This is Info file tar.info, produced by Makeinfo version 1.67 from the
input file tar.texi.

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

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

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

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

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

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

   This file documents GNU `tar', which is a utility used to store,
backup, and transport files.  `tar' is a tape (or disk) archiver.  This
manual documents the release 1.12.


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

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

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

`--create' (`-c')
     Creating an empty archive would have some kind of elegance.  One
     can initialize an empty archive and later use `--append' (`-r')
     for adding all members.  Some applications would not welcome
     making an exception in the way of adding the first archive member.
     On the other hand, many people reported that it is dangerously
     too easy for `tar' to destroy a magnetic tape with an empty
     archive(1).  The two most common errors are:

       1. Mistakingly using `create' instead of `extract', when the
          intent was to extract the full contents of an archive.  This
          error is likely: keys `c' and `x' are right next ot each
          other on the QWERTY keyboard.  Instead of being unpacked, the
          archive then gets wholly destroyed.  When users speak about
          "exploding" an archive, they usually mean something else :-).

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

     So, recognizing the likelihood and the catastrophical nature of
     these errors, GNU `tar' now takes some distance from elegance, and
     cowardly refuses to create an archive when `--create' (`-c')
     option is given, there are no arguments besides options, and
     `--files-from=FILE-OF-NAMES' (`-T FILE-OF-NAMES') option is *not*
     used.  Observe:

          $ tar c
          tar: Cowardly refusing to create an empty archive
          Try `tar --help' for more information.
          $ tar cf foo
          tar: Cowardly refusing to create an empty archive
          Try `tar --help' for more information.

     To get around the cautiousness of GNU `tar' and nevertheless
     create an archive with nothing in it, one may still use, as the
     value for the `--files-from=FILE-OF-NAMES' (`-T FILE-OF-NAMES')
     option, a file with no names in it, as shown in the following
     commands:

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

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

     When archives created on Unix are extracted on MS-DOS and
     MS-Windows 3.x, several problems pop up due to their
     incompatibilities with the Unix filesystem.  GNU `tar' has several
     trick up its sleeve, to overcome such problems:

        * Illegal characters in file names MS-DOS filesystem disallows
          certain characters from appearing in a file name.  Except on
          Windows 9X, it also doesn't allow file names with a leading
          dot and file names with more that a single dot in the basename
          of the file.

          GNU `tar' tries to repair such names automatically, by
          replacing the offending characters with legal ones.  For
          example, `.emacs' is exracted as `_emacs', `foo.bar.c' will
          be converted to `foo_bar.c', `lost+found' will become
          `lost_found', etc.  (`tar' even tries to be intelligent by
          treating some cases specially; for example, files which end
          with `.c++' are extracted as `.cxx'.)

          When the file is thus renamed, `tar' will print a message to
          that effect when operating verbosely (*note verbose::.).
          Note that if a directory needs to be renamed, `tar' will
          print such a message for all the files in that directory.

        * DOS device names MS-DOS and MS-Windows define its own
          reserved names of character devices.  For example, the
          console device is `CON', the printer device is `PRN', the
          serial port device is `AUX', etc.

          These names are special on DOS, in that files with these
          names, if they exist in any directory, are inaccessible: the
          device driver will intercept any file-related call before the
          filesystem ever sees it.  (That's because MS-DOS wants to
          create an illusion that the devices are present in every
          directory; in contrast, on Unix the devices live under a
          special directory called `/dev'.)

          When archives created on Unix are unpacked on MS-DOS, they
          might include files with these "forbidden" names, such as
          `prn.txt' or `aux.c'.  Trying to extract such files will lead
          to crashes, system wedges, and other atrocities, as the
          device drivers will try to interpret the data they get and
          act on it (e.g., some devices are read-only, and don't know
          how to handle incoming data).

          Therefore, `tar' checks every file name it is about to
          extract, and if it refers to a character device, `tar'
          changes that file's name by prepending an underscore `_' to
          it; thus, `aux.c' will be extracted as `_aux.c'.  (Actually,
          `tar' tries to prepend up to 2 underscore characters, each
          time checking if the new name is a regular file.  If both
          attempts are insuccessful, `tar' will print an error message
          and refuse to extract that file.)

          As with illegal characters, `tar' announces each renamed file
          when under verbose operation.

`--list' (`-t')
     GNU `tar' now shows dates as `1996-11-09', while it used to show
     them as `Nov 11 1996'.  (One can revert to the old behavior by
     defining `USE_OLD_CTIME' in `src/list.c' before reinstalling.) But
     preferrably, people you should get used to ISO 8601 dates.  Local
     American dates should be made available again with full date
     localisation support, once ready.  In the meantime, programs not
     being localisable for dates should prefer international dates,
     that's really the way to go.

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

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

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


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

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

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

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

* Menu:

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


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

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

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

   In the last chapter, you learned about the first three operations to
`tar'.  This chapter presents the remaining five operations to `tar':
`--append', `--update', `--concatenate', `--delete', and `--compare'.

   You are not likely to use these operations as frequently as those
covered in the last chapter; however, since they perform specialized
functions, they are quite useful when you do need to use them.  We will
give examples using the same directory and files that you created in
the last chapter.  As you may recall, the directory is called
`practice', the files are `jazz', `blues', `folk', `rock', and the two
archive files you created are `collection.tar' and `music.tar'.

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

   Unless we state otherwise, all practicing you do and examples you
follow in this chapter will take place in the `practice' directory that
you created in the previous chapter; see *Note prepare for examples::.
(Below in this section, we will remind you of the state of the examples
where the last chapter left them.)

   The five operations that we will cover in this chapter are:

`--append'
`-r'
     Add new entries to an archive that already exists.

`--update'
`-r'
     Add more recent copies of archive members to the end of an
     archive, if they exist.

`--concatenate'
`--catenate'
`-A'
     Add one or more pre-existing archives to the end of another
     archive.

`--delete'
     Delete items from an archive (does not work on tapes).

`--compare'
`--diff'
`-d'
     Compare archive members to their counterparts in the file system.


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

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

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

     $ ls
     blues   collection.tar   folk   jazz

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

     $ tar -tvf collection.tar
     jazz
     blues
     folk

The archive file `../music.tar', in the parent directory, looks like
this:

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


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

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

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

   If you want to add files to an existing archive, you don't need to
create a new archive; you can use `--append' (`-r').  The archive must
already exist in order to use `--append'.  (A related operation is the
`--update' operation; you can use this to add newer versions of archive
members to an existing archive.  To learn how to do this with
`--update', *note update::..)

   If you use `--append' (`-r') to add a file to an archive which
already contains a member with the same name, then the old member is
not deleted.  What does happen, however, is somewhat complex.  `tar'
*allows* you to have infinite numbers of archive members with the same
name.  Some 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 other members would end up in the working directory.  This
is because `--extract' extracts an archive in the order the members
appeared in the archive; the most recently archived members will be
extracted last.  Additionally, an extracted member will *overwrite* a
file of the same name which existed in the directory already, and `tar'
will not prompt you about this.  Thus, only the most recently archived
member will end up being extracted, as it will overwrite the one
extracted before it, and so on.  (One way of extracting all of the
members wiithout overwriting them is to use the `--backup' option, see
*Note backup::.)

   Other than using `--backup', there are a few additional ways to get
around this.

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

* Menu:

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


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

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

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

   The simplest way to add a file to an already existing archive is the
`--append' (`-r') operation, which writes specified files into the
archive whether or not they are already among the archived files.  When
you use `--append', you *must* specify file name arguments, as there is
no default.  If you specify a file that already exists in the archive,
another copy of the file will be added to the end of the archive.  As
with other operations, the member names of the newly added files will
be exactly the same as their names given on the command line (except
that backslashes in MS-DOS file names are mirrored to Unix-style
forward slashes).  The `--verbose' (`-v') option will print out the
names of the files as they are written into the archive.

   `--append' cannot be performed on some tape drives, unfortunately,
due to deficiencies in the formats those tape drives use.  The archive
must be a valid `tar' archive, or else the results of using this
operation will be unpredictable.  *Note Media::.

   To demonstrate use of `--append' to add a file to an archive, create
a file called `rock' in the `practice' directory.  Make sure you are in
the `practice' directory.  Then, run the following `tar' command to add
`rock' to `collection.tar':

     $ tar --append --file=collection.tar rock

If you now use the `--list' (`-t') operation, you will see that `rock'
has been added to the archive:

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


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

Multiple Files with the Same Name
.................................

   You can use `--append' (`-r') to add copies of files which have been
updated since the archive was created.  (However, we do not recommend
doing this since there is another `tar' option called `--update'; *note
update::. for more information.  We describe this use of `--append'
here for the sake of completeness.)   When you extract the archive, the
older version will be effectively lost.  This works because files are
extracted from an archive in the order in which they were archived.
Thus, when the archive is extracted, a file archived later in time will
overwrite a file of the same name which was archived earlier, even
though the older version of the file will remain in the archive unless
you delete all versions of the file.

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

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

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

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

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

The newest version of `blues' is now at the end of the archive (note
the different creation dates and file sizes).  If you extract the
archive, the older version of the file `blues' will be overwritten by
the newer version.  You can confirm this by extracting the archive and
running `ls' on the directory.  *Note Writing:: for more information.
(*Please note:* This is the case unless you employ the `--backup'
option; .)


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

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

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

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

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

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

* Menu:

* how to update::


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

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

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

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

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

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

   (The reason `tar' does not overwrite the older file when updating it
is because writing to the middle of a section of tape is a difficult
process.  Tapes are not designed to get data inserted in their middle;
doing so would destroy any data after the place you write.  *Note
Media:: for more information about tapes.)

   `--update' (`-u') is not suitable for performing backups for two
reasons: it does not change directory content entries, and it lengthens
the archive every time it is used.  The GNU `tar' options intended
specifically for backups are more efficient.  If you need to run
backups, please consult *Note Backups::.


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

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

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

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

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

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

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

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

   We can concatenate these two archives with `tar':

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

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

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

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

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

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

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

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


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

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

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

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

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

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

   To delete all versions of the file `blues' from the archive
`collection.tar' in the `practice' directory, make sure you are in that
directory, and then,

     $ tar --list --file=collection.tar
     jazz
     blues
     folk
     rock
     blues
     blues
     classical
     $ tar --delete --file=collection.tar blues
     $ tar --list --file=collection.tar
     jazz
     folk
     rock
     classical
     $

   The `--delete' option has been reported to work properly when `tar'
acts as a filter from `stdin' to `stdout'.


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

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

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

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

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

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

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

     $ tar --compare --file=bluesrock.tar rock blues funk
     rock
     blues
     funk: File does not exist

If the file in the archive is a link, `tar' may have a different error
message, such as:

     funk: Does not exist

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

   Some filesystems, such as the FAT and VFAT used on MS-DOS and
MS-Windows, only maintain file modification times up to the nearest even
second.  `tar' compiled for such systems knows about this, and doesn't
report difference in mod times unless it is larger than one second.
But if you run `tar' on Unix and your files reside on a FAT disk (via
some network interface), you could have some false alarms in this case.


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

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

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

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

* Menu:

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


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

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

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

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

   The `--read-full-records' (`-B') option is turned on by default when
`tar' reads an archive from standard input, or from a remote machine.
This is because on BSD Unix systems, attempting to read a pipe returns
however much happens to be in the pipe, even if it is less than was
requested.  If this option were not enabled, `tar' would fail as soon
as it read an incomplete record from the pipe.

   If you're not sure of the blocking factor of an archive, you can
read the archive by specifying `--read-full-records' (`-B') and
`--blocking-factor=BLOCKS' (`-b BLOCKS'), using a blocking factor
larger than what the archive uses.  This lets you avoid having to
determine the blocking factor of an archive.  *Note Blocking Factor::.

* Menu:

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


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

Reading Full Records
....................

`--read-full-records'
`-B'
     Use in conjunction with `--extract' (`--get', `-x') to read an
     archive which contains incomplete records, or one which has a
     blocking factor less than the one specified.


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

Ignoring Blocks of Zeros
........................

   Normally, `tar' stops reading when it encounters a block of zeros
between file entries (which usually indicates the end of the archive).
`--ignore-zeros' (`-i') allows `tar' to completely read an archive
which contains a block of zeros before the end (i.e. a damaged archive,
or one which was created by `cat'-ing several archives together).

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

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


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

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

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


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

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

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

* Menu:

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


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

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

   Normally, `tar' writes extracted files into the file system without
regard to the files already on the system; i.e., files with the same
names as archive members are overwritten when the archive is extracted.
If the name of a corresponding file name is a symbolic link, the file
pointed to by the symbolic link will be overwritten instead of the
symbolic link itself (if this is possible).  Moreover, special devices,
empty directories and even symbolic links are automatically removed if
they are found to be in the way of the proper extraction.

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

   The `--unlink-first' (`-U') option removes existing files, symbolic
links, empty directories, devices, etc., *prior* to extracting over
them.  This could take care of several situations where overwriting
files is undesirable.

   One such situation is when the archive includes a regular file
whereas the filesystem has a symbolic link by the same name.  Using
this option will prevent `tar' from overwriting the file to which the
symbolic link points by the contents of an extracted file, since the
link itself is removed prior to the extraction.

   Another situation is with overwriting a running program's executable
file.  On some systems, the backing store for the executable *is* the
original program file; overwriting it while the program runs could lead
to segmentation violation and other crashes.  You could use the
`--unlink-first' (`-U') option to prevent such woes when extracting
arbitrary executables over currently running copies.  Note that if
something goes wrong with the extraction and you *did* use this option,
you might end up with no file at all.  Without this option, if
something goes wrong with the extraction, the existing file is not
overwritten and preserved.  (If you want to be safe either way, use
both `--unlink-first' (`-U') and `--backup'; *Note backup::.)

   If you specify the `--recursive-unlink' option, `tar' removes
*anything* that keeps you from extracting a file as far as current
permissions will allow it.  This could include removal of the contents
of a full directory hierarchy.  For example, someone using this feature
may be very surprised at the results when extracting a directory entry
from the archive.  This option can be dangerous; be very aware of what
you are doing if you choose to use it.

   Without `--recursive-unlink', GNU `tar' silently overwrites
non-directories, but will never overwrite existing non-empty
directories.

   One especially nasty subtlety with both `--unlink-first' (`-U') and
`--recursive-unlink' options happens on filesystems which silently
truncate the names of the files in the archive when they are extracted
to the file system.  (MS-DOS and MS-Windows 3.X are two notorious
examples.)  In such case, `tar' might incidentally delete a file or a
directory which has nothing to do with the extracted file, only because
their names are the same after truncation.  We recommend not to use
these two options on such filesystems.  In fact, we recommend that if
you unpack on MS-DOS archives which were created on Unix, you *always*
use either `--keep-old-files' (`-k') or `--backup', to prevent
overwriting existing files due to file name truncation.  *Note backup::.

* Menu:

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


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

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

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


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

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

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


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

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

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

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


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

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

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

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

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


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

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

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

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


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

Writing to Standard Output
..........................

   To write the extracted files to the standard output, instead of
creating the files on the file system, use `--to-stdout' (`-O') in
conjunction with `--extract' (`--get', `-x').  This option is useful if
you are extracting files to send them through a pipe, and do not need to
preserve them in the file system.  You might need to use this option to
browse one or more files with a pager such as `less', or if you want
the extracted file to have another name on the file system.

   If you extract multiple members, they appear on standard output
concatenated, in the order they are found in the archive.

`--to-stdout'
`-O'
     Writes files to the standard output.  Used in conjunction with
     `--extract' (`--get', `-x').  Extract files to standard output.
     When this option is used, instead of creating the files specified,
     `tar' writes the contents of the files extracted to its standard
     output.  This may be useful if you are only extracting the files
     in order to send them through a pipe.  This option is meaningless
     with `--list' (`-t').


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

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

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


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

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

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

* Menu:

* Starting File::
* Same Order::


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

Starting File
.............

`--starting-file=NAME'
`-K NAME'
     Starts an operation in the middle of an archive.  Use in
     conjunction with `--extract' (`--get', `-x') or `--list' (`-t').

   If a previous attempt to extract files failed due to lack of disk
space, you can use `--starting-file=NAME' (`-K NAME') to start
extracting only after member NAME of the archive.  This assumes, of
course, that there is now free space, or that you are now extracting
into a different file system.  (You could also choose to suspend `tar',
remove unnecessary files from the file system, and then restart the
same `tar' operation.  In this case, `--starting-file=NAME' (`-K NAME')
is not necessary.  *Note Inc Dumps::, *Note interactive::, and
*Note exclude::.)


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

Same Order
..........

`--same-order'
`--preserve-order'
`-s'
     To process large lists of file names on machines with small
     amounts of memory.  Use in conjunction with `--compare' (`--diff',
     `-d'), `--list' (`-t') or `--extract' (`--get', `-x').

   The `--same-order' (`--preserve-order', `-s') option tells `tar'
that the list of file names to be listed or extracted is sorted in the
same order as the files in the archive.  This allows a large list of
names to be used, even on a small machine that would not otherwise be
able to hold all the names in memory at the same time.  Such a sorted
list can easily be created by running `tar -t' on the archive and
editing its output.

   This option is probably never needed on modern computer systems.


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

Backup options
==============

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

   Backup options may prove unexpectedly useful when extracting archives
containing many members having identical name, or when extracting
archives on systems having file name limitations, making different
members appear has having similar names through the side-effect of name
truncation.  (This is true only if we have a good scheme for truncated
backup names, which I'm not sure at all: I suspect work is needed in
this area.  The MS-DOS/MS-Windows version works with numbered backups,
even when file names are truncated.) When any existing file is backed
up before being overwritten by extraction, then clashing files are
automatically 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.  We recommend that you
*always* use numbered backups when unpacking archives on MS-DOS and on
other systems that have file name limitations.

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

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

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

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

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

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

    `t'
    `numbered'
          Always make numbered backups.

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

    `never'
    `simple'
          Always make simple backups.

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

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


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

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

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

   You can easily use archive files to transport a group of files from
one system to another: put all relevant files into an archive on one
computer system, transfer the archive to another system, and extract
the contents there.  The basic transfer medium might be magnetic tape,
Internet FTP, or even electronic mail (though you must encode the
archive with `uuencode' in order to transport it properly by mail).
Both machines do not have to use the same operating system, as long as
they both support the `tar' program.

   For example, here is how you might copy a directory's contents from
one disk to another, while preserving the dates, modes, owners and
link-structure of all the files therein.  In this case, the transfer
medium is a "pipe", which is one a Unix redirection mechanism:

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

The command also works using short option forms:

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

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


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

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

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

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

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

