without regard to the files already on the system; i.e., files with the
same names as archive members are overwritten when the archive is
extracted. If the name of a corresponding file name is a symbolic link,
the file pointed to by the symbolic link will be overwritten instead of
the symbolic link itself (if this is possible). Moreover, special
devices, empty directories and even symbolic links are automatically
removed if they are found to be on the way of the proper extraction.
To prevent tar from extracting an archive member from an
archive if doing so will overwrite a file in the file system, use --keep-old-files
(-k) in conjunction with `--extract'. When
this option is specified, tar will report an error stating
the name of the files in conflict instead of overwriting the file with
the corresponding extracted archive member.
@FIXME{these two P's have problems. i don't understand what they're trying to talk about well enough to fix them; i may have just made them worse (in particular the first of the two). waiting to talk with hag.}
The --unlink-first (-U) option removes existing files, symbolic links, empty directories, devices, etc., prior to extracting over them. In particular, using this option will prevent replacing an already existing symbolic link by the name of an extracted file, since the link itself is removed prior to the extraction, rather than the file it points to. On some systems, the backing store for the executable is the original program text. You could use the --unlink-first (-U) option to prevent segmentation violations or other woes when extracting arbitrary executables over currently running copies. Note that if something goes wrong with the extraction and you did use this option, you might end up with no file at all. Without this option, if something goes wrong with the extraction, the existing file is not overwritten and preserved.
@FIXME{huh?} If you specify the --recursive-unlink option, tar
removes anything that keeps you from extracting a file as far
as current permissions will allow it. This could include removal of the
contents of a full directory hierarchy. For example, someone using this
feature may be very surprised at the results when extracting a
directory entry from the archive. This option can be dangerous; be very
aware of what you are doing if you choose to use it.
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 then, maybe the site removed the link and renamed the whole
hierarchy from `/usr/local2' to `/usr/local'. Such
things happen all the time. I guess it would not be welcome at all that
GNU tar removes the whole hierarchy just to make room for
the link to be reinstated (unless it also simultaneously
restores the full `/usr/local2', of course! GNU tar
is indeed able to remove a whole hierarchy to reestablish a symbolic
link, for example, but only if --recursive-unlink
is specified to allow this behaviour. In any case, single files are
silently removed.
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) operation. @FIXME{Should be aliased to ignore-umask.}
@FIXME{Following paragraph needs to be rewritten: why doesnt' this cat files together, why is this useful. is it really useful with more than one file?}
To write the extracted files to the standard output, instead of creating the files on the file system, use --to-stdout (-O) in conjunction with --extract (--get, -x). This option is useful if you are extracting files to send them through a pipe, and do not need to preserve them in the file system. If you extract multiple members, they appear on standard output concatenated, in the order they are found in the archive.
tar writes the contents of the files
extracted to its standard output. This may be useful if you are only
extracting the files in order to send them through a pipe. This option
is meaningless with --list (-t).
@FIXME{Why would you want to do such a thing, how are files separated on the standard output? is this useful with more that one file? Are pipes the real reason?}
@FIXME{the various macros in the front of the manual think that this option goes in this section. i have no idea; i only know it's nowhere else in the book...}
If a previous attempt to extract files failed due to lack of disk
space, you can use --starting-file=name (-K name)
to start extracting only after member name of the archive.
This assumes, of course, that there is now free space, or that you are
now extracting into a different file system. (You could also choose to
suspend tar, remove unnecessary files from the file
system, and then restart the same tar operation. In this
case, --starting-file=name (-K name)
is not necessary. See section Using tar
to Perform Incremental Dumps, See section Asking
for Confirmation During Operations, and section Excluding Some Files.)
@FIXME{we don't need/want --preserve to exist any more (from melissa: ie, don't want that *version* of the option to exist, or don't want the option to exist in either version?}
@FIXME{i think this explanation is lacking.}
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 has having similar names through the side-effect of name truncation. (This is true only if we have a good scheme for truncated backup names, which I'm not sure at all: I suspect work is needed in this area.) When any existing file is backed up before being overwritten by extraction, then clashing files are automatically be renamed to be unique, and the true name is kept for only the last file of a series of clashing files. By using verbose mode, users may track exactly what happens.
At the detail level, some decisions are still experimental, and may change in the future, we are waiting comments from our users. So, please do not learn to depend blindly on the details of the backup features. For example, currently, directories themselves are never renamed through using these options, so, extracting a file over a directory still has good chances to fail. Also, backup options apply to created archives, not only to extracted members. For created archives, backups will not be attempted when the archive is a block or character device, or when it refers to a remote file.
For the sake of simplicity and efficiency, backups are made by renaming old files prior to creation or extraction, and not by copying. The original name is restored if the file creation fails. If a failure occurs after a partial extraction of a file, both the backup and the partially extracted file are kept.
SIMPLE_BACKUP_SUFFIX environment variable
is used. And if SIMPLE_BACKUP_SUFFIX is not set, the
default is `~', just as in Emacs.
VERSION_CONTROL
environment variable is used. And if VERSION_CONTROL
is not set, the default backup type is `existing'. This option corresponds to the Emacs variable `version-control';
the same values for method are accepted as in Emacs.
This options also more descriptive name. The valid methods
(unique abbreviations are accepted):
Some people express the desire to always use the op-backup option, by defining some kind of alias or script. This is not as easy as one may thing, due to the fact old style options should appear first and consume arguments a bit inpredictably for an alias or script. But, if you are ready to give up using old style options, you may resort to using something like (a Bourne shell function here):
tar () { /usr/local/bin/tar --backup $*; }
tar Usages@UNREVISED
@FIXME{Using Unix file linking capability to recreate directory
structures--linking files into one subdirectory and then tarring
that directory.}
@FIXME{Nice hairy example using absolute-names, newer, etc.}
You can easily use archive files to transport a group
of files from one system to another: put all relevant files into an
archive on one computer system, transfer the archive to another system,
and extract the contents there. The basic transfer medium might be
magnetic tape, Internet FTP, or even electronic mail (though you must
encode the archive with uuencode in order to transport
it properly by mail). Both machines do not have to use the same
operating system, as long as they both support the tar
program.
For example, here is how you might copy a directory's contents from one disk to another, while preserving the dates, modes, owners and link-structure of all the files therein. In this case, the transfer medium is a pipe, which is one a Unix redirection mechanism:
$ cd sourcedir; tar -cf - . | (cd targetdir; tar -xf -)
The command also works using short option forms:
@FIXME{The following using standard input/output correct??}
$ cd sourcedir; tar --create --file=- . | (cd targetdir; tar --extract --file=-)
This is one of the easiest methods to transfer a tar
archive.
You have now seen how to use all eight of the operations available to tar,
and a number of the possible options. The next chapter explains how to
choose and change file and archive names, how to use files to store
names of other files which you can then call as arguments to tar
(this can help you save time if you expect to archive the same list of
files a number of times), and how to @FIXME{in case it's not obvious,
i'm making this up in some sense based on my imited memory of what the
next chapter *really* does. i just wanted to flesh out this final
section a little bit so i'd remember to sitck it in here. :-)}
If there are too many files to conveniently list on the command line,
you can list the names in a file, and tar will read that
file. See section Reading Names from a File.
There are various ways of causing tar to skip over some
files, and not archive them. See section Choosing
Files and Names for tar.
@UNREVISED
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@UNREVISED
Full dumps should only be made when no other people or
programs are modifying files in the filesystem. If files are modified
while tar is making the backup, they may not be stored
properly in the archive, in which case you won't be able to restore
them if you have to. (Files not being modified are written with no
trouble, and do not corrupt the entire archive.)
You will want to use the --label=archive-label (-V archive-label) option to give the archive a volume label, so you can tell what this archive is even if the label falls off the tape, or anything like that.
Unless the filesystem you are dumping is guaranteed to fit on one volume, you will need to use the --multi-volume (-M) option. Make sure you have enough tapes on hand to complete the backup.
If you want to dump each filesystem separately you will need to use the --one-file-system
(-l) option to prevent tar from crossing
filesystem boundaries when storing (sub)directories.
The --incremental (-G) option is not needed, since this is a complete copy of everything in the filesystem, and a full restore from this backup would only be done onto a completely empty disk.
Unless you are in a hurry, and trust the tar program (and
your tapes), it is a good idea to use the --verify (-W)
option, to make sure your files really made it onto the dump properly.
This will also detect cases where the file was modified while (or just
after) it was being archived. Not all media (notably cartridge tapes)
are capable of being verified, unfortunately.
--listed-incremental=snapshot-file (-g snapshot-file) take a file name argument always. If the file doesn't exist, run a level zero dump, creating the file. If the file exists, uses that file to see what has changed.
--incremental (-G) @FIXME{look it up}
--incremental (-G) handle old GNU-format incremental backup.
This option should only be used when creating an incremental backup of
a filesystem. When the --incremental (-G) option
is used, tar writes, at the beginning of the archive, an
entry for each of the directories that will be operated on. The entry
for a directory includes a list of all the files in the directory at
the time the dump was done, and a flag for each file indicating whether
the file is going to be put in the archive. This information is used
when doing a complete incremental restore.
Note that this option causes tar to create a non-standard
archive that may not be readable by non-GNU versions of the tar
program.
The --incremental (-G) option means the archive is an incremental backup. Its meaning depends on the command that it modifies.
If the --incremental (-G) option is used with --list
(-t), tar will list, for each directory in the
archive, the list of files in that directory at the time the archive
was created. This information is put out in a format that is not easy
for humans to read, but which is unambiguous for a program: each file
name is preceded by either a `Y' if the file is present in
the archive, an `N' if the file is not included in the
archive, or a `D' if the file is a directory (and is
included in the archive). Each file name is terminated by a null
character. The last file is followed by an additional null and a
newline to indicate the end of the data.
If the --incremental (-G) option is used with --extract (--get, -x), then when the entry for a directory is found, all files that currently exist in that directory but are not listed in the archive are deleted from the directory.
This behavior is convenient when you are restoring a damaged file system from a succession of incremental backups: it restores the entire state of the file system to that which obtained when the backup was made. If you don't use --incremental (-G), the file system will probably fill up with files that shouldn't exist any more.
--listed-incremental=snapshot-file (-g snapshot-file) handle new GNU-format incremental backup. This option handles new GNU-format incremental backup. It has much the same effect as --incremental (-G), but also the time when the dump is done and the list of directories dumped is written to the given file. When restoring, only files newer than the saved time are restored, and the direcotyr list is used to speed up operations.
--listed-incremental=snapshot-file (-g snapshot-file)
acts like --incremental (-G), but when used in
conjunction with --create (-c) will also cause tar
to use the file file, which contains information about the
state of the filesystem at the time of the last backup, to decide which
files to include in the archive being created. That file will then be
updated by tar. If the file file does not exist
when this option is specified, tar will create it, and
include all appropriate files in the archive.
The file, which is archive independent, contains the date it was last
modified and a list of devices, inode numbers and directory names. tar
will archive files with newer mod dates or inode change times, and
directories with an unchanged inode number and device but a changed
directory name. The file is updated after the files to be archived are
determined, but before the new archive is actually created.
GNU tar actually writes the file twice: once before the
data and written, and once after.
tar to Perform
Incremental Dumps@UNREVISED
You will need to use the `-N date'
option to tell tar to only store files that have been
modified since date. date should be the date and
time of the last full/incremental dump.
A standard scheme is to do a monthly (full) dump once a month, a weekly dump once a week of everything since the last monthly and a daily every day of everything since the last (weekly or monthly) dump.
Here is a copy of the script used to dump the filesystems of the
machines here at the Free Software Foundation. This script is run via cron
late at night when people are least likely to be using the machines.
This script dumps several filesystems from several machines at once
(via NFS). The operator is responsible for ensuring that all the
machines will be up at the time the dump happens. If a machine is not
running, its files will not be dumped, and the next day's incremental
dump will not store files that would have gone onto that dump.
#!/bin/csh # Dump thingie set now = `date` set then = `cat date.nfs.dump` /u/hack/bin/tar -c -G -v\ -f /dev/rtu20\ -b 126\ -N "$then"\ -V "Dump from $then to $now"\ /alpha-bits/gp\ /gnu/hack\ /hobbes/u\ /spiff/u\ /sugar-bombs/u echo $now > date.nfs.dump mt -f /dev/rtu20 rew
Output from this script is stored in a file, for the operator to read later.
This script uses the file `date.nfs.dump' to store the date/time of the last dump.
Since this is a streaming tape drive, no attempt to verify the archive
is done. This is also why the high blocking factor (126) is used. The
tape drive must also be rewound by the mt command after
the dump is made.
@UNREVISED
--incremental (-G) is used in conjunction with --create (-c), --extract (--get, -x) or --list (-t) when backing up and restoring file systems. An archive cannot be extracted or listed with the --incremental (-G) option specified unless it was created with the option specified. This option should only be used by a script, not by the user, and is usually disregarded in favor of --listed-incremental=snapshot-file (-g snapshot-file), which is described below.
--incremental (-G) in conjunction with --create
(-c) causes tar to write, at the beginning of
the archive, an entry for each of the directories that will be
archived. The entry for a directory includes a list of all the files in
the directory at the time the archive was created and a flag for each
file indicating whether or not the file is going to be put in the
archive.
Note that this option causes tar to create a non-standard
archive that may not be readable by non-GNU versions of the tar
program.
--incremental (-G) in conjunction with --extract
(--get, -x) causes tar to read the
lists of directory contents previously stored in the archive, delete
files in the file system that did not exist in their directories when
the archive was created, and then extract the files in the archive.
This behavior is convenient when restoring a damaged file system from a succession of incremental backups: it restores the entire state of the file system to that which obtained when the backup was made. If --incremental (-G) isn't specified, the file system will probably fill up with files that shouldn't exist any more.
--incremental (-G) in conjunction with --list
(-t), causes tar to print, for each directory
in the archive, the list of files in that directory at the time the
archive was created. This information is put out in a format that is
not easy for humans to read, but which is unambiguous for a program:
each file name is preceded by either a `Y' if the file is
present in the archive, an `N' if the file is not included
in the archive, or a `D' if the file is a directory (and
is included in the archive). Each file name is terminated by a null
character. The last file is followed by an additional null and a
newline to indicate the end of the data.
--listed-incremental=snapshot-file (-g snapshot-file)
acts like --incremental (-G), but when used in
conjunction with --create (-c) will also cause tar
to use the file snapshot-file, which contains information
about the state of the file system at the time of the last backup, to
decide which files to include in the archive being created. That file
will then be updated by tar. If the file file
does not exist when this option is specified, tar will
create it, and include all appropriate files in the archive.
The file file, which is archive independent, contains the
date it was last modified and a list of devices, inode numbers and
directory names. tar will archive files with newer mod
dates or inode change times, and directories with an unchanged inode
number and device but a changed directory name. The file is updated
after the files to be archived are determined, but before the new
archive is actually created.
Despite it should be obvious that a device has a non-volatile value,
NFS devices have non-dependable values when an automounter gets in the
picture. This led to a great deal of spurious redumping in incremental
dumps, so it is somewhat useless to compare two NFS devices numbers
over time. So tar now considers all NFS devices as being
equal when it comes to comparing directories; this is fairly gross, but
there does not seem to be a better way to go.
@FIXME{this section needs to be written}
@UNREVISED
An archive containing all the files in the file system is called a full backup or full dump. You could insure your data by creating a full dump every day. This strategy, however, would waste a substantial amount of archive media and user time, as unchanged files are daily re-archived.
It is more efficient to do a full dump only occasionally. To back up files between full dumps, you can a incremental dump. A level one dump archives all the files that have changed since the last full dump.
A typical dump strategy would be to perform a full dump once a week, and a level one dump once a day. This means some versions of files will in fact be archived more than once, but this dump strategy makes it possible to restore a file system to within one day of accuracy by only extracting two archives--the last weekly (full) dump and the last daily (level one) dump. The only information lost would be in files changed or created since the last daily backup. (Doing dumps more than once a day is usually not worth the trouble).
GNU tar comes with scripts you can use to do full and
level-one dumps. Using scripts (shell programs) to perform backups and
restoration is a convenient and reliable alternative to typing out file
name lists and tar commands by hand.
Before you use these scripts, you need to edit the file `backup-specs', which specifies parameters used by the backup scripts and by the restore script. @FIXME{There is no such restore script!}. @FIXME-xref{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. @FIXME{There is
no such restore script!}. 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.
@FIXME-xref{Scripted Restoration}, for more information on running the
restoration script. @FIXME-xref{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. @FIXME{There is no
such restore script!}. See section Using tar
to Perform Incremental Dumps, and See section Using tar to Perform Incremental
Dumps, before making such an attempt.
@FIXME{shorten node names}
@UNREVISED
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.
@FIXME{This about backup scripts needs to be written: BS is a shell script .... thus ... `backup-specs' is in shell script syntax.}
@FIXME-xref{Script Syntax}, for an explanation of this syntax.
@FIXME{Whats a parameter .... looked at by the backup scripts ... which will be expecting to find ... now syntax ... value is linked to lame ... `backup-specs' specifies the following parameters:}
tar writes the archive to. This device
should be attached to the host on which the dump scripts are run.
@FIXME{examples for all ...}
tar will use when writing the dump
archive. See section The Blocking Factor
of an Archive.
tar on, and should
normally be the host that actually contains the file system. However,
the host machine must have GNU tar installed, and must
be able to access the directory containing the backup scripts and their
support files using the same file name that is used on the machine
where the scripts are run (ie. what pwd will print
when in that directory on that machine). If the host that contains the
file system does not have this capability, you can specify another host
as long as it can access the file system through NFS.
@UNREVISED
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]"
@UNREVISED
`backup-specs' is in shell script syntax. The following conventions should be considered when editing the script: @FIXME{"conventions?"}
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.
@UNREVISED
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' (@FIXME-pxref{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. @FIXME{There is no such restore script!}.
@FIXME-xref{Scripted Restoration}. @FIXME{Have file names changed?}
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. @FIXME-xref{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, what time the backup was made, and any error messages that were generated, as well as how much space was left in the media volume after the last volume of the archive was written. You should check this log file after every backup. The file name is `log-mmm-ddd-yyyy-level-1' or `log-mmm-ddd-yyyy-full'.
The script also prints the name of each system being dumped to the standard output.
@UNREVISED
Warning: The GNU tar distribution does not
provide any such restore script yet. This section is only
listed here for documentation maintenance purposes. In any case, all
contents is subject to change as things develop.
@FIXME{A section on non-scripted restore may be a good idea.}
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. @FIXME-xref{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 section Using tar to Perform
Incremental Dumps, and section Using tar
to Perform Incremental Dumps, for an explanation of how the script
makes that determination.
@FIXME{this may be an option, not a given}
tar@UNREVISED
@FIXME{Melissa (still) Doesn't Really Like This "Intro" Paragraph!!!}
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.
@FIXME{should the title of this section actually be, "naming an archive"?}
By default, tar uses an archive file name that was
compiled when it was built on the system; usually this name refers to
some physical tape drive on the machine. However, the person who
installed tar on the system may not set the default to a
meaningful value as far as most users are concerned. As a result, you
will usually want to tell tar where to find (or create)
the archive. The --file=archive-name (-f archive-name)
option allows you to either specify or name a file to use as the
archive instead of the default archive file location.
For example, in this tar command,
$ tar -cvf collection.tar blues folk jazz
`collection.tar' is the name of the archive. It must directly
follow the `-f' option, since whatever directly follows `-f'
will end up naming the archive. If you neglect to specify an
archive name, you may end up overwriting a file in the working
directory with the archive you create since tar will use
this file's name for the archive name.
An archive can be saved as a file in the file system, sent through a pipe or over a network, or written to an I/O device such as a tape, floppy disk, or CD write drive.
If you do not name the archive, tar
uses the value of the environment variable TAPE as the
file name for the archive. If that is not available, tar
uses a default, compiled-in archive name, usually that for tape unit
zero (ie. `/dev/tu00'). tar always needs an
archive name.
If you use `-' as an archive-name, tar
reads the archive from standard input (when listing or extracting
files), or writes it to standard output (when creating an archive). If
you use `-' as an archive-name when modifying an
archive, tar reads the original archive from its standard
input and writes the entire new archive to its standard output.
@FIXME{might want a different example here; this is already used in "notable tar usages".}
$ cd sourcedir; tar -cf - . | (cd targetdir; tar -xf -)
@FIXME{help!}
To specify an archive file on a device attached to a remote machine, use the following:
--file=hostname:/dev/file name
tar will complete the remote connection, if possible, and
prompt you for a username and password. If you use `--file=@hostname:/dev/file
name', tar will complete the remote
connection, if possible, using your username as the username on the
remote machine.
If the archive file name includes a colon (`:'), then it
is assumed to be a file on another machine. If the archive file is `user@host:file',
then file is used on the host host. The remote
host is accessed using the rsh program, with a username of user.
If the username is omitted (along with the `@' sign), then
your user name will be used. (This is the normal rsh
behavior.) It is necessary for the remote machine, in addition to
permitting your rsh access, to have the `/usr/ucb/rmt'
program installed. If you need to use a file whose name includes a
colon, then the remote tape drive behavior can be inhibited by using
the --force-local option.
@FIXME{i know we went over this yesterday, but bob (and now i do again, too) thinks it's out of the middle of nowhere. it doesn't seem to tie into what came before it well enough <<i moved it now, is it better here?>>. bob also comments that if Amanda isn't free software, we shouldn't mention it..}
When the archive is being created to `/dev/null', GNU tar
tries to minimize input and output operations. The Amanda backup
system, when used with GNU tar, has an initial sizing pass
which uses this feature.
File Name arguments specify which files in
the file system tar operates on, when creating or adding
to an archive, or which archive members tar operates on,
when reading or deleting from an archive. See section The Five Advanced tar Operations.
To specify file names, you can include them as the last arguments on the command line, as follows:
tar operation [option1 option2 ...] [file name-1 file name-2 ...]
If you specify a directory name as a file name argument, all the files
in that directory are operated on by tar.
If you do not specify files when tar is invoked with --create
(-c), tar operates on all the non-directory
files in the working directory. If you specify either --list
(-t) or --extract (--get, -x), tar
operates on all the archive members in the archive. If you specify any
operation other than one of these three, tar does nothing.
By default, tar takes file names from the command line.
However, there are other ways to specify file or member names, or to
modify the manner in which tar selects the files or
members upon which to operate; @FIXME{add xref here}. In general, these
methods work both for specifying the names of files and archive
members.
@UNREVISED
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.
@FIXME{add bob's example, from his message on 2-10-97}
The following example shows how to use find to generate a
list of files smaller than 400K in length and put that list into a file
called `small-files'. You can then use the `-T'
option to tar to specify the files from that file, `small-files',
to create the archive `little.tgz'. (The `-z'
option to tar compresses the archive with gzip;
see section Creating and Reading Compressed
Archives for more information.)
$ find . -size -400 -print > small-files $ tar -c -v -z -T small-files -f little.tgz
@FIXME{say more here to conclude the example/section?}
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 just just like `-print',
except that it separates files with a NUL rather than with a
newline. You can then run tar with both the `--null'
and `-T' options to specify that tar get the
files from that file, `long-files', to create the archive `big.tgz'.
The `--null' option to tar will cause tar
to recognize the NUL separator between files.
$ find . -size +800 -print0 > long-files $ tar -c -v --null --files-from=long-files --file=big.tar
@FIXME{say anything else here to conclude the section?}
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 regular expressions. Thus if tar
is called as `tar -c -X foo .' and the file `foo'
contains a single line `*.o', no files whose names end in `.o'
will be added to the archive.
@FIXME{do the exclude options files need to have stuff separated by newlines the same as the files-from option does?}
exclude
Options@FIXME{put in for the editor's/editors' amusement, but should be taken out in the final draft, just in case! : }
Some 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.
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.
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.
@FIXME{the next few paragraphs need work.}
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.
@FIXME{need to add a sentence or so here to make this clear for those who don't have dan around.}
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.
The --after-date=date (--newer=date, -N date)
option causes tar to only work on files whose modification
or inode-changed times are newer than the date given. If you
use this option when creating or appending to an archive, the archive
will only include new files. If you use `--after-date'
when extracting an archive, tar will only extract files
newer than the date you specify.
If you only want tar to make the date comparison based on
modification of the actual contents of the file (rather than inode
changes), then use the --newer-mtime=date option.
You may use these options with any operation. Note that these options
differ from the --update (-u) operation in that
they allow you to specify a particular date against which tar
can compare when deciding whether or not to archive the files.
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 section Date input
formats; remember that the entire date argument must be quoted if
it contains any spaces.)
Gurus would say that --after-date=date (--newer=date, -N date)
tests both the mtime (time the contents of the file were
last modified) and ctime (time the file's status was last
changed: owner, permissions, etc) fields, while --newer-mtime=date
tests only mtime field.
To be precise, --after-date=date (--newer=date, -N date)
checks both mtime and ctime and
processes the file if either one is more recent than date,
while --newer-mtime=date only checks mtime
and disregards ctime. Neither uses atime
(the last time the contents of the file were looked at).
Date specifiers can have embedded spaces. Because of this, you may need to quote date arguments to keep the shell from parsing them as separate arguments.
@FIXME{Need example of --newer-mtime with quoted argument.}
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 section The Incremental Options.
To select files newer than the modification time of a file that already
exists, you can use the `--reference' (`-r')
option of GNU date, available in GNU shell utilities 1.13
or later. It returns the timestamp of that already existing file; this
timestamp expands to become the referent date which `--newer'
uses to determine which files to archive. For example, you could say,
$ tar -cf archive.tar --newer="`date -r file`" /home
which tells @FIXME{need to fill this in!}.
@FIXME{arrggh! this is still somewhat confusing to me. :-< }
@FIXME{show dan bob's comments, from 2-10-97}
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 section Reading Names from a File for more
information on using find with tar, or look.
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
@FIXME{needs more explanation or a cite to another info file} as they
usually do not want all the files in a directory. They then use the
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.
@FIXME{example here}
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 operation.
The `--one-file-system' option causes tar to
modify its normal behavior in archiving the contents of directories. If
a file in a directory is not on the same filesystem as the directory
itself, then tar will not archive that file. If the file
is a directory itself, tar will not archive anything
beneath it; in other words, tar will not cross mount
points.
It is reported that using this option, the mount point is is archived, but nothing under it.
This option is useful for making full or incremental archival backups of a file system. If this option is used in conjunction with --verbose (-v), files that are excluded are mentioned by name on the standard error.
@FIXME{need to read over this node now for continuity; i've switched things around some.}
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 in the same archive.
Note that the file `cherry' is recorded in the archive under the precise name `cherry', not `food/cherry'. Thus, the archive will contain three files that all appear to have come from the same directory; if the archive is extracted with plain `tar --extract', all three files will be written in the current directory.
Contrast this with the command,
$ tar -c -f jams.tar grape prune -C food red/cherry
which records the third file in the archive under the name `red/cherry' so that, if the archive is extracted using `tar --extract', the third file will be written in a subdirectory named `orange-colored'.
You can use the `--directory' option to make the archive independent of the original name of the directory holding the files. The following command places the files `/etc/passwd', `/etc/hosts', and `/lib/libc.a' into the archive `foo.tar':
$ tar -c -f foo.tar -C /etc passwd hosts -C /lib libc.a
However, the names of the archive members will be exactly what they were on the command line: `passwd', `hosts', and `libc.a'. They will not appear to be related by file name to the original directories where those files were located.
Note that `--directory' options are interpreted
consecutively. If `--directory' specifies a relative file
name, it is interpreted relative to the then current directory, which
might not be the same as the original current working directory of tar,
due to a previous `--directory' option.
@FIXME{dan: does this mean that you *can* use the short option form, but you can *not* use the long option form with --files-from? or is this totally screwed?}
When using `--files-from' (see section Reading Names from a File), 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.)
@UNREVISED
By default, GNU tar drops a leading `/' on
input or output. This option turns off this behavior; it is equivalent
to changing to the root directory before running tar
(except it also turns off the usual warning message).
When tar extracts archive members from an archive, it
strips any leading slashes (`/') from the member name.
This causes absolute member names in the archive to be treated as
relative file names. This allows you to have such members extracted
wherever you want, instead of being restricted to extracting the member
in the exact directory named in the archive. For example, if the
archive member has the name `/etc/passwd', tar
will extract it as if the name were really `etc/passwd'.
Other tar programs do not do this. As a result, if you
create an archive whose member names start with a slash, they will be
difficult for other people with a non-GNU tar program to
use. Therefore, GNU tar also strips leading slashes from
member names when putting members into the archive. For example, if you
ask tar to add the file `/bin/ls' to an archive,
it will do so, but the member name will be `bin/ls'.
If you use the --absolute-names (-P) option, tar
will do neither of these transformations.
To archive or extract files relative to the root directory, specify the --absolute-names (-P) option.
Normally, tar acts on files relative to the working
directory--ignoring superior directory names when archiving, and
ignoring leading slashes when extracting.
When you specify --absolute-names (-P), tar
stores file names including all superior directory names, and
preserves leading slashes. If you only invoked tar from
the root directory you would never need the --absolute-names
(-P) option, but using this option may be more convenient
than switching to root.
@FIXME{Should be an example in the tutorial/wizardry section using this to transfer files between systems.}
@FIXME{Is write access an issue?}
@FIXME{this is still horrible; need to talk with dan on monday.}
tar prints out a message about removing the `/'
from file names. This message appears once per GNU tar
invocation. It represents something which ought to be told; ignoring
what it means can cause very serious surprises, later.
Some people, nevertheless, do not want to see this message. Wanting to
play really dangerously, one may of course redirect tar
standard error to the sink. For example, under sh:
$ tar -c -f archive.tar /home 2> /dev/null
Another solution, both nicer and simpler, would be to change to the `/' directory first, and then avoid absolute notation. For example:
$ (cd / && tar -c -f archive.tar home) $ tar -c -f archive.tar -C / home
... 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.
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.
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.
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.
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 effects of relative items accumulate. Here are some examples:
1 year 1 year ago 3 years 2 days
The unit of time displacement may be selected by the string `year' or `month' for moving by whole years or months. These are fuzzy units, as years and months are not all of equal duration. More precise units are `fortnight' which is worth 14 days, `week' worth 7 days, `day' worth 24 hours, `hour' worth 60 minutes, `minute' or `min' worth 60 seconds, and `second' or `sec' worth one second. An `s' suffix on these units is accepted and ignored.
The unit of time may be preceded by a multiplier, given as an optionally signed number. Unsigned numbers are taken as positively signed. No number at all implies 1 for a multiplier. Following a relative item by the string `ago' is equivalent to preceding the unit by a multiplicator with value -1.
The string `tomorrow' is worth one day in the future (equivalent to `day'), the string `yesterday' is worth one day in the past (equivalent to `day ago').
The strings `now' or `today' are relative items corresponding to zero-valued time displacement, these strings come from the fact a zero-valued time displacement represents the current time when not otherwise change by previous items. They may be used to stress other items, like in `12:00 today'. The string `this' also has the meaning of a zero-valued time displacement, but is preferred in date strings like `this thursday'.
When a relative item makes the resulting date to cross the boundary between DST and non-DST (or vice-versa), the hour is adjusted according to the local time.
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 section 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.
getdate
getdate was originally implemented by
Steven M. Bellovin (`smb@research.att.com') while at the
University of North Carolina at Chapel Hill. The code was later tweaked
by a couple of people on Usenet, then completely overhauled by Rich
$alz (`rsalz@bbn.com') and Jim Berets (`jberets@bbn.com')
in August, 1990. Various revisions for the GNU system were made by
David MacKenzie, Jim Meyering, and others.
This chapter was originally produced by Fran@,{c}ois Pinard (`pinard@iro.umontreal.ca') from the `getdate.y' source code, and then edited by K. Berry (`kb@cs.umb.edu').
@FIXME{need an intro here}
tar
Archives More Portable
Creating a tar archive on a particular system that is
meant to be useful later on many other machines and with other versions
of tar is more challenging than you might think. tar
archive formats have been evolving since the first versions of Unix.
Many such formats are around, and are not always comptible with each
other. This section discusses a few problems, and gives some advice
about making tar archives more portable.
One golden rule is simplicity. For example, limit your tar
archives to contain only regular files and directories, avoiding other
kind of special files. Do not attempt to save sparse files or
contiguous files as such. Let's discuss a few more problems, in turn.
Use straight file and directory names, made up of printable ASCII characters, avoiding colons, slashes, backslashes, spaces, and other dangerous characters. Avoid deep directory nesting. Accounting for oldish System V machines, limit your file and directory names to 14 characters or less.
If you intend to have your tar archives to be read under
MSDOS, you should not rely on case distinction for file names, and you
might use the GNU doschk program for helping you further
diagnosing illegal MSDOS names, which are even more limited than System
V's.
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 with using this option.
In most cases, a new format archive can be read by an old
tar program without serious trouble, so this option
should seldom be needed. On the other hand, most modern 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 tar
GNU tar was based on an early draft of the POSIX 1003.1 ustar
standard. GNU extensions to tar, such as the support for
file names longer than 100 characters, use portions of the tar
header record which were specified in that POSIX draft as unused.
Subsequent changes in POSIX have allocated the same parts of the header
record for other purposes. As a result, GNU tar is
incompatible with the current POSIX spec, and with tar
programs that follow it.
We plan to reimplement these GNU extensions in a new way which is
upward compatible with the latest POSIX tar format, but we
don't know when this will be done.
In the mean time, there is simply no telling what might happen if you
read a GNU tar archive, which uses the GNU extensions,
using some other tar program. So if you want to read the
archive with another tar program, be sure to write it
using the `--old-archive' option (`-o').
@FIXME{is there a way to tell which flavor of tar was used to write a particular archive before you try to read it?}
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 did not intimately studied yet.
Given a truly POSIX archive with names having more than 100 characters,
I guess that GNU tar up to 1.11.8 will process it as if it
were an old V7 archive, and be fooled by some fields which are coded
differently. So, the question is to decide if the next generation of
GNU tar should produce POSIX format by default, whenever
possible, producing archives older versions of GNU tar
might not be able to read correctly. I fear that we will have to
suffer such a choice one of these days, if we want GNU tar
to go closer to POSIX. We can rush it. Another possibility is to
produce the current GNU tar format by default for a few
years, but have GNU tar versions from some 1.POSIX
and up able to recognize all three formats, and let older GNU tar
fade out slowly. Then, we could switch to producing POSIX format by
default, with not much harm to those still having (very old at that
time) GNU tar versions prior to 1.POSIX.
POSIX format cannot represent very long names, volume headers,
splitting of files in multi-volumes, sparse files, and incremental
dumps; these would be all disallowed if --posix or POSIXLY_CORRECT.
Otherwise, if tar is given long names, or `-[VMSgG]',
then it should automatically go non-POSIX. I think this is easily
granted without much discussion.
Another point is that only mtime is stored in POSIX
archives, while GNU tar currently also store atime
and ctime. If we want GNU tar to go closer
to POSIX, my choice would be to drop atime and ctime
support on average. On the other hand, I perceive that full dumps or
incremental dumps need atime and ctime
support, so for those special applications, POSIX has to be avoided
altogether.
A few users requested that --sparse (-S) be
always active by default, I think that before replying to them, we have
to decide if we want GNU tar to go closer to POSIX on
average, while producing files. My choice would be to go closer to
POSIX in the long run. Besides possible double reading, I do not see
any point of not trying to save files as sparse when creating archives
which are neither POSIX nor old-V7, so the actual --sparse (-S)
would become selected by default when producing such archives, whatever
the reason is. So, --sparse (-S) alone might be
redefined to force GNU-format archives, and recover its previous
meaning from this fact.
GNU-format as it exists now can easily fool other POSIX tar,
as it uses fields which POSIX considers to be part of the file name
prefix. I wonder if it would not be a good idea, in the long run, to
try changing GNU-format so any added field (like ctime, atime,
file offset in subsequent volumes, or sparse file descriptions) be
wholly and always pushed into an extension block, instead of using
space in the POSIX header block. I could manage to do that portably
between future GNU 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 while
creating archives, as per POSIX standards. On reading, GNU tar
computes both checksums and accept any. It is somewhat worrying that a
lot of people may go around doing backup of their files using faulty
(or at least non-standard) software, not learning about it until it's
time to restore their missing files with an incompatible file
extractor, or vice versa.
GNU tar compute checksums both ways, and accept any on
read, so GNU tar can read Sun tapes even with their wrong checksums.
GNU tar produces the standard checksum, however, raising
incompatibilities with Sun. That is to say, GNU tar has
not been modified to produce incorrect archives to be read by
buggy tar's. I've been told that more recent Sun tar
now read standard archives, so maybe Sun did a similar patch, after
all?
The story seems to be that when Sun first imported tar
sources on their system, they recompiled it without realizing that the
checksums were computed differently, because of a change in the default
signing of char's in their compiler. So they started
computing checksums wrongly. When they later realized their mistake,
they merely decided to stay compatible with it, and with themselves
afterwards. Presumably, but I do not really know, HP-UX has chosen that
their tar archives to be compatible with Sun's. The
current standards do not favor Sun tar format. In any
case, it now falls on the shoulders of SunOS and HP-UX users to get a tar
able to read the good archives they receive.
gzip.
@FIXME{ach; these two bits orig from "compare" (?). where to put?} Some format parameters must be taken into consideration when modifying an archive: @FIXME{???}. Compressed archives cannot be modified.
You can use `--gzip' and `--gunzip' on
physical devices (tape drives, etc.) and remote files as well as on
normal files; data to or from such devices or remote files is reblocked
by another copy of the tar program to enforce the
specified (or default) record size. The default compression parameters
are used; if you need to override them, avoid the --gzip (--gunzip, --ungzip, -z)
option and run gzip explicitly. (Or set the `GZIP'
environment variable.)
The --gzip (--gunzip, --ungzip, -z) option does not work with the --multi-volume (-M) option, or with the --update (-u), --append (-r), --concatenate (--catenate, -A), or --delete operations.
It is not exact to say that GNU tar is to work in concert
with gzip in a way similar to zip, say.
Surely, it is possible that tar and gzip be
done with a single call, like in:
$ tar cfz archive.tar.gz subdir
to save all of `subdir' into a gzip'ed
archive. Later you can do:
$ tar xfz archive.tar.gz
to explode and unpack.
The difference is that the whole archive is compressed. With zip,
archive members are archived individually. tar's method
yields better compression. On the other hand, one can view the contents
of a zip archive without having to decompress it. As for
the tar and gzip tandem, you need to
decompress the archive to see its contents. However, this may be done
without needing disk space, by using pipes internally:
$ tar tfz archive.tar.gz
About corrupted compressed archives: gzip'ed
files have no redundancy, for maximum compression. The adaptive nature
of the compression scheme means that the compression tables are
implicitly spread all over the archive. If you lose a few blocks, the
dynamic construction of the compression tables becomes unsychronized,
and there is little chance that you could recover later in the archive.
There are pending suggestions for having a per-volume or per-file
compression in GNU tar. This would allow for viewing the
contents without decompression, and for resynchronizing decompression
at every volume or file, in case of corrupted archives. Doing so, we
might loose some compressibility. But this would have make recovering
easier. So, there are pros and cons. We'll see!
compress. Otherwise like --gzip
(--gunzip, --ungzip, -z).
--compress (--uncompress, -Z) stores
an archive in compressed format. This option is useful in saving time
over networks and space in pipes, and when storage space is at a
premium. --compress (--uncompress, -Z)
causes tar to compress when writing the archive, or to
uncompress when reading the archive.
To perform compression and uncompression on the archive, tar
runs the compress utility. tar uses the
default compression parameters; if you need to override them, avoid the --compress
(--uncompress, -Z) option and run the compress
utility explicitly. It is useful to be able to call the compress
utility from within tar because the compress
utility by itself cannot access remote tape drives.
The --compress (--uncompress, -Z)
option will not work in conjunction with the --multi-volume (-M)
option or the --append (-r), --update (-u), --append
(-r) and --delete operations. See section The Five Advanced tar Operations,
for more information on these operations.
If there is no compress utility available, tar will report
an error. Please note that the compress
program may be covered by a patent, and therefore we recommend you
stop using it.
tar will compress (when
writing an archive), or uncompress (when reading an archive). Used in
conjunction with the --create (-c), --extract
(--get, -x), --list (-t)
and --compare (--diff, -d)
operations.
You can have archives be compressed by using the --gzip (--gunzip, --ungzip, -z)
option. This will arrange for tar to use the gzip
program to be used to compress or uncompress the archive wren writing
or reading it.
To use the older, obsolete, compress program, use the --compress
(--uncompress, -Z) option. The GNU Project
recommends you not use compress, because there is a patent
covering the algorithm it uses. You could be sued for patent
infringment merely by running compress.
I have one question, or maybe it's a suggestion if there isn't a way to
do it now. I would like to use --gzip (--gunzip, --ungzip, -z),
but I'd also like the output to be fed through a program like GNU ecc
(actually, right now that's `exactly' what I'd like to
use :-)), basically adding ECC protection on top of compression. It
seems as if this should be quite easy to do, but I can't work out
exactly how to go about it. Of course, I can pipe the standard output
of tar through ecc, but then I lose (though I
haven't started using it yet, I confess) the ability to have tar
use rmt for it's I/O (I think).
I think the most straightforward thing would be to let me specify a general set of filters outboard of compression (preferably ordered, so the order can be automatically reversed on input operations, and with the options they require specifiable), but beggars shouldn't be choosers and anything you decide on would be fine with me.
By the way, I like ecc but if (as the comments say) it
can't deal with loss of block sync, I'm tempted to throw some time at
adding that capability. Supposing I were to actually do such a thing
and get it (apparantly) working, do you accept contributed changes to
utilities like that? (Leigh Clayton `loc@soliton.com', May
1995).
Isn't that exactly the role of the --use-compress-prog=program option? I never tried it myself, but I suspect you may want to write a prog script or program able to filter stdin to stdout to way you want. It should recognize the `-d' option, for when extraction is needed rather than creation.
It has been reported that if one writes compressed data (through the --gzip (--gunzip, --ungzip, -z) or --compress (--uncompress, -Z) options) to a DLT and tries to use the DLT compression mode, the data will actually get bigger and one will end up with less space on the tape.
This option causes all files to be put in the archive to be tested for
sparseness, and handled specially if they are. The --sparse (-S)
option is useful when many dbm files, for example, are
being backed up. Using this option dramatically decreases the amount of
space needed to store such a file.
In later versions, this option may be removed, and the testing and treatment of sparse files may be done automatically with any special GNU options. For now, it is an option needing to be specified on the command line with the creation or updating of an archive.
Files in the filesystem occasionally have "holes." A hole in
a file is a section of the file's contents which was never written. The
contents of a hole read as all zeros. On many operating systems, actual
disk storage is not allocated for holes, but they are counted in the
length of the file. If you archive such a file, tar could
create an archive longer than the original. To have tar
attempt to recognize the holes in a file, use --sparse (-S).
When you use the --sparse (-S) option, then, for
any file using less disk space than would be expected from its length, tar
searches the file for consecutive stretches of zeros. It then records
in the archive for the file where the consecutive stretches of zeros
are, and only archives the "real contents" of the file. On
extraction (using --sparse (-S) is not needed on
extraction) any such files have hols created wherever the continuous
stretches of zeros were found. Thus, if you use --sparse (-S), tar
archives won't take more space than the original.
A file is sparse if it contains blocks of zeros whose existence is
recorded, but that have no space allocated on disk. When you specify
the --sparse (-S) option in conjunction with the --create
(-c) operation, tar tests all files for
sparseness while archiving. If tar finds a file to be
sparse, it uses a sparse representation of the file in the archive. See
section How to Create Archives, for more
information about creating archives.
--sparse (-S) is useful when archiving files, such as dbm files, likely to contain many nulls. This option dramatically decreases the amount of space needed to store such an archive.
Please Note: Always use --sparse (-S) when performing file system backups, to avoid archiving the expanded forms of files stored sparsely in the system.
Even if your system has no sparse files currently, some may be created in the future. If you use --sparse (-S) while making file system backups as a matter of course, you can be assured the archive will never take more space on the media than the files take on disk (otherwise, archiving a disk filled with sparse files might take hundreds of tapes). @FIXME-xref{incremental when node name is set.}
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 whole disk file to locate the holes, and
so, even if sparse files use little space on disk and in the archive,
they may sometimes require inordinate amount of time for reading and
examining all-zero blocks of a file. Although it works, it's painfully
slow for a large (sparse) file, even though the resulting tar archive
may be small. (One user reports that dumping a `core' file of
over 400 megabytes, but with only about 3 megabytes of actual data,
took about 9 minutes on a Sun Sparstation ELC, with full CPU
utilisation.)
This reading is required in all cases and is not related to the fact the --sparse (-S) option is used or not, so by merely not using the option, you are not saving time(6).
Programs like dump do not have to read the entire file; by
examining the file system directly, they can determine in advance
exactly where the holes are and thus avoid reading through them. The
only data it need read are the actual allocated data blocks. GNU tar
uses a more portable and straightforward archiving approach, it would
be fairly difficult that it does otherwise. Elizabeth Zwicky writes to `comp.unix.internals',
on 1990-12-10:
What I did say is that you cannot tell the difference between a hole
and an equivalent number of nulls without reading raw blocks. st_blocks
at best tells you how many holes there are; it doesn't tell you where.
Just as programs may, conceivably, care what st_blocks is
(care to name one that does?), they may also care where the holes are
(I have no examples of this one either, but it's equally imaginable).
I conclude from this that good archivers are not portable. One can arguably conclude that if you want a portable program, you can in good conscience restore files with as many holes as possible, since you can't get it right.
@UNREVISED
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
section Performing Backups and Restoring Files),
but it is good enough for some purposes.
Handling of file attributes
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).
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)
(@FIXME{same-owner?}), 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.
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
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).
@UNREVISED
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 section Updating an Archive, and to learn more about having more than one archive member with the same name, see @FIXME-xref{-backup node, when it's written}.
In addition to entries describing archive members, an archive may
contain entries which tar itself uses to store
information. See section Including a Label in
the Archive, for an example of such an archive entry.
A tar archive file contains a series of blocks. Each block
contains BLOCKSIZE bytes. Although this format may be
thought of as being on magnetic tape, other media are often used.
Each file archived is represented by a header block which describes the file, followed by zero or more blocks which give the contents of the file. At the end of the archive file there may be a block filled with binary zeros as an end-of-file marker. A reasonable system should write a block of zeros at the end, but must not assume that such a block exists when reading an archive.
The blocks may be blocked for physical I/O operations. Each
record of n blocks (where n is set by the --blocking-factor=512-size
(-b 512-size) option to tar) is
written with a single `write ()' operation. On magnetic
tapes, the result of such a write is a single record. When writing an
archive, the last record of blocks should be written at the full size,
with blocks after the zero block containing all zeros. When reading an
archive, a reasonable system should properly handle an archive whose
last record is shorter than the rest, or which contains garbage records
after a zero block.
The header block is defined in C as follows. In the GNU tar
distribution, this is part of file `src/tar.h':
/* GNU tar Archive Format description. */
/* If OLDGNU_COMPATIBILITY is not zero, tar produces archives which, by
default, are readable by older versions of GNU tar. This can be
overriden by using --posix; in this case, POSIXLY_CORRECT in environment
may be set for enforcing stricter conformance. If OLDGNU_COMPATIBILITY
is zero or undefined, tar will eventually produces archives which, by
default, POSIX compatible; then either using --posix or defining
POSIXLY_CORRECT enforces stricter conformance.
This #define will disappear in a few years. FP, June 1995. */
#define OLDGNU_COMPATIBILITY 1
/*---------------------------------------------.
| `tar' Header Block, from POSIX 1003.1-1990. |
`---------------------------------------------*/
/* POSIX header. */
struct posix_header
{ /* byte offset */
char name[100]; /* 0 */
char mode[8]; /* 100 */
char uid[8]; /* 108 */
char gid[8]; /* 116 */
char size[12]; /* 124 */
char mtime[12]; /* 136 */
char chksum[8]; /* 148 */
char typeflag; /* 156 */
char linkname[100]; /* 157 */
char magic[6]; /* 257 */
char version[2]; /* 263 */
char uname[32]; /* 265 */
char gname[32]; /* 297 */
char devmajor[8]; /* 329 */
char devminor[8]; /* 337 */
char prefix[155]; /* 345 */
/* 500 */
};
#define TMAGIC "ustar" /* ustar and a null */
#define TMAGLEN 6
#define TVERSION "00" /* 00 and no null */
#define TVERSLEN 2
/* Values used in typeflag field. */
#define REGTYPE '0' /* regular file */
#define AREGTYPE '\0' /* regular file */
#define LNKTYPE '1' /* link */
#define SYMTYPE '2' /* reserved */
#define CHRTYPE '3' /* character special */
#define BLKTYPE '4' /* block special */
#define DIRTYPE '5' /* directory */
#define FIFOTYPE '6' /* FIFO special */
#define CONTTYPE '7' /* reserved */
/* Bits used in the mode field, values in octal. */
#define TSUID 04000 /* set UID on execution */
#define TSGID 02000 /* set GID on execution */
#define TSVTX 01000 /* reserved */
/* file permissions */
#define TUREAD 00400 /* read by owner */
#define TUWRITE 00200 /* write by owner */
#define TUEXEC 00100 /* execute/search by owner */
#define TGREAD 00040 /* read by group */
#define TGWRITE 00020 /* write by group */
#define TGEXEC 00010 /* execute/search by group */
#define TOREAD 00004 /* read by other */
#define TOWRITE 00002 /* write by other */
#define TOEXEC 00001 /* execute/search by other */
/*-------------------------------------.
| `tar' Header Block, GNU extensions. |
`-------------------------------------*/
/* In GNU tar, SYMTYPE is for to symbolic links, and CONTTYPE is for
contiguous files, so maybe disobeying the `reserved' comment in POSIX
header description. I suspect these were meant to be used this way, and
should not have really been `reserved' in the published standards. */
/* *BEWARE* *BEWARE* *BEWARE* that the following information is still
boiling, and may change. Even if the OLDGNU format description should be
accurate, the so-called GNU format is not yet fully decided. It is
surely meant to use only extensions allowed by POSIX, but the sketch
below repeats some ugliness from the OLDGNU format, which should rather
go away. Sparse files should be saved in such a way that they do *not*
require two passes at archive creation time. Huge files get some POSIX
fields to overflow, alternate solutions have to be sought for this. */
/* Descriptor for a single file hole. */
struct sparse
{ /* byte offset */
char offset[12]; /* 0 */
char numbytes[12]; /* 12 */
/* 24 */
};
/* Sparse files are not supported in POSIX ustar format. For sparse files
with a POSIX header, a GNU extra header is provided which holds overall
sparse information and a few sparse descriptors. When an old GNU header
replaces both the POSIX header and the GNU extra header, it holds some
sparse descriptors too. Whether POSIX or not, if more sparse descriptors
are still needed, they are put into as many successive sparse headers as
necessary. The following constants tell how many sparse descriptors fit
in each kind of header able to hold them. */
#define SPARSES_IN_EXTRA_HEADER 16
#define SPARSES_IN_OLDGNU_HEADER 4
#define SPARSES_IN_SPARSE_HEADER 21
/* The GNU extra header contains some information GNU tar needs, but not
foreseen in POSIX header format. It is only used after a POSIX header
(and never with old GNU headers), and immediately follows this POSIX
header, when typeflag is a letter rather than a digit, so signaling a GNU
extension. */
struct extra_header
{ /* byte offset */
char atime[12]; /* 0 */
char ctime[12]; /* 12 */
char offset[12]; /* 24 */
char realsize[12]; /* 36 */
char longnames[4]; /* 48 */
char unused_pad1[68]; /* 52 */
struct sparse sp[SPARSES_IN_EXTRA_HEADER];
/* 120 */
char isextended; /* 504 */
/* 505 */
};
/* Extension header for sparse files, used immediately after the GNU extra
header, and used only if all sparse information cannot fit into that
extra header. There might even be many such extension headers, one after
the other, until all sparse information has been recorded. */
struct sparse_header
{ /* byte offset */
struct sparse sp[SPARSES_IN_SPARSE_HEADER];
/* 0 */
char isextended; /* 504 */
/* 505 */
};
/* The old GNU format header conflicts with POSIX format in such a way that
POSIX archives may fool old GNU tar's, and POSIX tar's might well be
fooled by old GNU tar archives. An old GNU format header uses the space
used by the prefix field in a POSIX header, and cumulates information
normally found in a GNU extra header. With an old GNU tar header, we
never see any POSIX header nor GNU extra header. Supplementary sparse
headers are allowed, however. */
struct oldgnu_header
{ /* byte offset */
char unused_pad1[345]; /* 0 */
char atime[12]; /* 345 */
char ctime[12]; /* 357 */
char offset[12]; /* 369 */
char longnames[4]; /* 381 */
char unused_pad2; /* 385 */
struct sparse sp[SPARSES_IN_OLDGNU_HEADER];
/* 386 */
char isextended; /* 482 */
char realsize[12]; /* 483 */
/* 495 */
};
/* OLDGNU_MAGIC uses both magic and version fields, which are contiguous.
Found in an archive, it indicates an old GNU header format, which will be
hopefully become obsolescent. With OLDGNU_MAGIC, uname and gname are
valid, though the header is not truly POSIX conforming. */
#define OLDGNU_MAGIC "ustar " /* 7 chars and a null */
/* The standards committee allows only capital A through capital Z for
user-defined expansion. */
/* This is a dir entry that contains the names of files that were in the
dir at the time the dump was made. */
#define GNUTYPE_DUMPDIR 'D'
/* Identifies the *next* file on the tape as having a long linkname. */
#define GNUTYPE_LONGLINK 'K'
/* Identifies the *next* file on the tape as having a long name. */
#define GNUTYPE_LONGNAME 'L'
/* This is the continuation of a file that began on another volume. */
#define GNUTYPE_MULTIVOL 'M'
/* For storing filenames that do not fit into the main header. */
#define GNUTYPE_NAMES 'N'
/* This is for sparse files. */
#define GNUTYPE_SPARSE 'S'
/* This file is a tape/volume header. Ignore it on extraction. */
#define GNUTYPE_VOLHDR 'V'
/*--------------------------------------.
| tar Header Block, overall structure. |
`--------------------------------------*/
/* tar files are made in basic blocks of this size. */
#define BLOCKSIZE 512
enum archive_format
{
DEFAULT_FORMAT, /* format to be decided later */
V7_FORMAT, /* old V7 tar format */
OLDGNU_FORMAT, /* GNU format as per before tar 1.12 */
POSIX_FORMAT, /* restricted, pure POSIX format */
GNU_FORMAT /* POSIX format with GNU extensions */
};
union block
{
char buffer[BLOCKSIZE];
struct posix_header header;
struct extra_header extra_header;
struct oldgnu_header oldgnu_header;
struct sparse_header sparse_header;
};
/* End of Format description. */
All characters in header blocks are represented by using 8-bit characters in the local variant of ASCII. Each field within the structure is contiguous; that is, there is no padding used within the structure. Each character on the archive medium is stored contiguously.
Bytes representing the contents of files (after the header block of
each file) are not translated in any way and are not constrained to
represent characters in any character set. The tar format
does not distinguish text files from binary files, and no translation
of file contents is performed.
The name, linkname, magic, uname,
and gname are null-terminated character strings. All other
fileds are zero-filled octal numbers in ASCII. Each numeric field of
width w contains w minus 2 digits, a space, and a
null, except size, and mtime, which do not
contain the trailing null.
The name field is the file name of the file, with
directory names (if any) preceding the file name, separated by slashes.
@FIXME{how big a name before field overflows?}
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.
@FIXME-xref{Modifiers}, 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.
@UNREVISED
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@UNREVISED
@FIXME{Reorganize the following material}
The cpio archive formats, like tar, do have
maximum pathname lengths. The binary and old ASCII formats have a max
path length of 256, and the new ASCII and CRC ASCII formats have a max
path length of 1024. GNU cpio can read and write archives
with arbitrary pathname lengths, but other cpio
implementations may crash unexplainedly trying to read them.
tar handles symbolic links in the form in which it comes
in BSD; cpio doesn't handle symbolic links in the form in
which it comes in System V prior to SVR4, and some vendors may have
added symlinks to their system without enhancing cpio to
know about them. Others may have enhanced it in a way other than the
way I did it at Sun, and which was adopted by AT&T (and which is, I
think, also present in the cpio that Berkeley picked up
from AT&T and put into a later BSD release--I think I gave them my
changes).
(SVR4 does some funny stuff with tar; basically, its cpio
can handle tar format input, and write it on output, and
it probably handles symbolic links. They may not have bothered doing
anything to enhance tar as a result.)
cpio handles special files; traditional tar
doesn't.
tar comes with V7, System III, System V, and BSD source; cpio
comes only with System III, System V, and later BSD (4.3-tahoe and
later).
tar's way of handling multiple hard links to a file can
handle file systems that support 32-bit inumbers (e.g., the BSD file
system); cpios way requires you to play some games (in its
"binary" format, i-numbers are only 16 bits, and in its
"portable ASCII" format, they're 18 bits--it would have to
play games with the "file system ID" field of the header to
make sure that the file system ID/i-number pairs of different files
were always different), and I don't know which cpios, if
any, play those games. Those that don't might get confused and think
two files are the same file when they're not, and make hard links
between them.
tars way of handling multiple hard links to a file places
only one copy of the link on the tape, but the name attached to that
copy is the only one you can use to retrieve the file; cpios
way puts one copy for every link, but you can retrieve it using any of
the names.
What type of check sum (if any) is used, and how is this calculated.
See the attached manual pages for tar and cpio
format. tar uses a checksum which is the sum of all the
bytes in the tar header for a file; cpio uses
no checksum.
If anyone knows why cpio was made when tar
was present at the unix scene,
It wasn't. cpio first showed up in PWB/UNIX 1.0; no
generally-available version of UNIX had tar at the time. I
don't know whether any version that was generally available within
AT&T had tar, or, if so, whether the people
within AT&T who did cpio knew about it.
On restore, if there is a corruption on a tape tar will
stop at that point, while cpio will skip over it and try
to restore the rest of the files.
The main difference is just in the command syntax and header format.
tar is a little more tape-oriented in that everything is
blocked to start on a record boundary.
Is there any differences between the ability to recover crashed archives between the two of them. (Is there any chance of recovering crashed archives at all.)
Theoretically it should be easier under tar since the
blocking lets you find a header with some variation of `dd skip=nn'.
However, modern cpio's and variations have an option to
just search for the next file header after an error with a reasonable
chance of re-syncing. However, lots of tape driver software won't allow
you to continue past a media error which should be the only reason for
getting out of sync unless a file changed sizes while you were writing
the archive.
If anyone knows why cpio was made when tar
was present at the unix scene, please tell me about this too.
Probably because it is more media efficient (by not blocking everything
and using only the space needed for the headers where tar
always uses 512 bytes per file header) and it knows how to archive
special files.
You might want to look at the freely available alternatives. The major
ones are afio, GNU tar, and pax,
each of which have their own extensions with some backwards
compatibility.
Sparse files were tarred as sparse files (which you can
easily test, because the resulting archive gets smaller, and GNU cpio
can no longer read it).
@UNREVISED
A few special cases about tape handling warrant more detailed description. These special cases are discussed below.
Many complexities surround the use of tar on tape drives.
Since the creation and manipulation of archives located on magnetic
tape was the original purpose of tar, it contains many
features making such manipulation easier.
Archives are usually written on dismountable media--tape cartridges, mag tapes, or floppy disks.
The amount of data a tape or disk holds depends not only on its size, but also on how it is formatted. A 2400 foot long reel of mag tape holds 40 megabytes of data when formated at 1600 bits per inch. The physically smaller EXABYTE tape cartridge holds 2.3 gigabytes.
Magnetic media are re-usable--once the archive on a tape is no longer needed, the archive can be erased and the tape or disk used over. Media quality does deteriorate with use, however. Most tapes or disks should be disgarded when they begin to produce data errors. EXABYTE tape cartridges should be disgarded when they generate an error count (number of non-usable bits) of more than 10k.
Magnetic media are written and erased using magnetic fields, and should be protected from such fields to avoid damage to stored data. Sticking a floppy disk to a filing cabinet using a magnet is probably not a good idea.
@UNREVISED
This option is used to specify the file name of the archive tar
works on.
If the file name is `-', tar reads the
archive from standard input (when listing or extracting), or writes it
to standard output (when creating). If the `-' file name
is given when updating an archive, tar will read the
original archive from its standard input, and will write the entire new
archive to its standard output.
If the file name contains a `:', it is interpreted as `hostname:file
name'. If the hostname contains an at sign (@),
it is treated as `user@hostname:file name'. In either
case, tar will invoke the command rsh (or remsh)
to start up an `/etc/rmt' on the remote machine. If you give
an alternate login name, it will be given to the rsh.
Naturally, the remote machine must have an executable `/etc/rmt'.
This program is free software from the University of California, and a
copy of the source code can be found with the sources for tar;
it's compiled and installed by default.
If this option is not given, but the environment variable TAPE
is set, its value is used; otherwise, old versions of tar
used a default archive name (which was picked when tar
was compiled). The default is normally set up to be the first
tape drive or other transportable I/O medium on the system.
Starting with version 1.11.5, GNU tar uses standard input
and standard output as the default device, and I will not try anymore
supporting automatic device detection at installation time. This was
failing really in too many cases, it was hopeless. This is now
completely left to the installer to override standard input and
standard output for default device, if this seems preferrable to
him/her. Further, I think most actual usages of tar
are done with pipes or disks, not really tapes, cartridges or
diskettes.
Some users think that using standard input and output is running after trouble. This could lead to a nasty surprise on your screen if you forget to specify an output file name--especially if you are going through a network or terminal server capable of buffering large amounts of output. We had so many bug reports in that area of configuring default tapes automatically, and so many contradicting requests, that we finally consider the problem to be portably intractable. We could of course use something like `/dev/tape' as a default, but this is also running after various kind of trouble, going from hung processes to accidental destruction of real tapes. After having seen all this mess, using standard input and output as a default really sounds like the only clean choice left, and a very useful one too.
GNU tar reads and writes archive in records, I suspect
this is the main reason why block devices are preferred over character
devices. Most probably, block devices are more efficient too. The
installer could also check for `DEFTAPE' in `<sys/mtio.h>'.
rsh. This
option exists so that people who use something other than the standard rsh
(e.g., a Kerberized rsh) can access a remote device.
When this command is not used, the shell command found when the tar
program was installed is used instead. This is the first found of `/usr/ucb/rsh', `/usr/bin/remsh', `/usr/bin/rsh', `/usr/bsd/rsh'
or `/usr/bin/nsh'. The installer may have overriden this
by defining the environment variable RSH at
installation time.
tar
to write a multi-volume archive--one that may be larger
than will fit on the medium used to hold it. See section Archives Longer than One Tape or Disk.
In order to access the tape drive on a remote machine, tar
uses the remote tape server written at the University of California at
Berkeley. The remote tape server must be installed as `/etc/rmt'
on any machine whose tape drive you want to use. tar
calls `/etc/rmt' by running an rsh or remsh
to the remote machine, optionally using a different login name if one
is supplied.
A copy of the source for the remote tape server is provided. It is Copyright (C) 1983 by the Regents of the University of California, but can be freely distributed. Instructions for compiling and installing it are included in the `Makefile'.
Unless you use the --absolute-names (-P)
option, GNU tar will not allow you to create an archive
that contains absolute file names (a file name beginning with `/'.)
If you try, tar will automatically remove the leading `/'
from the file names it stores in the archive. It will also type a
warning message telling you what it is doing.
When reading an archive that was created with a different tar
program, GNU tar automatically extracts entries in the
archive which have absolute file names as if the file names were not
absolute. This is an important feature. A visitor here once gave a tar
tape to an operator to restore; the operator used Sun tar
instead of GNU tar, and the result was that it replaced
large portions of our `/bin' and friends with versions from
the tape; needless to say, we were unhappy about having to recover the
file system from backup tapes.
For example, if the archive contained a file `/usr/bin/computoy',
GNU tar would extract the file to `usr/bin/computoy',
relative to the current directory. If you want to extract the files in
an archive to the same absolute names that they had when the archive
was created, you should do a `cd /' before extracting the
files from the archive, or you should either use the --absolute-names
(-P) option, or use the command `tar -C / ...'.
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
@UNREVISED
Block and record terminology is rather confused, and it is also confusing to the expert reader. On the other hand, readers who are new to the field have a fresh mind, and they may safely skip the next two paragraphs, as the remainder of this manual uses those two terms in a quite consistent way.
John Gilmore, the writer of the public domain tar from
which GNU tar was originally derived, wrote (June 1995):
The nomenclature of tape drives comes from IBM, where I believe they
were invented for the IBM 650 or so. On IBM mainframes, what is
recorded on tape are tape blocks. The logical organization of data is
into records. There are various ways of putting records into blocks,
including F (fixed sized records), V
(variable sized records), FB (fixed blocked: fixed size
records, n to a block), VB (variable size
records, n to a block), VSB (variable spanned
blocked: variable sized records that can occupy more than one block),
etc. The JCL `DD RECFORM=' parameter
specified this to the operating system.
The Unix man page on tar was totally confused about this.
When I wrote PD TAR, I used the historically correct
terminology (tar writes data records, which are grouped
into blocks). It appears that the bogus terminology made it into POSIX
(no surprise here), and now Fran@,{c}ois has migrated that terminology
back into the source code too.
The term physical block means the basic transfer chunk from or
to a device, after which reading or writing may stop without anything
being lost. In this manual, the term block usually refers to a
disk physical block, assuming that each disk block is 512
bytes in length. It is true that some disk devices have different
physical blocks, but tar ignore these differences in its
own format, which is meant to be portable, so a tar block
is always 512 bytes in length, and block always mean a tar
block. The term logical block often represents the basic
chunk of allocation of many disk blocks as a single entity, which the
operating system treats somewhat atomically; this concept is only
barely used in GNU tar.
The term physical record is another way to speak of a physical
block, those two terms are somewhat interchangeable. In this manual,
the term record usually refers to a tape physical block, assuming
that the tar archive is kept on magnetic tape. It is true
that archives may be put on disk or used with pipes, but nevertheless, tar
tries to read and write the archive one record at a time,
whatever the medium in use. One record is made up of an integral number
of blocks, and this operation of putting many disk blocks into a single
tape block is called reblocking, or more simply, blocking.
The term logical record refers to the logical organization of
many characters into something meaningful to the application. The term unit
record describes a small set of characters which are transmitted
whole to or by the application, and often refers to a line of text.
Those two last terms are unrelated to what we call a record in
GNU tar.
When writing to tapes, tar writes the contents of the
archive in chunks known as records. To change the default
blocking factor, use the --blocking-factor=512-size
(-b 512-size) option. Each record will then be
composed of 512-size blocks. (Each tar block is
512 bytes. See section The Standard Format.)
Each file written to the archive uses at least one full record. As a
result, using a larger record size can result in more wasted space for
small files. On the other hand, a larger record size can often be read
and written much more efficiently.
Further complicating the problem is that some tape drives ignore the blocking entirely. For these, a larger record size can still improve performance (because the software layers above the tape drive still honor the blocking), but not as dramatically as on tape drives that honor blocking.
When reading an archive, tar can usually figure out the
record size on itself. When this is the case, and a non-standard record
size was used when the archive was created, tar will print
a message about a non-standard blocking factor, and then operate
normally. On some tape devices, however, tar cannot figure
out the record size itself. On most of those, you can specify a
blocking factor (with --blocking-factor=512-size (-b 512-size))
larger than the actual blocking factor, and then use the --read-full-records
(-B) option. (If you specify a blocking factor with --blocking-factor=512-size
(-b 512-size) and don't use the --read-full-records
(-B) option, then tar will not attempt to
figure out the recording size itself.) On some devices, you must always
specify the record size exactly with --blocking-factor=512-size
(-b 512-size) when reading, because tar
cannot figure it out. In any case, use --list (-t)
before doing any extractions to see whether tar is reading
the archive correctly.
tar blocks are all fixed size (512 bytes), and its scheme
for putting them into records is to put a whole number of them (one or
more) into each record. tar records are all the same size;
at the end of the file there's a block containing all zeros, which is
how you tell that the remainder of the last record(s) are garbage.
In a standard tar file (no options), the block size is 512
and the record size is 10240, for a blocking factor of 20. What the --blocking-factor=512-size
(-b 512-size) option does is sets the blocking
factor, changing the record size while leaving the block size at 512
bytes. 20 was fine for ancient 800 or 1600 bpi reel-to-reel tape
drives; most tape drives these days prefer much bigger records in order
to stream and not waste tape. When writing tapes for myself, some tend
to use a factor of the order of 2048, say, giving a record size of
around one megabyte.
If you use a blocking factor larger than 20, older tar
programs might not be able to read the archive, so we recommend this
as a limit to use in practice. GNU tar, however, will
support arbitrarily large record sizes, limited only by the amount of
virtual memory or the physical characteristics of the tape device.
Format parameters specify how an archive is written on the archive media. The best choice of format parameters will vary depending on the type and number of files being archived, and on the media used to store the archive.
To specify format parameters when accessing or creating an archive, you
can use the options described in the following sections. If you do not
specify any format parameters, tar uses default
parameters. You cannot modify a compressed archive. If you create an
archive with the --blocking-factor=512-size (-b 512-size)
option specified (see section The Blocking
Factor of an Archive), you must specify that blocking-factor when
operating on the archive. See section Controlling
the Archive Format, for other examples of format parameter
considerations.
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=512-size (-b 512-size) 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 section How to Create Archives, for information on
writing archives.
@FIXME{Need example of using a cartridge with blocking factor=126 or more.}
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 section How
to List Archives, for more information on the --list (-t)
operation. See section Options to Help Read
Archives, for a more detailed explanation of that option.
Device blocking
tar,
will do reads and writes of the archive in records of block*512
bytes. This is true even when the archive is compressed. Some devices
requires that all write operations be a multiple of a certain size, and
so, tar pads the archive out to the next record
boundary. The default blocking factor is set when tar
is compiled, and is typically 20. Blocking factors larger than 20
cannot be read by very old versions of tar, or by some
newer versions of tar running on old machines with
small address spaces. With a magnetic tape, larger records give faster
throughput and fit more data on a tape (because there are fewer
inter-record gaps). If the archive is in a disk file or a pipe, you may
want to specify a smaller blocking factor, since a large one will
result in a large number of null bytes at the end of the archive. When
writing cartridge or other streaming tapes, a much larger blocking
factor (say 126 or more) will greatly increase performance. However,
you must specify the same blocking factor when reading or updating the
archive. Apparently, Exabyte drives have a physical block size of 8K
bytes. If we choose our blocksize as a multiple of 8k bytes, then the
problem seems to 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.
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.
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.
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
@FIXME{Appropriate options should be moved here from elsewhere.}
When handling various tapes or cartridges, you have to take care of selecting a proper blocking, that is, the number of disk blocks you put together as a single tape block on the tape, without intervening tape gaps. A tape gap is a small landing area on the tape with no information on it, used for decelerating the tape to a full stop, and for later regaining the reading or writing speed. When the tape driver starts reading a record, the record has to be read whole without stopping, as a tape gap is needed to stop the tape motion without loosing information.
Using higher blocking (putting more disk blocks per
tape block) will use the tape more efficiently as there will be less
tape gaps. But reading such tapes may be more difficult for the system,
as more memory will be required to receive at once the whole record.
Further, if there is a reading error on a huge record, this is less
likely that the system will succeed in recovering the information. So,
blocking should not be too low, nor it should be too high. tar
uses by default a blocking of 20 for historical reasons, and it does
not really matter when reading or writing to disk. Current tape
technology would easily 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 depends of the buffering techniques used inside modern
tape controllers. Some imposes a minimum blocking, or a maximum
blocking. Others request blocking to be some exponent of two.
So, there is no fixed rule for blocking. But blocking at read time should ideally be the same as blocking used at write time. At one place I know, with a wide variety of equipment, they found it best to use a blocking of 32 to guarantee that their tapes are fully interchangeable.
I was also told that, for recycled tapes, prior erasure (by the same drive unit that will be used to create the archives) sometimes lowers the error rates observed at rewriting time.
I might also use `--number-blocks' instead of `--block-number', so `--block' will then expand to `--blocking-factor' unambiguously.
@FIXME{Appropriate options should be moved here from elsewhere.}
Most tape devices have two entries in the `/dev' directory, or entries that come in pairs, which differ only in the minor number for this device. Let's take for example `/dev/tape', which often points to the only or usual tape device of a given system. There might be a corresponding `/dev/nrtape' or `/dev/ntape'. The simpler name is the rewinding version of the device, while the name having `nr' in it is the no rewinding version of the same device.
A rewinding tape device will bring back the tape to its beginning point
automatically when this device is opened or closed. Since tar
opens the archive file before using it and closes it afterwards, this
means that a simple:
$ tar cf /dev/tape directory
will reposition the tape to its beginning both prior and after saving directory contents to it, thus erasing prior tape contents and making it so that any subsequent write operation will destroy what has just been saved.
So, a rewinding device is normally meant to hold one
and only one file. If you want to put more than one tar
archive on a given tape, you will need to avoid using the rewinding
version of the tape device. You will also have to pay special attention
to tape positioning. Errors in 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:
$ mt -f /dev/nrtape rewind $ tar cf /dev/nrtape directory-1
Tape marks are special magnetic patterns
written on the tape media, which are later recognizable by the reading
hardware. These marks are used after each file, when there are many on
a single tape. An empty file (that is to say, two tape marks in a row)
signal the logical end of the tape, after which no file exist. Usually,
non-rewinding tape device drivers will react to the close request
issued by tar by first writing two tape marks after your
archive, and by backspacing over one of these. So, if you remove the
tape at that time from the tape drive, it is properly terminated. But
if you write another file at the current position, the second tape mark
will be erased by the new information, leaving only one tape mark
between files.
So, you may now save directory-2 as a second archive after the first on the same tape by issuing the command:
$ tar cf /dev/nrtape directory-2
and so on for all the archives you want to put on the same tape.
Another usual case is that you do not write all the archives the same day, and you need to remove and store the tape between two archive sessions. In general, you must remember how many files are already saved on your tape. Suppose your tape already has 16 files on it, and that you are ready to write the 17th. You have to take care of skipping the first 16 tape marks before saving directory-17, say, by using these commands:
$ mt -f /dev/nrtape rewind $ mt -f /dev/nrtape fsf 16 $ tar cf /dev/nrtape directory-17
In all the previous examples, we put aside blocking considerations, but you should do the proper things for that as well. See section Blocking.
@UNREVISED
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. @FIXME{There is no such restore script!}.
@FIXME-xref{Scripted Restoration}). See section The mt Utility, 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@UNREVISED
@FIXME{Is it true that this only works on non-block devices? should explain the difference, (fixed or variable).} See section The Blocking Factor of an Archive.
You can use the mt utility to advance or rewind a tape
past a specified number of archive files on the tape. This will allow
you to move to the beginning of an archive before extracting or reading
it, or to the end of all the archives before writing a new one.
@FIXME{Why isn't there an "advance 'til you find two tape marks
together"?}
The syntax of the mt command is:
mt [-f tapename] operation [number]
where tapename is the name of the tape device, number is the number of times an operation is performed (with a default of one), and operation is one of the following:
@FIXME{is there any use for record operations?}
@FIXME{Is there a better way to frob the spacing on the list?}
If you don't specify a tapename, mt uses the
environment variable TAPE; if TAPE does not exist, 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 an operation
failed.
@FIXME{New node on how to find an archive?}
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 section Including
a Label in the Archive. @FIXME-xref{Matching Format Parameters}.
@FIXME{fix cross references} `tar --list --label' will
cause tar to print the label.
@FIXME{Program to list all the labels on a tape?}
@UNREVISED
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 only type `y' 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=1024-size
(-L 1024-size) 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 section Including
a Label in the Archive, 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
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 section How to Create Archives). 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 described 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 section Extracting
an Entire Archive, 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 operations, you need to use the entire archive.
If a multi-volume archive was labeled using --label=archive-label
(-V archive-label) (see section Including a Label in the Archive) when it
was created, tar will not automatically label volumes
which are added later. To label subsequent volumes, specify --label=archive-label
(-V archive-label) again in conjunction with the --append
(-r), --update (-u) or --concatenate
(--catenate, -A) operation.
@FIXME{There should be a sample program here, including an exit before end. Is the exit status even checked in tar? :-(}
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.
@UNREVISED
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) (@FIXME-pxref{Using Multiple Tapes}), then the volume label will have `Volume nnn' appended to the name you give, where nnn is the number of the volume of the archive. (If you use the --label=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 section Including a Label in the Archive.
When tar writes an archive to tape, it creates a single
tape file. If multiple archives are written to the same tape, one after
the other, they each get written as separate tape files. When
extracting, it is necessary to position the tape at the right place
before running tar. To do this, use the mt
command. For more information on the mt command and on
the organization of tapes into a sequence of tape files, see section The mt Utility.
People seem to often do:
--label="some-prefix `date +some-format`"
or such, for pushing a common date in all volumes or an archive set.
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.
@FIXME{Should the arg to --label be a quoted string?? No.}
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) operation to include a label entry in the archive as it is being created.
If you create an archive using both --label=archive-label (-V archive-label) and --multi-volume (-M), each volume of the archive will have an archive label of the form `archive-label Volume n', where n is 1 for the first volume, 2 for the next, and so on. @FIXME-xref{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 section Excluding Some Files,
for a precise description of how match is attempted(7). 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 \
--volume="Daily backup for `date +%Y-%m-%d`"
Also note that each label has its own date and time, which corresponds
to when GNU tar initially attempted to write it, often
soon after the operator launches tar or types the carriage
return telling that the next tape is ready. Comparing date labels does
give an idea of tape throughput only if the delays for rewinding tapes
and the operator switching them were negligible, which is ususally not
the case.
@FIXME{was --volume}
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) operation.
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 explicitely 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 section Comparing Archive Members with the File System.
Note that these two options have a slightly different intent. The --compare
(--diff, -d) option 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) operation, 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.
Jump to: - - a - b - c - d - e - f - g - h - i - j - l - m - n - o - p - r - s - t - u - v - w - y
--list with file name
arguments
getdate
cat vs concatenate
concatenate vs cat
tar
--list
with
--delete and
tar program
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'
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.
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.
This document was generated on 7 November 1998 using the texi2html translator version 1.52.