path: root/doc/tar.info-2

This is tar.info, produced by makeinfo version 4.8 from tar.texi.

   This manual is for GNU `tar' (version 1.17, 8 June 2007), which
creates and extracts files from archives.

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

     Permission is granted to copy, distribute and/or modify this
     document under the terms of the GNU Free Documentation License,
     Version 1.1 or any later version published by the Free Software
     Foundation; with no Invariant Sections, with the Front-Cover Texts
     being "A GNU Manual," and with the Back-Cover Texts as in (a)
     below.  A copy of the license is included in the section entitled
     "GNU Free Documentation License".

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

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

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


File: tar.info,  Node: Portability,  Next: cpio,  Prev: Attributes,  Up: Formats

8.3 Making `tar' Archives More Portable
=======================================

Creating a `tar' archive on a particular system that is meant to be
useful later on many other machines and with other versions of `tar' is
more challenging than you might think.  `tar' archive formats have been
evolving since the first versions of Unix.  Many such formats are
around, and are not always compatible with each other.  This section
discusses a few problems, and gives some advice about making `tar'
archives more portable.

   One golden rule is simplicity.  For example, limit your `tar'
archives to contain only regular files and directories, avoiding other
kind of special files.  Do not attempt to save sparse files or
contiguous files as such.  Let's discuss a few more problems, in turn.

* Menu:

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


File: tar.info,  Node: Portable Names,  Next: dereference,  Up: Portability

8.3.1 Portable Names
--------------------

Use portable file and member names.  A name is portable if it contains
only ASCII letters and digits, `/', `.', `_', and `-'; it cannot be
empty, start with `-' or `//', or contain `/-'.  Avoid deep directory
nesting.  For portability to old Unix hosts, limit your file name
components to 14 characters or less.

   If you intend to have your `tar' archives to be read under MSDOS,
you should not rely on case distinction for file names, and you might
use the GNU `doschk' program for helping you further diagnosing illegal
MSDOS names, which are even more limited than System V's.


File: tar.info,  Node: dereference,  Next: old,  Prev: Portable Names,  Up: Portability

8.3.2 Symbolic Links
--------------------

Normally, when `tar' archives a symbolic link, it writes a block to the
archive naming the target of the link.  In that way, the `tar' archive
is a faithful record of the file system contents.  `--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.


File: tar.info,  Node: old,  Next: ustar,  Prev: dereference,  Up: Portability

8.3.3 Old V7 Archives
---------------------

Certain old versions of `tar' cannot handle additional information
recorded by newer `tar' programs.  To create an archive in V7 format
(not ANSI), which can be read by these old versions, specify the
`--format=v7' option in conjunction with the `--create' (`-c') (`tar'
also accepts `--portability' or `--old-archive' for this option).  When
you specify it, `tar' leaves out information about directories, pipes,
fifos, contiguous files, and device files, and specifies file ownership
by group and user IDs instead of group and user names.

   When updating an archive, do not use `--format=v7' unless the
archive was created using this option.

   In most cases, a _new_ format archive can be read by an _old_ `tar'
program without serious trouble, so this option should seldom be
needed.  On the other hand, most modern `tar's are able to read old
format archives, so it might be safer for you to always use
`--format=v7' for your distributions.  Notice, however, that `ustar'
format is a better alternative, as it is free from many of `v7''s
drawbacks.


File: tar.info,  Node: ustar,  Next: gnu,  Prev: old,  Up: Portability

8.3.4 Ustar Archive Format
--------------------------

Archive format defined by POSIX.1-1988 specification is called `ustar'.
Although it is more flexible than the V7 format, it still has many
restrictions (*Note ustar: Formats, for the detailed description of
`ustar' format).  Along with V7 format, `ustar' format is a good choice
for archives intended to be read with other implementations of `tar'.

   To create archive in `ustar' format, use `--format=ustar' option in
conjunction with the `--create' (`-c').


File: tar.info,  Node: gnu,  Next: posix,  Prev: ustar,  Up: Portability

8.3.5 GNU and old GNU `tar' format
----------------------------------

GNU `tar' was based on an early draft of the POSIX 1003.1 `ustar'
standard.  GNU extensions to `tar', such as the support for file names
longer than 100 characters, use portions of the `tar' header record
which were specified in that POSIX draft as unused.  Subsequent changes
in POSIX have allocated the same parts of the header record for other
purposes.  As a result, GNU `tar' format is incompatible with the
current POSIX specification, and with `tar' programs that follow it.

   In the majority of cases, `tar' will be configured to create this
format by default.  This will change in future releases, since we plan
to make `POSIX' format the default.

   To force creation a GNU `tar' archive, use option `--format=gnu'.


File: tar.info,  Node: posix,  Next: Checksumming,  Prev: gnu,  Up: Portability

8.3.6 GNU `tar' and POSIX `tar'
-------------------------------

Starting from version 1.14 GNU `tar' features full support for
POSIX.1-2001 archives.

   A POSIX conformant archive will be created if `tar' was given
`--format=posix' (`--format=pax') option.  No special option is
required to read and extract from a POSIX archive.

* Menu:

* PAX keywords:: Controlling Extended Header Keywords.


File: tar.info,  Node: PAX keywords,  Up: posix

8.3.6.1 Controlling Extended Header Keywords
............................................

`--pax-option=KEYWORD-LIST'
     Handle keywords in PAX extended headers.  This option is
     equivalent to `-o' option of the `pax' utility.

   KEYWORD-LIST is a comma-separated list of keyword options, each
keyword option taking one of the following forms:

`delete=PATTERN'
     When used with one of archive-creation commands, this option
     instructs `tar' to omit from extended header records that it
     produces any keywords matching the string PATTERN.

     When used in extract or list mode, this option instructs tar to
     ignore any keywords matching the given PATTERN in the extended
     header records.  In both cases, matching is performed using the
     pattern matching notation described in POSIX 1003.2, 3.13 (*note
     wildcards::).  For example:

          --pax-option delete=security.*

     would suppress security-related information.

`exthdr.name=STRING'
     This keyword allows user control over the name that is written
     into the ustar header blocks for the extended headers.  The name
     is obtained from STRING after making the following substitutions:

     Meta-character    Replaced By
     -------------------------------------------------------- 
     %d                The directory name of the file,
                       equivalent to the result of the
                       `dirname' utility on the translated
                       file name.
     %f                The name of the file with the
                       directory information stripped,
                       equivalent to the result of the
                       `basename' utility on the translated
                       file name.
     %p                The process ID of the `tar' process.
     %%                A `%' character.

     Any other `%' characters in STRING produce undefined results.

     If no option `exthdr.name=string' is specified, `tar' will use the
     following default value:

          %d/PaxHeaders.%p/%f

`globexthdr.name=STRING'
     This keyword allows user control over the name that is written into
     the ustar header blocks for global extended header records.  The
     name is obtained from the contents of STRING, after making the
     following substitutions:

     Meta-character    Replaced By
     -------------------------------------------------------- 
     %n                An integer that represents the
                       sequence number of the global
                       extended header record in the
                       archive, starting at 1.
     %p                The process ID of the `tar' process.
     %%                A `%' character.

     Any other `%' characters in STRING produce undefined results.

     If no option `globexthdr.name=string' is specified, `tar' will use
     the following default value:

          $TMPDIR/GlobalHead.%p.%n

     where `$TMPDIR' represents the value of the TMPDIR environment
     variable.  If TMPDIR is not set, `tar' uses `/tmp'.

`KEYWORD=VALUE'
     When used with one of archive-creation commands, these
     keyword/value pairs will be included at the beginning of the
     archive in a global extended header record.  When used with one of
     archive-reading commands, `tar' will behave as if it has
     encountered these keyword/value pairs at the beginning of the
     archive in a global extended header record.

`KEYWORD:=VALUE'
     When used with one of archive-creation commands, these
     keyword/value pairs will be included as records at the beginning
     of an extended header for each file.  This is effectively
     equivalent to KEYWORD=VALUE form except that it creates no global
     extended header records.

     When used with one of archive-reading commands, `tar' will behave
     as if these keyword/value pairs were included as records at the
     end of each extended header; thus, they will override any global or
     file-specific extended header record keywords of the same names.
     For example, in the command:

          tar --format=posix --create \
              --file archive --pax-option gname:=user .

     the group name will be forced to a new value for all files stored
     in the archive.


File: tar.info,  Node: Checksumming,  Next: Large or Negative Values,  Prev: posix,  Up: Portability

8.3.7 Checksumming Problems
---------------------------

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.


File: tar.info,  Node: Large or Negative Values,  Next: Other Tars,  Prev: Checksumming,  Up: Portability

8.3.8 Large or Negative Values
------------------------------

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

The above sections suggest to use `oldest possible' archive format if
in doubt.  However, sometimes it is not possible.  If you attempt to
archive a file whose metadata cannot be represented using required
format, GNU `tar' will print error message and ignore such a file.  You
will than have to switch to a format that is able to handle such
values.  The format summary table (*note Formats::) will help you to do
so.

   In particular, when trying to archive files larger than 8GB or with
timestamps not in the range 1970-01-01 00:00:00 through 2242-03-16
12:56:31 UTC, you will have to chose between GNU and POSIX archive
formats.  When considering which format to choose, bear in mind that
the GNU format uses two's-complement base-256 notation to store values
that do not fit into standard ustar range.  Such archives can generally
be read only by a GNU `tar' implementation.  Moreover, they sometimes
cannot be correctly restored on another hosts even by GNU `tar'.  For
example, using two's complement representation for negative time stamps
that assumes a signed 32-bit `time_t' generates archives that are not
portable to hosts with differing `time_t' representations.

   On the other hand, POSIX archives, generally speaking, can be
extracted by any tar implementation that understands older ustar
format.  The only exception are files larger than 8GB.


File: tar.info,  Node: Other Tars,  Prev: Large or Negative Values,  Up: Portability

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

In previous sections you became acquainted with various quirks
necessary to make your archives portable.  Sometimes you may need to
extract archives containing GNU-specific members using some third-party
`tar' implementation or an older version of GNU `tar'.  Of course your
best bet is to have GNU `tar' installed, but if it is for some reason
impossible, this section will explain how to cope without it.

   When we speak about "GNU-specific" members we mean two classes of
them: members split between the volumes of a multi-volume archive and
sparse members.  You will be able to always recover such members if the
archive is in PAX format.  In addition split members can be recovered
from archives in old GNU format.  The following subsections describe
the required procedures in detail.

* Menu:

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


File: tar.info,  Node: Split Recovery,  Next: Sparse Recovery,  Up: Other Tars

8.3.9.1 Extracting Members Split Between Volumes
................................................

If a member is split between several volumes of an old GNU format
archive most third party `tar' implementation will fail to extract it.
To extract it, use `tarcat' program (*note Tarcat::).  This program is
available from GNU `tar' home page
(http://www.gnu.org/software/tar/utils/tarcat.html).  It concatenates
several archive volumes into a single valid archive.  For example, if
you have three volumes named from `vol-1.tar' to `vol-3.tar', you can
do the following to extract them using a third-party `tar':

     $ tarcat vol-1.tar vol-2.tar vol-3.tar | tar xf -

   You could use this approach for most (although not all) PAX format
archives as well.  However, extracting split members from a PAX archive
is a much easier task, because PAX volumes are constructed in such a
way that each part of a split member is extracted to a different file
by `tar' implementations that are not aware of GNU extensions.  More
specifically, the very first part retains its original name, and all
subsequent parts are named using the pattern:

     %d/GNUFileParts.%p/%f.%n

where symbols preceeded by `%' are "macro characters" that have the
following meaning:

Meta-character     Replaced By
------------------------------------------------------------ 
%d                 The directory name of the file,
                   equivalent to the result of the
                   `dirname' utility on its full name.
%f                 The file name of the file, equivalent
                   to the result of the `basename' utility
                   on its full name.
%p                 The process ID of the `tar' process that
                   created the archive.
%n                 Ordinal number of this particular part.

   For example, if the file `var/longfile' was split during archive
creation between three volumes, and the creator `tar' process had
process ID `27962', then the member names will be:

     var/longfile
     var/GNUFileParts.27962/longfile.1
     var/GNUFileParts.27962/longfile.2

   When you extract your archive using a third-party `tar', these files
will be created on your disk, and the only thing you will need to do to
restore your file in its original form is concatenate them in the
proper order, for example:

     $ cd var
     $ cat GNUFileParts.27962/longfile.1 \
       GNUFileParts.27962/longfile.2 >> longfile
     $ rm -f GNUFileParts.27962

   Notice, that if the `tar' implementation you use supports PAX format
archives, it will probably emit warnings about unknown keywords during
extraction.  They will look like this:

     Tar file too small
     Unknown extended header keyword 'GNU.volume.filename' ignored.
     Unknown extended header keyword 'GNU.volume.size' ignored.
     Unknown extended header keyword 'GNU.volume.offset' ignored.

You can safely ignore these warnings.

   If your `tar' implementation is not PAX-aware, you will get more
warnings and more files generated on your disk, e.g.:

     $ tar xf vol-1.tar
     var/PaxHeaders.27962/longfile: Unknown file type 'x', extracted as
     normal file
     Unexpected EOF in archive
     $ tar xf vol-2.tar
     tmp/GlobalHead.27962.1: Unknown file type 'g', extracted as normal file
     GNUFileParts.27962/PaxHeaders.27962/sparsefile.1: Unknown file type
     'x', extracted as normal file

   Ignore these warnings.  The `PaxHeaders.*' directories created will
contain files with "extended header keywords" describing the extracted
files.  You can delete them, unless they describe sparse members.  Read
further to learn more about them.


File: tar.info,  Node: Sparse Recovery,  Prev: Split Recovery,  Up: Other Tars

8.3.9.2 Extracting Sparse Members
.................................

Any `tar' implementation will be able to extract sparse members from a
PAX archive.  However, the extracted files will be "condensed", i.e.,
any zero blocks will be removed from them.  When we restore such a
condensed file to its original form, by adding zero blocks (or "holes")
back to their original locations, we call this process "expanding" a
compressed sparse file.

   To expand a file, you will need a simple auxiliary program called
`xsparse'.  It is available in source form from GNU `tar' home page
(http://www.gnu.org/software/tar/utils/xsparse.html).

   Let's begin with archive members in "sparse format version 1.0"(1),
which are the easiest to expand.  The condensed file will contain both
file map and file data, so no additional data will be needed to restore
it.  If the original file name was `DIR/NAME', then the condensed file
will be named `DIR/GNUSparseFile.N/NAME', where N is a decimal
number(2).

   To expand a version 1.0 file, run `xsparse' as follows:

     $ xsparse `cond-file'

where `cond-file' is the name of the condensed file.  The utility will
deduce the name for the resulting expanded file using the following
algorithm:

  1. If `cond-file' does not contain any directories, `../cond-file'
     will be used;

  2. If `cond-file' has the form `DIR/T/NAME', where both T and NAME
     are simple names, with no `/' characters in them, the output file
     name will be `DIR/NAME'.

  3. Otherwise, if `cond-file' has the form `DIR/NAME', the output file
     name will be `NAME'.

   In the unlikely case when this algorithm does not suit your needs,
you can explicitly specify output file name as a second argument to the
command:

     $ xsparse `cond-file' `out-file'

   It is often a good idea to run `xsparse' in "dry run" mode first.
In this mode, the command does not actually expand the file, but
verbosely lists all actions it would be taking to do so.  The dry run
mode is enabled by `-n' command line argument:

     $ xsparse -n /home/gray/GNUSparseFile.6058/sparsefile
     Reading v.1.0 sparse map
     Expanding file `/home/gray/GNUSparseFile.6058/sparsefile' to
     `/home/gray/sparsefile'
     Finished dry run

   To actually expand the file, you would run:

     $ xsparse /home/gray/GNUSparseFile.6058/sparsefile

The program behaves the same way all UNIX utilities do: it will keep
quiet unless it has simething important to tell you (e.g. an error
condition or something).  If you wish it to produce verbose output,
similar to that from the dry run mode, use `-v' option:

     $ xsparse -v /home/gray/GNUSparseFile.6058/sparsefile
     Reading v.1.0 sparse map
     Expanding file `/home/gray/GNUSparseFile.6058/sparsefile' to
     `/home/gray/sparsefile'
     Done

   Additionally, if your `tar' implementation has extracted the
"extended headers" for this file, you can instruct `xstar' to use them
in order to verify the integrity of the expanded file.  The option `-x'
sets the name of the extended header file to use.  Continuing our
example:

     $ xsparse -v -x /home/gray/PaxHeaders.6058/sparsefile \
       /home/gray/GNUSparseFile.6058/sparsefile
     Reading extended header file
     Found variable GNU.sparse.major = 1
     Found variable GNU.sparse.minor = 0
     Found variable GNU.sparse.name = sparsefile
     Found variable GNU.sparse.realsize = 217481216
     Reading v.1.0 sparse map
     Expanding file `/home/gray/GNUSparseFile.6058/sparsefile' to
     `/home/gray/sparsefile'
     Done

   An "extended header" is a special `tar' archive header that precedes
an archive member and contains a set of "variables", describing the
member properties that cannot be stored in the standard `ustar' header.
While optional for expanding sparse version 1.0 members, the use of
extended headers is mandatory when expanding sparse members in older
sparse formats: v.0.0 and v.0.1 (The sparse formats are described in
detail in *note Sparse Formats::.)  So, for these formats, the question
is: how to obtain extended headers from the archive?

   If you use a `tar' implementation that does not support PAX format,
extended headers for each member will be extracted as a separate file.
If we represent the member name as `DIR/NAME', then the extended header
file will be named `DIR/PaxHeaders.N/NAME', where N is an integer
number.

   Things become more difficult if your `tar' implementation does
support PAX headers, because in this case you will have to manually
extract the headers.  We recommend the following algorithm:

  1. Consult the documentation of your `tar' implementation for an
     option that prints "block numbers" along with the archive listing
     (analogous to GNU `tar''s `-R' option).  For example, `star' has
     `-block-number'.

  2. Obtain verbose listing using the `block number' option, and find
     block numbers of the sparse member in question and the member
     immediately following it.  For example, running `star' on our
     archive we obtain:

          $ star -t -v -block-number -f arc.tar
          ...
          star: Unknown extended header keyword 'GNU.sparse.size' ignored.
          star: Unknown extended header keyword 'GNU.sparse.numblocks' ignored.
          star: Unknown extended header keyword 'GNU.sparse.name' ignored.
          star: Unknown extended header keyword 'GNU.sparse.map' ignored.
          block        56:  425984 -rw-r--r--  gray/users Jun 25 14:46 2006 GNUSparseFile.28124/sparsefile
          block       897:   65391 -rw-r--r--  gray/users Jun 24 20:06 2006 README
          ...

     (as usual, ignore the warnings about unknown keywords.)

  3. Let SIZE be the size of the sparse member, BS be its block number
     and BN be the block number of the next member.  Compute:

          N = BS - BN - SIZE/512 - 2

     This number gives the size of the extended header part in tar
     "blocks".  In our example, this formula gives: `897 - 56 - 425984
     / 512 - 2 = 7'.

  4. Use `dd' to extract the headers:

          dd if=ARCHIVE of=HNAME bs=512 skip=BS count=N

     where ARCHIVE is the archive name, HNAME is a name of the file to
     store the extended header in, BS and N are computed in previous
     steps.

     In our example, this command will be

          $ dd if=arc.tar of=xhdr bs=512 skip=56 count=7

   Finally, you can expand the condensed file, using the obtained
header:

     $ xsparse -v -x xhdr GNUSparseFile.6058/sparsefile
     Reading extended header file
     Found variable GNU.sparse.size = 217481216
     Found variable GNU.sparse.numblocks = 208
     Found variable GNU.sparse.name = sparsefile
     Found variable GNU.sparse.map = 0,2048,1050624,2048,...
     Expanding file `GNUSparseFile.28124/sparsefile' to `sparsefile'
     Done

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

   (1) *Note PAX 1::.

   (2) technically speaking, N is a "process ID" of the `tar' process
which created the archive (*note PAX keywords::).


File: tar.info,  Node: cpio,  Prev: Portability,  Up: Formats

8.4 Comparison of `tar' and `cpio'
==================================

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

The `cpio' archive formats, like `tar', do have maximum file name
lengths.  The binary and old ASCII formats have a maximum file length
of 256, and the new ASCII and CRC ASCII formats have a max file length
of 1024.  GNU `cpio' can read and write archives with arbitrary file
name lengths, but other `cpio' implementations may crash unexplainedly
trying to read them.

   `tar' handles symbolic links in the form in which it comes in BSD;
`cpio' doesn't handle symbolic links in the form in which it comes in
System V prior to SVR4, and some vendors may have added symlinks to
their system without enhancing `cpio' to know about them.  Others may
have enhanced it in a way other than the way I did it at Sun, and which
was adopted by AT&T (and which is, I think, also present in the `cpio'
that Berkeley picked up from AT&T and put into a later BSD 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);
`cpio's way requires you to play some games (in its "binary" format,
i-numbers are only 16 bits, and in its "portable ASCII" format, they're
18 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 `cpio's,
if any, play those games.  Those that don't might get confused and
think two files are the same file when they're not, and make hard links
between them.

   `tar's way of handling multiple hard links to a file places only one
copy of the link on the tape, but the name attached to that copy is the
_only_ one you can use to retrieve the file; `cpio's way puts one copy
for every link, but you can retrieve it using any of the names.

     What type of check sum (if any) is used, and how is this
     calculated.

   See the attached manual pages for `tar' and `cpio' format.  `tar'
uses a checksum which is the sum of all the bytes in the `tar' header
for a file; `cpio' uses no checksum.

     If anyone knows why `cpio' was made 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 resyncing.
However, lots of tape driver software won't allow you to continue past
a media error which should be the only reason for getting out of sync
unless a file changed sizes while you were writing the archive.

     If anyone knows why `cpio' was made 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 `tar'red as sparse files (which you can easily
test, because the resulting archive gets smaller, and GNU `cpio' can no
longer read it).


File: tar.info,  Node: Media,  Next: Changes,  Prev: Formats,  Up: Top

9 Tapes and Other Archive Media
*******************************

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

A few special cases about tape handling warrant more detailed
description.  These special cases are discussed below.

   Many complexities surround the use of `tar' on tape drives.  Since
the creation and manipulation of archives located on magnetic tape was
the original purpose of `tar', it contains many features making such
manipulation easier.

   Archives are usually written on dismountable media--tape cartridges,
mag tapes, or floppy disks.

   The amount of data a tape or disk holds depends not only on its size,
but also on how it is formatted.  A 2400 foot long reel of mag tape
holds 40 megabytes of data when formatted at 1600 bits per inch.  The
physically smaller EXABYTE tape cartridge holds 2.3 gigabytes.

   Magnetic media are re-usable--once the archive on a tape is no longer
needed, the archive can be erased and the tape or disk used over.
Media quality does deteriorate with use, however.  Most tapes or disks
should be discarded when they begin to produce data errors.  EXABYTE
tape cartridges should be discarded when they generate an "error count"
(number of non-usable bits) of more than 10k.

   Magnetic media are written and erased using magnetic fields, and
should be protected from such fields to avoid damage to stored data.
Sticking a floppy disk to a filing cabinet using a magnet is probably
not a good idea.

* Menu:

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


File: tar.info,  Node: Device,  Next: Remote Tape Server,  Up: Media

9.1 Device Selection and Switching
==================================

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

`-f [HOSTNAME:]FILE'
`--file=[HOSTNAME:]FILE'
     Use archive file or device FILE on HOSTNAME.

   This option is used to specify the file name of the archive `tar'
works on.

   If the file name is `-', `tar' reads the archive from standard input
(when listing or extracting), or writes it to standard output (when
creating).  If the `-' file name is given when updating an archive,
`tar' will read the original archive from its standard input, and will
write the entire new archive to its standard output.

   If the file name contains a `:', it is interpreted as `hostname:file
name'.  If the HOSTNAME contains an "at" sign (`@'), it is treated as
`user@hostname:file name'.  In either case, `tar' will invoke the
command `rsh' (or `remsh') to start up an `/usr/libexec/rmt' on the
remote machine.  If you give an alternate login name, it will be given
to the `rsh'.  Naturally, the remote machine must have an executable
`/usr/libexec/rmt'.  This program is free software from the University
of California, and a copy of the source code can be found with the
sources for `tar'; it's compiled and installed by default.  The exact
path to this utility is determined when configuring the package.  It is
`PREFIX/libexec/rmt', where PREFIX stands for your installation prefix.
This location may also be overridden at runtime by using
`rmt-command=COMMAND' option (*Note --rmt-command: Option Summary, for
detailed description of this option.  *Note Remote Tape Server::, for
the description of `rmt' command).

   If this option is not given, but the environment variable `TAPE' is
set, its value is used; otherwise, old versions of `tar' used a default
archive name (which was picked when `tar' was compiled).  The default
is normally set up to be the "first" tape drive or other transportable
I/O medium on the system.

   Starting with version 1.11.5, GNU `tar' uses standard input and
standard output as the default device, and I will not try anymore
supporting automatic device detection at installation time.  This was
failing really in too many cases, it was hopeless.  This is now
completely left to the installer to override standard input and
standard output for default device, if this seems preferable.  Further,
I think _most_ actual usages of `tar' are done with pipes or disks, not
really tapes, cartridges or diskettes.

   Some users think that using standard input and output is running
after trouble.  This could lead to a nasty surprise on your screen if
you forget to specify an output file 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>'.

`--force-local'
     Archive file is local even if it contains a colon.

`--rsh-command=COMMAND'
     Use remote COMMAND instead of `rsh'.  This option exists so that
     people who use something other than the standard `rsh' (e.g., a
     Kerberized `rsh') can access a remote device.

     When this command is not used, the shell command found when the
     `tar' program was installed is used instead.  This is the first
     found of `/usr/ucb/rsh', `/usr/bin/remsh', `/usr/bin/rsh',
     `/usr/bsd/rsh' or `/usr/bin/nsh'.  The installer may have
     overridden this by defining the environment variable `RSH' _at
     installation time_.

`-[0-7][lmh]'
     Specify drive and density.

`-M'
`--multi-volume'
     Create/list/extract multi-volume archive.

     This option causes `tar' to write a "multi-volume" archive--one
     that may be larger than will fit on the medium used to hold it.
     *Note Multi-Volume Archives::.

`-L NUM'
`--tape-length=NUM'
     Change tape after writing NUM x 1024 bytes.

     This option might be useful when your tape drivers do not properly
     detect end of physical tapes.  By being slightly conservative on
     the maximum tape length, you might avoid the problem entirely.

`-F FILE'
`--info-script=FILE'
`--new-volume-script=FILE'
     Execute `file' at end of each tape.  This implies `--multi-volume'
     (`-M').  *Note info-script::, for a detailed description of this
     option.


File: tar.info,  Node: Remote Tape Server,  Next: Common Problems and Solutions,  Prev: Device,  Up: Media

9.2 The Remote Tape Server
==========================

In order to access the tape drive on a remote machine, `tar' uses the
remote tape server written at the University of California at Berkeley.
The remote tape server must be installed as `PREFIX/libexec/rmt' on
any machine whose tape drive you want to use.  `tar' calls `rmt' by
running an `rsh' or `remsh' to the remote machine, optionally using a
different login name if one is supplied.

   A copy of the source for the remote tape server is provided.  It is
Copyright (C) 1983 by the Regents of the University of California, but
can be freely distributed.  It is compiled and installed by default.

   Unless you use the `--absolute-names' (`-P') option, GNU `tar' will
not allow you to create an archive that contains absolute file names (a
file name beginning with `/'.) If you try, `tar' will automatically
remove the leading `/' from the file names it stores in the archive.
It will also type a warning message telling you what it is doing.

   When reading an archive that was created with a different `tar'
program, GNU `tar' automatically extracts entries in the archive which
have absolute file names as if the file names were not absolute.  This
is an important feature.  A visitor here once gave a `tar' tape to an
operator to restore; the operator used Sun `tar' instead of GNU `tar',
and the result was that it replaced large portions of our `/bin' and
friends with versions from the tape; needless to say, we were unhappy
about having to recover the file system from backup tapes.

   For example, if the archive contained a file `/usr/bin/computoy',
GNU `tar' would extract the file to `usr/bin/computoy', relative to the
current directory.  If you want to extract the files in an archive to
the same absolute names that they had when the archive was created, you
should do a `cd /' before extracting the files from the archive, or you
should either use the `--absolute-names' option, or use the command
`tar -C / ...'.

   Some versions of Unix (Ultrix 3.1 is known to have this problem),
can claim that a short write near the end of a tape succeeded, when it
actually failed.  This will result in the -M option not working
correctly.  The best workaround at the moment is to use a significantly
larger blocking factor than the default 20.

   In order to update an archive, `tar' must be able to backspace the
archive in order to reread or rewrite a record that was just read (or
written).  This is currently possible only on two kinds of files: normal
disk files (or any other file that can be backspaced with `lseek'), and
industry-standard 9-track magnetic tape (or any other kind of tape that
can be backspaced with the `MTIOCTOP' `ioctl'.

   This means that the `--append', `--concatenate', and `--delete'
commands will not work on any other kind of file.  Some media simply
cannot be backspaced, which means these commands and options will never
be able to work on them.  These non-backspacing media include pipes and
cartridge tape drives.

   Some other media can be backspaced, and `tar' will work on them once
`tar' is modified to do so.

   Archives created with the `--multi-volume', `--label', and
`--incremental' (`-G') options may not be readable by other version of
`tar'.  In particular, restoring a file that was split over a volume
boundary will require some careful work with `dd', if it can be done at
all.  Other versions of `tar' may also create an empty file whose name
is that of the volume header.  Some versions of `tar' may create normal
files instead of directories archived with the `--incremental' (`-G')
option.


File: tar.info,  Node: Common Problems and Solutions,  Next: Blocking,  Prev: Remote Tape Server,  Up: Media

9.3 Some Common Problems and their Solutions
============================================

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


File: tar.info,  Node: Blocking,  Next: Many,  Prev: Common Problems and Solutions,  Up: Media

9.4 Blocking
============

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

"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 Franc,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.  *Note Standard::.)  Each file written to the archive uses at
least one full record.  As a result, using a larger record size can
result in more wasted space for small files.  On the other hand, a
larger record size can often be read and written much more efficiently.

   Further complicating the problem is that some tape drives ignore the
blocking entirely.  For these, a larger record size can still improve
performance (because the software layers above the tape drive still
honor the blocking), but not as dramatically as on tape drives that
honor blocking.

   When reading an archive, `tar' can usually figure out the record
size on itself.  When this is the case, and a non-standard record size
was used when the archive was created, `tar' will print a message about
a non-standard blocking factor, and then operate normally.  On some
tape devices, however, `tar' cannot figure out the record size itself.
On most of those, you can specify a blocking factor (with
`--blocking-factor') larger than the actual blocking factor, and then
use the `--read-full-records' (`-B') option.  (If you specify a
blocking factor with `--blocking-factor' and don't use the
`--read-full-records' option, then `tar' will not attempt to figure out
the recording size itself.)  On some devices, you must always specify
the record size exactly with `--blocking-factor' when reading, because
`tar' cannot figure it out.  In any case, use `--list' (`-t') before
doing any extractions to see whether `tar' is reading the archive
correctly.

   `tar' blocks are all fixed size (512 bytes), and its scheme for
putting them into records is to put a whole number of them (one or
more) into each record.  `tar' records are all the same size; at the
end of the file there's a block containing all zeros, which is how you
tell that the remainder of the last record(s) are garbage.

   In a standard `tar' file (no options), the block size is 512 and the
record size is 10240, for a blocking factor of 20.  What the
`--blocking-factor' option does is sets the blocking factor, changing
the record size while leaving the block size at 512 bytes.  20 was fine
for ancient 800 or 1600 bpi reel-to-reel tape drives; most tape drives
these days prefer much bigger records in order to stream and not waste
tape.  When writing tapes for myself, some tend to use a factor of the
order of 2048, say, giving a record size of around one megabyte.

   If you use a blocking factor larger than 20, older `tar' programs
might not be able to read the archive, so we recommend this as a limit
to use in practice.  GNU `tar', however, will support arbitrarily large
record sizes, limited only by the amount of virtual memory or the
physical characteristics of the tape device.

* Menu:

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


File: tar.info,  Node: Format Variations,  Next: Blocking Factor,  Up: Blocking

9.4.1 Format Variations
-----------------------

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

Format parameters specify how an archive is written on the archive
media.  The best choice of format parameters will vary depending on the
type and number of files being archived, and on the media used to store
the archive.

   To specify format parameters when accessing or creating an archive,
you can use the options described in the following sections.  If you do
not specify any format parameters, `tar' uses default parameters.  You
cannot modify a compressed archive.  If you create an archive with the
`--blocking-factor' option specified (*note Blocking Factor::), you
must specify that blocking-factor when operating on the archive.  *Note
Formats::, for other examples of format parameter considerations.


File: tar.info,  Node: Blocking Factor,  Prev: Format Variations,  Up: Blocking

9.4.2 The Blocking Factor of an Archive
---------------------------------------

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

The data in an archive is grouped into blocks, which are 512 bytes.
Blocks are read and written in whole number multiples called "records".
The number of blocks in a record (i.e., the size of a record in units
of 512 bytes) is called the "blocking factor".  The
`--blocking-factor=512-SIZE' (`-b 512-SIZE') option specifies the
blocking factor of an archive.  The default blocking factor is
typically 20 (i.e., 10240 bytes), but can be specified at installation.
To find out the blocking factor of an existing archive, use `tar
--list --file=ARCHIVE-NAME'.  This may not work on some devices.

   Records are separated by gaps, which waste space on the archive
media.  If you are archiving on magnetic tape, using a larger blocking
factor (and therefore larger records) provides faster throughput and
allows you to fit more data on a tape (because there are fewer gaps).
If you are archiving on cartridge, a very large blocking factor (say
126 or more) greatly increases performance.  A smaller blocking factor,
on the other hand, may be useful when archiving small files, to avoid
archiving lots of nulls as `tar' fills out the archive to the end of
the record.  In general, the ideal record size depends on the size of
the inter-record gaps on the tape you are using, and the average size
of the files you are archiving.  *Note create::, for information on
writing archives.

   Archives with blocking factors larger than 20 cannot be read by very
old versions of `tar', or by some newer versions of `tar' running on
old machines with small address spaces.  With GNU `tar', the blocking
factor of an archive is limited only by the maximum record size of the
device containing the archive, or by the amount of available virtual
memory.

   Also, on some systems, not using adequate blocking factors, as
sometimes imposed by the device drivers, may yield unexpected
diagnostics.  For example, this has been reported:

     Cannot write to /dev/dlt: Invalid argument

In such cases, it sometimes happen that the `tar' bundled by the system
is aware of block size idiosyncrasies, while GNU `tar' requires an
explicit specification for the block size, which it cannot guess.  This
yields some people to consider GNU `tar' is misbehaving, because by
comparison, `the bundle `tar' works OK'.  Adding `-b 256', for example,
might resolve the problem.

   If you use a non-default blocking factor when you create an archive,
you must specify the same blocking factor when you modify that archive.
Some archive devices will also require you to specify the blocking
factor when reading that archive, however this is not typically the
case.  Usually, you can use `--list' (`-t') without specifying a
blocking factor--`tar' reports a non-default record size and then lists
the archive members as it would normally.  To extract files from an
archive with a non-standard blocking factor (particularly if you're not
sure what the blocking factor is), you can usually use the
`--read-full-records' (`-B') option while specifying a blocking factor
larger then the blocking factor of the archive (i.e., `tar --extract
--read-full-records --blocking-factor=300'.  *Note list::, for more
information on the `--list' (`-t') operation.  *Note Reading::, for a
more detailed explanation of that option.

`--blocking-factor=NUMBER'
`-b NUMBER'
     Specifies the blocking factor of an archive.  Can be used with any
     operation, but is usually not necessary with `--list' (`-t').

   Device blocking

`-b BLOCKS'
`--blocking-factor=BLOCKS'
     Set record size to BLOCKS * 512 bytes.

     This option is used to specify a "blocking factor" for the archive.
     When reading or writing the archive, `tar', will do reads and
     writes of the archive in records of BLOCK*512 bytes.  This is true
     even when the archive is compressed.  Some devices requires that
     all write operations be a multiple of a certain size, and so, `tar'
     pads the archive out to the next record boundary.

     The default blocking factor is set when `tar' is compiled, and is
     typically 20.  Blocking factors larger than 20 cannot be read by
     very old versions of `tar', or by some newer versions of `tar'
     running on old machines with small address spaces.

     With a magnetic tape, larger records give faster throughput and fit
     more data on a tape (because there are fewer inter-record gaps).
     If the archive is in a disk file or a pipe, you may want to specify
     a smaller blocking factor, since a large one will result in a large
     number of null bytes at the end of the archive.

     When writing cartridge or other streaming tapes, a much larger
     blocking factor (say 126 or more) will greatly increase
     performance.  However, you must specify the same blocking factor
     when reading or updating the archive.

     Apparently, Exabyte drives have a physical block size of 8K bytes.
     If we choose our blocksize as a multiple of 8k bytes, then the
     problem seems to disappear.  Id est, we are using block size of
     112 right now, and we haven't had the problem since we switched...

     With GNU `tar' the blocking factor is limited only by the maximum
     record size of the device containing the archive, or by the amount
     of available virtual memory.

     However, deblocking or reblocking is virtually avoided in a special
     case which often occurs in practice, but which requires all the
     following conditions to be simultaneously true:
        * the archive is subject to a compression option,

        * the archive is not handled through standard input or output,
          nor redirected nor piped,

        * the archive is directly handled to a local disk, instead of
          any special device,

        * `--blocking-factor' is not explicitly specified on the `tar'
          invocation.

     If the output goes directly to a local disk, and not through
     stdout, then the last write is not extended to a full record size.
     Otherwise, reblocking occurs.  Here are a few other remarks on this
     topic:

        * `gzip' will complain about trailing garbage if asked to
          uncompress a compressed archive on tape, there is an option
          to turn the message off, but it breaks the regularity of
          simply having to use `PROG -d' for decompression.  It would
          be nice if gzip was silently ignoring any number of trailing
          zeros.  I'll ask Jean-loup Gailly, by sending a copy of this
          message to him.

        * `compress' does not show this problem, but as Jean-loup
          pointed out to Michael, `compress -d' silently adds garbage
          after the result of decompression, which tar ignores because
          it already recognized its end-of-file indicator.  So this bug
          may be safely ignored.

        * `gzip -d -q' will be silent about the trailing zeros indeed,
          but will still return an exit status of 2 which tar reports
          in turn.  `tar' might ignore the exit status returned, but I
          hate doing that, as it weakens the protection `tar' offers
          users against other possible problems at decompression time.
          If `gzip' was silently skipping trailing zeros _and_ also
          avoiding setting the exit status in this innocuous case, that
          would solve this situation.

        * `tar' should become more solid at not stopping to read a pipe
          at the first null block encountered.  This inelegantly breaks
          the pipe.  `tar' should rather drain the pipe out before
          exiting itself.

`-i'
`--ignore-zeros'
     Ignore blocks of zeros in archive (means EOF).

     The `--ignore-zeros' (`-i') option causes `tar' to ignore blocks
     of zeros in the archive.  Normally a block of zeros indicates the
     end of the archive, but when reading a damaged archive, or one
     which was created by concatenating several archives together, this
     option allows `tar' to read the entire archive.  This option is
     not on by default because many versions of `tar' write garbage
     after the zeroed blocks.

     Note that this option causes `tar' to read to the end of the
     archive file, which may sometimes avoid problems when multiple
     files are stored on a single physical tape.

`-B'
`--read-full-records'
     Reblock as we read (for reading 4.2BSD pipes).

     If `--read-full-records' is used, `tar' will not panic if an
     attempt to read a record from the archive does not return a full
     record.  Instead, `tar' will keep reading until it has obtained a
     full record.

     This option is turned on by default when `tar' is reading an
     archive from standard input, or from a remote machine.  This is
     because on BSD Unix systems, a read of a pipe will return however
     much happens to be in the pipe, even if it is less than `tar'
     requested.  If this option was not used, `tar' would fail as soon
     as it read an incomplete record from the pipe.

     This option is also useful with the commands for updating an
     archive.


   Tape blocking

   When handling various tapes or cartridges, you have to take care of
selecting a proper blocking, that is, the number of disk blocks you put
together as a single tape block on the tape, without intervening tape
gaps.  A "tape gap" is a small landing area on the tape with no
information on it, used for decelerating the tape to a full stop, and
for later regaining the reading or writing speed.  When the tape driver
starts reading a record, the record has to be read whole without
stopping, as a tape gap is needed to stop the tape motion without
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
accommodate higher blockings.  Sun recommends a blocking of 126 for
Exabytes and 96 for DATs.  We were told that for some DLT drives, the
blocking should be a multiple of 4Kb, preferably 64Kb (`-b 128') or 256
for decent performance.  Other manufacturers may use different
recommendations for the same tapes.  This might also depends of the
buffering techniques used inside modern tape controllers.  Some imposes
a minimum blocking, or a maximum blocking.  Others request blocking to
be some exponent of two.

   So, there is no fixed rule for blocking.  But blocking at read time
should ideally be the same as blocking used at write time.  At one place
I know, with a wide variety of equipment, they found it best to use a
blocking of 32 to guarantee that their tapes are fully interchangeable.

   I was also told that, for recycled tapes, prior erasure (by the same
drive unit that will be used to create the archives) sometimes lowers
the error rates observed at rewriting time.

   I might also use `--number-blocks' instead of `--block-number', so
`--block' will then expand to `--blocking-factor' unambiguously.


File: tar.info,  Node: Many,  Next: Using Multiple Tapes,  Prev: Blocking,  Up: Media

9.5 Many Archives on One Tape
=============================

Most tape devices have two entries in the `/dev' directory, or entries
that come in pairs, which differ only in the minor number for this
device.  Let's take for example `/dev/tape', which often points to the
only or usual tape device of a given system.  There might be a
corresponding `/dev/nrtape' or `/dev/ntape'.  The simpler name is the
_rewinding_ version of the device, while the name having `nr' in it is
the _no rewinding_ version of the same device.

   A rewinding tape device will bring back the tape to its beginning
point automatically when this device is opened or closed.  Since `tar'
opens the archive file before using it and closes it afterwards, this
means that a simple:

     $ tar cf /dev/tape DIRECTORY

will reposition the tape to its beginning both prior and after saving
DIRECTORY contents to it, thus erasing prior tape contents and making
it so that any subsequent write operation will destroy what has just
been saved.

   So, a rewinding device is normally meant to hold one and only one
file.  If you want to put more than one `tar' archive on a given tape,
you will need to avoid using the rewinding version of the tape device.
You will also have to pay special attention to tape positioning.
Errors in positioning may overwrite the valuable data already on your
tape.  Many people, burnt by past experiences, will only use rewinding
devices and limit themselves to one file per tape, precisely to avoid
the risk of such errors.  Be fully aware that writing at the wrong
position on a tape loses all information past this point and most
probably until the end of the tape, and this destroyed information
_cannot_ be recovered.

   To save DIRECTORY-1 as a first archive at the beginning of a tape,
and leave that tape ready for a second archive, you should use:

     $ mt -f /dev/nrtape rewind
     $ tar cf /dev/nrtape DIRECTORY-1

   "Tape marks" are special magnetic patterns written on the tape
media, which are later recognizable by the reading hardware.  These
marks are used after each file, when there are many on a single tape.
An empty file (that is to say, two tape marks in a row) signal the
logical end of the tape, after which no file exist.  Usually,
non-rewinding tape device drivers will react to the close request issued
by `tar' by first writing two tape marks after your archive, and by
backspacing over one of these.  So, if you remove the tape at that time
from the tape drive, it is properly terminated.  But if you write
another file at the current position, the second tape mark will be
erased by the new information, leaving only one tape mark between files.

   So, you may now save DIRECTORY-2 as a second archive after the first
on the same tape by issuing the command:

     $ tar cf /dev/nrtape DIRECTORY-2

and so on for all the archives you want to put on the same tape.

   Another usual case is that you do not write all the archives the same
day, and you need to remove and store the tape between two archive
sessions.  In general, you must remember how many files are already
saved on your tape.  Suppose your tape already has 16 files on it, and
that you are ready to write the 17th.  You have to take care of skipping
the first 16 tape marks before saving DIRECTORY-17, say, by using these
commands:

     $ mt -f /dev/nrtape rewind
     $ mt -f /dev/nrtape fsf 16
     $ tar cf /dev/nrtape DIRECTORY-17

   In all the previous examples, we put aside blocking considerations,
but you should do the proper things for that as well.  *Note Blocking::.

* Menu:

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


File: tar.info,  Node: Tape Positioning,  Next: mt,  Up: Many

9.5.1 Tape Positions and Tape Marks
-----------------------------------

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

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.  You can do it manually via `mt' utility
(*note mt::).  The `restore' script does that automatically (*note
Scripted Restoration::).

   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**----------------


File: tar.info,  Node: mt,  Prev: Tape Positioning,  Up: Many

9.5.2 The `mt' Utility
----------------------

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

*Note Blocking Factor::.

   You can use the `mt' utility to advance or rewind a tape past a
specified number of archive files on the tape.  This will allow you to
move to the beginning of an archive before extracting or reading it, or
to the end of all the archives before writing a new one.

   The syntax of the `mt' command is:

     mt [-f TAPENAME] OPERATION [NUMBER]

   where TAPENAME is the name of the tape device, NUMBER is the number
of times an operation is performed (with a default of one), and
OPERATION is one of the following:

`eof'
`weof'
     Writes NUMBER tape marks at the current position on the tape.

`fsf'
     Moves tape position forward NUMBER files.

`bsf'
     Moves tape position back NUMBER files.

`rewind'
     Rewinds the tape.  (Ignores NUMBER).

`offline'
`rewoff1'
     Rewinds the tape and takes the tape device off-line.  (Ignores
     NUMBER).

`status'
     Prints status information about the tape unit.


   If you don't specify a TAPENAME, `mt' uses the environment variable
`TAPE'; if `TAPE' is not set, `mt' will use the default device
specified in your `sys/mtio.h' file (`DEFTAPE' variable).  If this is
not defined, the program will display a descriptive error message and
exit with code 1.

   `mt' returns a 0 exit status when the operation(s) were successful,
1 if the command was unrecognized, and 2 if an operation failed.


File: tar.info,  Node: Using Multiple Tapes,  Next: label,  Prev: Many,  Up: Media

9.6 Using Multiple Tapes
========================

Often you might want to write a large archive, one larger than will fit
on the actual tape you are using.  In such a case, you can run multiple
`tar' commands, but this can be inconvenient, particularly if you are
using options like `--exclude=PATTERN' or dumping entire file systems.
Therefore, `tar' provides a special mode for creating multi-volume
archives.

   "Multi-volume" archive is a single `tar' archive, stored on several
media volumes of fixed size.  Although in this section we will often
call `volume' a "tape", there is absolutely no requirement for
multi-volume archives to be stored on tapes.  Instead, they can use
whatever media type the user finds convenient, they can even be located
on files.

   When creating a multi-volume archive, GNU `tar' continues to fill
current volume until it runs out of space, then it switches to next
volume (usually the operator is queried to replace the tape on this
point), and continues working on the new volume.  This operation
continues until all requested files are dumped.  If GNU `tar' detects
end of media while dumping a file, such a file is archived in split
form.  Some very big files can even be split across several volumes.

   Each volume is itself a valid GNU `tar' archive, so it can be read
without any special options.  Consequently any file member residing
entirely on one volume can be extracted or otherwise operated upon
without needing the other volume.  Sure enough, to extract a split
member you would need all volumes its parts reside on.

   Multi-volume archives suffer from several limitations.  In
particular, they cannot be compressed.

   GNU `tar' is able to create multi-volume archives of two formats
(*note Formats::): `GNU' and `POSIX'.

* Menu:

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


File: tar.info,  Node: Multi-Volume Archives,  Next: Tape Files,  Up: Using Multiple Tapes

9.6.1 Archives Longer than One Tape or Disk
-------------------------------------------

To create an archive that is larger than will fit on a single unit of
the media, use the `--multi-volume' (`-M') option in conjunction with
the `--create' option (*note create::).  A "multi-volume" archive can
be manipulated like any other archive (provided the `--multi-volume'
option is specified), but is stored on more than one tape or disk.

   When you specify `--multi-volume', `tar' does not report an error
when it comes to the end of an archive volume (when reading), or the
end of the media (when writing).  Instead, it prompts you to load a new
storage volume.  If the archive is on a magnetic tape, you should
change tapes when you see the prompt; if the archive is on a floppy
disk, you should change disks; etc.

`--multi-volume'
`-M'
     Creates a multi-volume archive, when used in conjunction with
     `--create' (`-c').  To perform any other operation on a
     multi-volume archive, specify `--multi-volume' in conjunction with
     that operation.  For example:

          $ tar --create --multi-volume --file=/dev/tape FILES

   The method `tar' uses to detect end of tape is not perfect, and
fails on some operating systems or on some devices.  If `tar' cannot
detect the end of the tape itself, you can use `--tape-length' option
to inform it about the capacity of the tape:

`--tape-length=SIZE'
`-L SIZE'
     Set maximum length of a volume.  The SIZE argument should then be
     the usable size of the tape in units of 1024 bytes.  This option
     selects `--multi-volume' automatically.  For example:

          $ tar --create --tape-length=41943040 --file=/dev/tape FILES

   When GNU `tar' comes to the end of a storage media, it asks you to
change the volume.  The built-in prompt for POSIX locale is(1):

     Prepare volume #N for `ARCHIVE' and hit return:

where N is the ordinal number of the volume to be created and ARCHIVE
is archive file or device name.

   When prompting for a new tape, `tar' accepts any of the following
responses:

`?'
     Request `tar' to explain possible responses

`q'
     Request `tar' to exit immediately.

`n FILE-NAME'
     Request `tar' to write the next volume on the file FILE-NAME.

`!'
     Request `tar' to run a subshell.  This option can be disabled by
     giving `--restrict' command line option to `tar'(2).

`y'
     Request `tar' to begin writing the next volume.

   (You should only type `y' after you have changed the tape; otherwise
`tar' will write over the volume it just finished.)

   The volume number used by `tar' in its tape-changing prompt can be
changed; if you give the `--volno-file=FILE-OF-NUMBER' option, then
FILE-OF-NUMBER should be an non-existing file to be created, or else, a
file already containing a decimal number.  That number will be used as
the volume number of the first volume written.  When `tar' is finished,
it will rewrite the file with the now-current volume number. (This does
not change the volume number written on a tape label, as per *note
label::, it _only_ affects the number used in the prompt.)

   If you want more elaborate behavior than this, you can write a
special "new volume script", that will be responsible for changing the
volume, and instruct `tar' to use it instead of its normal prompting
procedure:

`--info-script=SCRIPT-NAME'
`--new-volume-script=SCRIPT-NAME'
`-F SCRIPT-NAME'
     Specify the full name of the volume script to use.  The script can
     be used to eject cassettes, or to broadcast messages such as
     `Someone please come change my tape' when performing unattended
     backups.

   The SCRIPT-NAME is executed without any command line arguments.  It
inherits `tar''s shell environment.  Additional data is passed to it
via the following environment variables:

`TAR_VERSION'
     GNU `tar' version number.

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

`TAR_VOLUME'
     Ordinal number of the volume `tar' is about to start.

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

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

`TAR_FD'
     File descriptor which can be used to communicate the new volume
     name to `tar'.

   The volume script can instruct `tar' to use new archive name, by
writing in to file descriptor `$TAR_FD' (see below for an example).

   If the info script fails, `tar' exits; otherwise, it begins writing
the next volume.

   If you want `tar' to cycle through a series of files or tape drives,
there are three approaches to choose from.  First of all, you can give
`tar' multiple `--file' options.  In this case the specified files will
be used, in sequence, as the successive volumes of the archive.  Only
when the first one in the sequence needs to be used again will `tar'
prompt for a tape change (or run the info script).  For example,
suppose someone has two tape drives on a system named `/dev/tape0' and
`/dev/tape1'.  For having GNU `tar' to switch to the second drive when
it needs to write the second tape, and then back to the first tape,
etc., just do either of:

     $ tar --create --multi-volume --file=/dev/tape0 --file=/dev/tape1 FILES
     $ tar cMff /dev/tape0 /dev/tape1 FILES

   The second method is to use the `n' response to the tape-change
prompt.

   Finally, the most flexible approach is to use a volume script, that
writes new archive name to the file descriptor `$TAR_FD'.  For example,
the following volume script will create a series of archive files, named
`ARCHIVE-VOL', where ARCHIVE is the name of the archive being created
(as given by `--file' option) and VOL is the ordinal number of the
archive being created:

     #! /bin/sh
     echo Preparing volume $TAR_VOLUME of $TAR_ARCHIVE.

     name=`expr $TAR_ARCHIVE : '\(.*\)-.*'`
     case $TAR_SUBCOMMAND in
     -c)       ;;
     -d|-x|-t) test -r ${name:-$TAR_ARCHIVE}-$TAR_VOLUME || exit 1
     	  ;;
     *)        exit 1
     esac

     echo ${name:-$TAR_ARCHIVE}-$TAR_VOLUME >&$TAR_FD

   The same script cant be used while listing, comparing or extracting
from the created archive.  For example:

     # Create a multi-volume archive:
     $ tar -c -L1024 -f archive.tar -F new-volume .
     # Extract from the created archive:
     $ tar -x -f archive.tar -F new-volume .

Notice, that the first command had to use `-L' option, since otherwise
GNU `tar' will end up writing everything to file `archive.tar'.

   You can read each individual volume of a multi-volume archive as if
it were an archive by itself.  For example, to list the contents of one
volume, use `--list', without `--multi-volume' specified.  To extract
an archive member from one volume (assuming it is described that
volume), use `--extract', again without `--multi-volume'.

   If an archive member is split across volumes (i.e., its entry begins
on one volume of the media and ends on another), you need to specify
`--multi-volume' to extract it successfully.  In this case, you should
load the volume where the archive member starts, and use `tar --extract
--multi-volume'--`tar' will prompt for later volumes as it needs them.
*Note extracting archives::, for more information about extracting
archives.

   Multi-volume archives can be modified like any other archive.  To add
files to a multi-volume archive, you need to only mount the last volume
of the archive media (and new volumes, if needed).  For all other
operations, you need to use the entire archive.

   If a multi-volume archive was labeled using `--label=ARCHIVE-LABEL'
(*note label::) when it was created, `tar' will not automatically label
volumes which are added later.  To label subsequent volumes, specify
`--label=ARCHIVE-LABEL' again in conjunction with the `--append',
`--update' or `--concatenate' operation.

   Notice that multi-volume support is a GNU extension and the archives
created in this mode should be read only using GNU `tar'.  If you
absolutely have to process such archives using a third-party `tar'
implementation, read *note Split Recovery::.

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

   (1) If you run GNU `tar' under a different locale, the translation
to the locale's language will be used.

   (2) *Note --restrict::, for more information about this option


File: tar.info,  Node: Tape Files,  Next: Tarcat,  Prev: Multi-Volume Archives,  Up: Using Multiple Tapes

9.6.2 Tape Files
----------------

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

To give the archive a name which will be recorded in it, use the
`--label=VOLUME-LABEL' (`-V VOLUME-LABEL') option.  This will write a
special block identifying VOLUME-LABEL as the name of the archive to
the front of the archive which will be displayed when the archive is
listed with `--list'.  If you are creating a multi-volume archive with
`--multi-volume' (*note Using Multiple Tapes::), then the volume label
will have `Volume NNN' appended to the name you give, where NNN is the
number of the volume of the archive.  (If you use the
`--label=VOLUME-LABEL') option when reading an archive, it checks to
make sure the label on the tape matches the one you give. *Note label::.

   When `tar' writes an archive to tape, it creates a single tape file.
If multiple archives are written to the same tape, one after the
other, they each get written as separate tape files.  When extracting,
it is necessary to position the tape at the right place before running
`tar'.  To do this, use the `mt' command.  For more information on the
`mt' command and on the organization of tapes into a sequence of tape
files, see *note mt::.

   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.


File: tar.info,  Node: Tarcat,  Prev: Tape Files,  Up: Using Multiple Tapes

9.6.3 Concatenate Volumes into a Single Archive
-----------------------------------------------

Sometimes it is necessary to convert existing GNU `tar' multi-volume
archive to a single `tar' archive.  Simply concatenating all volumes
into one will not work, since each volume carries an additional
information at the beginning.  GNU `tar' is shipped with the shell
script `tarcat' designed for this purpose.

   The script takes a list of files comprising a multi-volume archive
and creates the resulting archive at the standard output.  For example:

     tarcat vol.1 vol.2 vol.3 | tar tf -

   The script implements a simple heuristics to determine the format of
the first volume file and to decide how to process the rest of the
files.  However, it makes no attempt to verify whether the files are
given in order or even if they are valid `tar' archives.  It uses `dd'
and does not filter its standard error, so you will usually see lots of
spurious messages.


File: tar.info,  Node: label,  Next: verify,  Prev: Using Multiple Tapes,  Up: Media

9.7 Including a Label in the Archive
====================================

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

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' operation to include a label entry in the archive as it
is being created.

`--label=ARCHIVE-LABEL'
`-V ARCHIVE-LABEL'
     Includes an "archive-label" at the beginning of the archive when
     the archive is being created, when used in conjunction with the
     `--create' operation.  Checks to make sure the archive label
     matches the one specified (when used in conjunction with any other
     operation.

   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. *Note Using
Multiple Tapes::, for information on creating multiple volume archives.

   The volume label will be displayed by `--list' along with the file
contents.  If verbose display is requested, it will also be explicitly
marked as in the example below:

     $ tar --verbose --list --file=iamanarchive
     V--------- 0 0        0 1992-03-07 12:01 iamalabel--Volume Header--
     -rw-r--r-- ringo user 40 1990-05-21 13:30 iamafilename

   However, `--list' option will cause listing entire contents of the
archive, which may be undesirable (for example, if the archive is
stored on a tape).  You can request checking only the volume by
specifying `--test-label' option.  This option reads only the first
block of an archive, so it can be used with slow storage devices.  For
example:

     $ tar --test-label --file=iamanarchive
     iamalabel

   If `--test-label' is used with a single command line argument, `tar'
compares the volume label with the argument.  It exits with code 0 if
the two strings match, and with code 2 otherwise.  In this case no
output is displayed.  For example:

     $ tar --test-label --file=iamanarchive 'iamalable'
     => 0
     $ tar --test-label --file=iamanarchive 'iamalable' alabel
     => 1

   If you request any operation, other than `--create', along with
using `--label' option, `tar' will first check if the archive label
matches the one specified and will refuse to proceed if it does not.
Use this as a safety precaution to avoid accidentally overwriting
existing archives.  For example, if you wish to add files to `archive',
presumably labeled with string `My volume', you will get:

     $ tar -rf archive --label 'My volume' .
     tar: Archive not labeled to match `My volume'

in case its label does not match.  This will work even if `archive' is
not labeled at all.

   Similarly, `tar' will refuse to list or extract the archive if its
label doesn't match the ARCHIVE-LABEL specified.  In those cases,
ARCHIVE-LABEL argument is interpreted as a globbing-style pattern which
must match the actual magnetic volume label.  *Note exclude::, for a
precise description of how match is attempted(1).  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' was once called `--volume', but is not available under
that name anymore.

   You can also use `--label' to get a common information on all tapes
of a series.  For having this information different in each series
created through a single script used on a regular basis, just manage to
get some date string as part of the label.  For example:

     $ tar cfMV /dev/tape "Daily backup for `date +%Y-%m-%d`"
     $ tar --create --file=/dev/tape --multi-volume \
          --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 usually not the
case.

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

   (1) 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'.


File: tar.info,  Node: verify,  Next: Write Protection,  Prev: label,  Up: Media

9.8 Verifying Data as It is Stored
==================================

`-W'
`--verify'
     Attempt to verify the archive after writing.

   This option causes `tar' to verify the archive after writing it.
Each volume is checked after it is written, and any discrepancies are
recorded on the standard error output.

   Verification requires that the archive be on a back-space-able
medium.  This means pipes, some cartridge tape drives, and some other
devices cannot be verified.

   You can insure the accuracy of an archive by comparing files in the
system with archive members.  `tar' can compare an archive to the file
system as the archive is being written, to verify a write operation, or
can compare a previously written archive, to insure that it is up to
date.

   To check for discrepancies in an archive immediately after it is
written, use the `--verify' (`-W') option in conjunction with the
`--create' operation.  When this option is specified, `tar' checks
archive members against their counterparts in the file system, and
reports discrepancies on the standard error.

   To verify an archive, you must be able to read it from before the end
of the last written entry.  This option is useful for detecting data
errors on some tapes.  Archives written to pipes, some cartridge tape
drives, and some other devices cannot be verified.

   One can explicitly compare an already made archive with the file
system by using the `--compare' (`--diff', `-d') option, instead of
using the more automatic `--verify' option.  *Note compare::.

   Note that these two options have a slightly different intent.  The
`--compare' option checks how identical are the logical contents of some
archive with what is on your disks, while the `--verify' option is
really for checking if the physical contents agree and if the recording
media itself is of dependable quality.  So, for the `--verify'
operation, `tar' tries to defeat all in-memory cache pertaining to the
archive, while it lets the speed optimization undisturbed for the
`--compare' option.  If you nevertheless use `--compare' for media
verification, you may have to defeat the in-memory cache yourself,
maybe by opening and reclosing the door latch of your recording unit,
forcing some doubt in your operating system about the fact this is
really the same volume as the one just written or read.

   The `--verify' option would not be necessary if drivers were indeed
able to detect dependably all write failures.  This sometimes require
many magnetic heads, some able to read after the writes occurred.  One
would not say that drivers unable to detect all cases are necessarily
flawed, as long as programming is concerned.

   The `--verify' (`-W') option will not work in conjunction with the
`--multi-volume' (`-M') option or the `--append' (`-r'), `--update'
(`-u') and `--delete' operations.  *Note Operations::, for more
information on these operations.

   Also, since `tar' normally strips leading `/' from file names (*note
absolute::), a command like `tar --verify -cf /tmp/foo.tar /etc' will
work as desired only if the working directory is `/', as `tar' uses the
archive's relative member names (e.g., `etc/motd') when verifying the
archive.


File: tar.info,  Node: Write Protection,  Prev: verify,  Up: Media

9.9 Write Protection
====================

Almost all tapes and diskettes, and in a few rare cases, even disks can
be "write protected", to protect data on them from being changed.  Once
an archive is written, you should write protect the media to prevent
the archive from being accidentally overwritten or deleted.  (This will
protect the archive from being changed with a tape or floppy 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.


File: tar.info,  Node: Changes,  Next: Configuring Help Summary,  Prev: Media,  Up: Top

Appendix A Changes
******************

This appendix lists some important user-visible changes between version
GNU `tar' 1.17 and previous versions. An up-to-date version of this
document is available at the GNU `tar' documentation page
(http://www.gnu.org/software/tar/manual/changes.html).

Use of globbing patterns when listing and extracting.
     Previous versions of GNU tar assumed shell-style globbing when
     extracting from or listing an archive.  For example:

          $ tar xf foo.tar '*.c'

     would extract all files whose names end in `.c'.  This behavior
     was not documented and was incompatible with traditional tar
     implementations.  Therefore, starting from version 1.15.91, GNU tar
     no longer uses globbing by default.  For example, the above
     invocation is now interpreted as a request to extract from the
     archive the file named `*.c'.

     To facilitate transition to the new behavior for those users who
     got used to the previous incorrect one, `tar' will print a warning
     if it finds out that a requested member was not found in the
     archive and its name looks like a globbing pattern.  For example:

          $ tar xf foo.tar  '*.c'
          tar: Pattern matching characters used in file names. Please,
          tar: use --wildcards to enable pattern matching, or --no-wildcards to
          tar: suppress this warning.
          tar: *.c: Not found in archive
          tar: Error exit delayed from previous errors

     To treat member names as globbing patterns, use -wildcards option.
     If you want to tar to mimic the behavior of versions prior to
     1.15.91, add this option to your `TAR_OPTIONS' variable.

     *Note wildcards::, for the detailed discussion of the use of
     globbing patterns by GNU `tar'.

Use of short option `-o'.
     Earlier versions of GNU `tar' understood `-o' command line option
     as a synonym for `--old-archive'.

     GNU `tar' starting from version 1.13.90 understands this option as
     a synonym for `--no-same-owner'.  This is compatible with UNIX98
     `tar' implementations.

     However, to facilitate transition, `-o' option retains its old
     semantics when it is used with one of archive-creation commands.
     Users are encouraged to use `--format=oldgnu' instead.

     It is especially important, since versions of GNU Automake up to
     and including 1.8.4 invoke tar with this option to produce
     distribution tarballs.  *Note v7: Formats, for the detailed
     discussion of this issue and its implications.

     .  *Note tar-v7: (automake)Options, for a description on how to
     use various archive formats with `automake'.

     Future versions of GNU `tar' will understand `-o' only as a
     synonym for `--no-same-owner'.

Use of short option `-l'
     Earlier versions of GNU `tar' understood `-l' option as a synonym
     for `--one-file-system'.  Since such usage contradicted to UNIX98
     specification and harmed compatibility with other implementation,
     it was declared deprecated in version 1.14.  However, to
     facilitate transition to its new semantics, it was supported by
     versions 1.15 and 1.15.90.  The present use of `-l' as a short
     variant of `--check-links' was introduced in version 1.15.91.

Use of options `--portability' and `--old-archive'
     These options are deprecated.  Please use `--format=v7' instead.

Use of option `--posix'
     This option is deprecated.  Please use `--format=posix' instead.


File: tar.info,  Node: Configuring Help Summary,  Next: Tar Internals,  Prev: Changes,  Up: Top

Appendix B Configuring Help Summary
***********************************

Running `tar --help' displays the short `tar' option summary (*note
help::). This summary is organized by "groups" of semantically close
options. The options within each group are printed in the following
order: a short option, eventually followed by a list of corresponding
long option names, followed by a short description of the option. For
example, here is an excerpt from the actual `tar --help' output:


 Main operation mode:

  -A, --catenate, --concatenate   append tar files to an archive
  -c, --create               create a new archive
  -d, --diff, --compare      find differences between archive and
                             file system
      --delete               delete from the archive

   The exact visual representation of the help output is configurable
via `ARGP_HELP_FMT' environment variable. The value of this variable is
a comma-separated list of "format variable" assignments. There are two
kinds of format variables. An "offset variable" keeps the offset of
some part of help output text from the leftmost column on the screen. A
"boolean" variable is a flag that toggles some output feature on or
off. Depending on the type of the corresponding variable, there are two
kinds of assignments:

Offset assignment
     The assignment to an offset variable has the following syntax:

          VARIABLE=VALUE

     where VARIABLE is the variable name, and VALUE is a numeric value
     to be assigned to the variable.

Boolean assignment
     To assign `true' value to a variable, simply put this variable
     name. To assign `false' value, prefix the variable name with
     `no-'. For example:

          # Assign `true' value:
          dup-args
          # Assign `false' value:
          no-dup-args

   Following variables are declared:

 -- Help Output: boolean dup-args
     If true, arguments for an option are shown with both short and long
     options, even when a given option has both forms, for example:

            -f ARCHIVE, --file=ARCHIVE use archive file or device ARCHIVE

     If false, then if an option has both short and long forms, the
     argument is only shown with the long one, for example:

            -f, --file=ARCHIVE         use archive file or device ARCHIVE

     and a message indicating that the argument is applicable to both
     forms is printed below the options. This message can be disabled
     using `dup-args-note' (see below).

     The default is false.

 -- Help Output: boolean dup-args-note
     If this variable is true, which is the default, the following
     notice is displayed at the end of the help output:

          Mandatory or optional arguments to long options are also
          mandatory or optional for any corresponding short options.

     Setting `no-dup-args-note' inhibits this message. Normally, only
     one of variables `dup-args' or `dup-args-note' should be set.

 -- Help Output: offset short-opt-col
     Column in which short options start. Default is 2.

          $ tar --help|grep ARCHIVE
            -f, --file=ARCHIVE   use archive file or device ARCHIVE
          $ ARGP_HELP_FMT=short-opt-col=6 tar --help|grep ARCHIVE
                -f, --file=ARCHIVE   use archive file or device ARCHIVE

 -- Help Output: offset long-opt-col
     Column in which long options start. Default is 6. For example:

          $ tar --help|grep ARCHIVE
            -f, --file=ARCHIVE   use archive file or device ARCHIVE
          $ ARGP_HELP_FMT=long-opt-col=16 tar --help|grep ARCHIVE
            -f,           --file=ARCHIVE   use archive file or device ARCHIVE

 -- Help Output: offset doc-opt-col
     Column in which "doc options" start.  A doc option isn't actually
     an option, but rather an arbitrary piece of documentation that is
     displayed in much the same manner as the options.  For example, in
     the description of `--format' option:

            -H, --format=FORMAT        create archive of the given format.

           FORMAT is one of the following:

              gnu                      GNU tar 1.13.x format
              oldgnu                   GNU format as per tar <= 1.12
              pax                      POSIX 1003.1-2001 (pax) format
              posix                    same as pax
              ustar                    POSIX 1003.1-1988 (ustar) format
              v7                       old V7 tar format

     the format names are doc options. Thus, if you set
     `ARGP_HELP_FMT=doc-opt-col=6' the above part of the help output
     will look as follows:

            -H, --format=FORMAT        create archive of the given format.

           FORMAT is one of the following:

                  gnu                      GNU tar 1.13.x format
                  oldgnu                   GNU format as per tar <= 1.12
                  pax                      POSIX 1003.1-2001 (pax) format
                  posix                    same as pax
                  ustar                    POSIX 1003.1-1988 (ustar) format
                  v7                       old V7 tar format

 -- Help Output: offset opt-doc-col
     Column in which option description starts. Default is 29.

          $ tar --help|grep ARCHIVE
            -f, --file=ARCHIVE         use archive file or device ARCHIVE
          $ ARGP_HELP_FMT=opt-doc-col=19 tar --help|grep ARCHIVE
            -f, --file=ARCHIVE   use archive file or device ARCHIVE
          $ ARGP_HELP_FMT=opt-doc-col=9 tar --help|grep ARCHIVE
            -f, --file=ARCHIVE
                     use archive file or device ARCHIVE

     Notice, that the description starts on a separate line if
     `opt-doc-col' value is too small.

 -- Help Output: offset header-col
     Column in which "group headers" are printed.  A group header is a
     descriptive text preceding an option group.  For example, in the
     following text:


      Main operation mode:

       -A, --catenate, --concatenate   append tar files to
                                  an archive
       -c, --create               create a new archive
      `Main operation mode:' is the group header.

     The default value is 1.

 -- Help Output: offset usage-indent
     Indentation of wrapped usage lines. Affects `--usage' output.
     Default is 12.

 -- Help Output: offset rmargin
     Right margin of the text output. Used for wrapping.


File: tar.info,  Node: Tar Internals,  Next: Genfile,  Prev: Configuring Help Summary,  Up: Top

Appendix C Tar Internals
************************

* Menu:

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


File: tar.info,  Node: Standard,  Next: Extensions,  Up: Tar Internals

Basic Tar Format
================

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

While an archive may contain many files, the archive itself is a single
ordinary file.  Like any other file, an archive file can be written to
a storage device such as a tape or disk, sent through a pipe or over a
network, saved on the active file system, or even stored in another
archive.  An archive file is not easy to read or manipulate without
using the `tar' utility or Tar mode in GNU Emacs.

   Physically, an archive consists of a series of file entries
terminated by an end-of-archive entry, which consists of two 512 blocks
of zero bytes.  A file entry usually describes one of the files in the
archive (an "archive member"), and consists of a file header and the
contents of the file.  File headers contain file names and statistics,
checksum information which `tar' uses to detect file corruption, and
information about file types.

   Archives are permitted to have more than one member with the same
member name.  One way this situation can occur is if more than one
version of a file has been stored in the archive.  For information
about adding new versions of a file to an archive, see *note update::.

   In addition to entries describing archive members, an archive may
contain entries which `tar' itself uses to store information.  *Note
label::, for an example of such an archive entry.

   A `tar' archive file contains a series of blocks.  Each block
contains `BLOCKSIZE' bytes.  Although this format may be thought of as
being on magnetic tape, other media are often used.

   Each file archived is represented by a header block which describes
the file, followed by zero or more blocks which give the contents of
the file.  At the end of the archive file there are two 512-byte blocks
filled with binary zeros as an end-of-file marker.  A reasonable system
should write such end-of-file marker at the end of an archive, but must
not assume that such a block exists when reading an archive.  In
particular GNU `tar' always issues a warning if it does not encounter
it.

   The blocks may be "blocked" for physical I/O operations.  Each
record of N blocks (where N is set by the `--blocking-factor=512-SIZE'
(`-b 512-SIZE') option to `tar') is written with a single `write ()'
operation.  On magnetic tapes, the result of such a write is a single
record.  When writing an archive, the last record of blocks should be
written at the full size, with blocks after the zero block containing
all zeros.  When reading an archive, a reasonable system should
properly handle an archive whose last record is shorter than the rest,
or which contains garbage records after a zero block.

   The header block is defined in C as follows.  In the GNU `tar'
distribution, this is part of file `src/tar.h':


     /* tar Header Block, from POSIX 1003.1-1990.  */

     /* POSIX header.  */

     struct posix_header
     {                              /* byte offset */
       char name[100];               /*   0 */
       char mode[8];                 /* 100 */
       char uid[8];                  /* 108 */
       char gid[8];                  /* 116 */
       char size[12];                /* 124 */
       char mtime[12];               /* 136 */
       char chksum[8];               /* 148 */
       char typeflag;                /* 156 */
       char linkname[100];           /* 157 */
       char magic[6];                /* 257 */
       char version[2];              /* 263 */
       char uname[32];               /* 265 */
       char gname[32];               /* 297 */
       char devmajor[8];             /* 329 */
       char devminor[8];             /* 337 */
       char prefix[155];             /* 345 */
                                     /* 500 */
     };

     #define TMAGIC   "ustar"        /* ustar and a null */
     #define TMAGLEN  6
     #define TVERSION "00"           /* 00 and no null */
     #define TVERSLEN 2

     /* Values used in typeflag field.  */
     #define REGTYPE  '0'            /* regular file */
     #define AREGTYPE '\0'           /* regular file */
     #define LNKTYPE  '1'            /* link */
     #define SYMTYPE  '2'            /* reserved */
     #define CHRTYPE  '3'            /* character special */
     #define BLKTYPE  '4'            /* block special */
     #define DIRTYPE  '5'            /* directory */
     #define FIFOTYPE '6'            /* FIFO special */
     #define CONTTYPE '7'            /* reserved */

     #define XHDTYPE  'x'            /* Extended header referring to the
                                        next file in the archive */
     #define XGLTYPE  'g'            /* Global extended header */

     /* Bits used in the mode field, values in octal.  */
     #define TSUID    04000          /* set UID on execution */
     #define TSGID    02000          /* set GID on execution */
     #define TSVTX    01000          /* reserved */
                                     /* file permissions */
     #define TUREAD   00400          /* read by owner */
     #define TUWRITE  00200          /* write by owner */
     #define TUEXEC   00100          /* execute/search by owner */
     #define TGREAD   00040          /* read by group */
     #define TGWRITE  00020          /* write by group */
     #define TGEXEC   00010          /* execute/search by group */
     #define TOREAD   00004          /* read by other */
     #define TOWRITE  00002          /* write by other */
     #define TOEXEC   00001          /* execute/search by other */

     /* tar Header Block, GNU extensions.  */

     /* In GNU tar, SYMTYPE is for to symbolic links, and CONTTYPE is for
        contiguous files, so maybe disobeying the `reserved' comment in POSIX
        header description.  I suspect these were meant to be used this way, and
        should not have really been `reserved' in the published standards.  */

     /* *BEWARE* *BEWARE* *BEWARE* that the following information is still
        boiling, and may change.  Even if the OLDGNU format description should be
        accurate, the so-called GNU format is not yet fully decided.  It is
        surely meant to use only extensions allowed by POSIX, but the sketch
        below repeats some ugliness from the OLDGNU format, which should rather
        go away.  Sparse files should be saved in such a way that they do *not*
        require two passes at archive creation time.  Huge files get some POSIX
        fields to overflow, alternate solutions have to be sought for this.  */

     /* Descriptor for a single file hole.  */

     struct sparse
     {                              /* byte offset */
       char offset[12];              /*   0 */
       char numbytes[12];            /*  12 */
                                     /*  24 */
     };

     /* Sparse files are not supported in POSIX ustar format.  For sparse files
        with a POSIX header, a GNU extra header is provided which holds overall
        sparse information and a few sparse descriptors.  When an old GNU header
        replaces both the POSIX header and the GNU extra header, it holds some
        sparse descriptors too.  Whether POSIX or not, if more sparse descriptors
        are still needed, they are put into as many successive sparse headers as
        necessary.  The following constants tell how many sparse descriptors fit
        in each kind of header able to hold them.  */

     #define SPARSES_IN_EXTRA_HEADER  16
     #define SPARSES_IN_OLDGNU_HEADER 4
     #define SPARSES_IN_SPARSE_HEADER 21

     /* Extension header for sparse files, used immediately after the GNU extra
        header, and used only if all sparse information cannot fit into that
        extra header.  There might even be many such extension headers, one after
        the other, until all sparse information has been recorded.  */

     struct sparse_header
     {                              /* byte offset */
       struct sparse sp[SPARSES_IN_SPARSE_HEADER];
                                     /*   0 */
       char isextended;              /* 504 */
                                     /* 505 */
     };

     /* The old GNU format header conflicts with POSIX format in such a way that
        POSIX archives may fool old GNU tar's, and POSIX tar's might well be
        fooled by old GNU tar archives.  An old GNU format header uses the space
        used by the prefix field in a POSIX header, and cumulates information
        normally found in a GNU extra header.  With an old GNU tar header, we
        never see any POSIX header nor GNU extra header.  Supplementary sparse
        headers are allowed, however.  */

     struct oldgnu_header
     {                              /* byte offset */
       char unused_pad1[345];        /*   0 */
       char atime[12];               /* 345 Incr. archive: atime of the file */
       char ctime[12];               /* 357 Incr. archive: ctime of the file */
       char offset[12];              /* 369 Multivolume archive: the offset of
                                        the start of this volume */
       char longnames[4];            /* 381 Not used */
       char unused_pad2;             /* 385 */
       struct sparse sp[SPARSES_IN_OLDGNU_HEADER];
                                     /* 386 */
       char isextended;              /* 482 Sparse file: Extension sparse header
                                        follows */
       char realsize[12];            /* 483 Sparse file: Real size*/
                                     /* 495 */
     };

     /* OLDGNU_MAGIC uses both magic and version fields, which are contiguous.
        Found in an archive, it indicates an old GNU header format, which will be
        hopefully become obsolescent.  With OLDGNU_MAGIC, uname and gname are
        valid, though the header is not truly POSIX conforming.  */
     #define OLDGNU_MAGIC "ustar  "  /* 7 chars and a null */

     /* The standards committee allows only capital A through capital Z for
        user-defined expansion.  Other letters in use include:

        'A' Solaris Access Control List
        'E' Solaris Extended Attribute File
        'I' Inode only, as in 'star'
        'N' Obsolete GNU tar, for file names that do not fit into the main header.
        'X' POSIX 1003.1-2001 eXtended (VU version)  */

     /* This is a dir entry that contains the names of files that were in the
        dir at the time the dump was made.  */
     #define GNUTYPE_DUMPDIR 'D'

     /* Identifies the *next* file on the tape as having a long linkname.  */
     #define GNUTYPE_LONGLINK 'K'

     /* Identifies the *next* file on the tape as having a long name.  */
     #define GNUTYPE_LONGNAME 'L'

     /* This is the continuation of a file that began on another volume.  */
     #define GNUTYPE_MULTIVOL 'M'

     /* This is for sparse files.  */
     #define GNUTYPE_SPARSE 'S'

     /* This file is a tape/volume header.  Ignore it on extraction.  */
     #define GNUTYPE_VOLHDR 'V'

     /* Solaris extended header */
     #define SOLARIS_XHDTYPE 'X'

     /* Jo"rg Schilling star header */

     struct star_header
     {                              /* byte offset */
       char name[100];               /*   0 */
       char mode[8];                 /* 100 */
       char uid[8];                  /* 108 */
       char gid[8];                  /* 116 */
       char size[12];                /* 124 */
       char mtime[12];               /* 136 */
       char chksum[8];               /* 148 */
       char typeflag;                /* 156 */
       char linkname[100];           /* 157 */
       char magic[6];                /* 257 */
       char version[2];              /* 263 */
       char uname[32];               /* 265 */
       char gname[32];               /* 297 */
       char devmajor[8];             /* 329 */
       char devminor[8];             /* 337 */
       char prefix[131];             /* 345 */
       char atime[12];               /* 476 */
       char ctime[12];               /* 488 */
                                     /* 500 */
     };

     #define SPARSES_IN_STAR_HEADER      4
     #define SPARSES_IN_STAR_EXT_HEADER  21

     struct star_in_header
     {
       char fill[345];       /*   0  Everything that is before t_prefix */
       char prefix[1];       /* 345  t_name prefix */
       char fill2;           /* 346  */
       char fill3[8];        /* 347  */
       char isextended;      /* 355  */
       struct sparse sp[SPARSES_IN_STAR_HEADER]; /* 356  */
       char realsize[12];    /* 452  Actual size of the file */
       char offset[12];      /* 464  Offset of multivolume contents */
       char atime[12];       /* 476  */
       char ctime[12];       /* 488  */
       char mfill[8];        /* 500  */
       char xmagic[4];       /* 508  "tar" */
     };

     struct star_ext_header
     {
       struct sparse sp[SPARSES_IN_STAR_EXT_HEADER];
       char isextended;
     };

   All characters in header blocks are represented by using 8-bit
characters in the local variant of ASCII.  Each field within the
structure is contiguous; that is, there is no padding used within the
structure.  Each character on the archive medium is stored contiguously.

   Bytes representing the contents of files (after the header block of
each file) are not translated in any way and are not constrained to
represent characters in any character set.  The `tar' format does not
distinguish text files from binary files, and no translation of file
contents is performed.

   The `name', `linkname', `magic', `uname', and `gname' are
null-terminated character strings.  All other fields are zero-filled
octal numbers in ASCII.  Each numeric field of width W contains W minus
1 digits, and a null.

   The `name' field is the file name of the file, with directory names
(if any) preceding the file name, separated by slashes.

   The `mode' field provides nine bits specifying file permissions and
three bits to specify the Set UID, Set GID, and Save Text ("sticky")
modes.  Values for these bits are defined above.  When special
permissions are required to create a file with a given mode, and the
user restoring files from the archive does not hold such permissions,
the mode bit(s) specifying those special permissions are ignored.
Modes which are not supported by the operating system restoring files
from the archive will be ignored.  Unsupported modes should be faked up
when creating or updating an archive; e.g., the group permission could
be copied from the _other_ permission.

   The `uid' and `gid' fields are the numeric user and group ID of the
file owners, respectively.  If the operating system does not support
numeric user or group IDs, these fields should be ignored.

   The `size' field is the size of the file in bytes; linked files are
archived with this field specified as zero.

   The `mtime' field is the data modification time of the file at the
time it was archived.  It is the ASCII representation of the octal
value of the last time the file's contents were modified, represented
as an integer number of seconds since January 1, 1970, 00:00
Coordinated Universal Time.

   The `chksum' field is the ASCII representation of the octal value of
the simple sum of all bytes in the header block.  Each 8-bit byte in
the header is added to an unsigned integer, initialized to zero, the
precision of which shall be no less than seventeen bits.  When
calculating the checksum, the `chksum' field is treated as if it were
all blanks.

   The `typeflag' field specifies the type of file archived.  If a
particular implementation does not recognize or permit the specified
type, the file will be extracted as if it were a regular file.  As this
action occurs, `tar' issues a warning to the standard error.

   The `atime' and `ctime' fields are used in making incremental
backups; they store, respectively, the particular file's access and
status change times.

   The `offset' is used by the `--multi-volume' (`-M') option, when
making a multi-volume archive.  The offset is number of bytes into the
file that we need to restart at to continue the file on the next tape,
i.e., where we store the location that a continued file is continued at.

   The following fields were added to deal with sparse files.  A file
is "sparse" if it takes in unallocated blocks which end up being
represented as zeros, i.e., no useful data.  A test to see if a file is
sparse is to look at the number blocks allocated for it versus the
number of characters in the file; if there are fewer blocks allocated
for the file than would normally be allocated for a file of that size,
then the file is sparse.  This is the method `tar' uses to detect a
sparse file, and once such a file is detected, it is treated
differently from non-sparse files.

   Sparse files are often `dbm' files, or other database-type files
which have data at some points and emptiness in the greater part of the
file.  Such files can appear to be very large when an `ls -l' is done
on them, when in truth, there may be a very small amount of important
data contained in the file.  It is thus undesirable to have `tar' think
that it must back up this entire file, as great quantities of room are
wasted on empty blocks, which can lead to running out of room on a tape
far earlier than is necessary.  Thus, sparse files are dealt with so
that these empty blocks are not written to the tape.  Instead, what is
written to the tape is a description, of sorts, of the sparse file:
where the holes are, how big the holes are, and how much data is found
at the end of the hole.  This way, the file takes up potentially far
less room on the tape, and when the file is extracted later on, it will
look exactly the way it looked beforehand.  The following is a
description of the fields used to handle a sparse file:

   The `sp' is an array of `struct sparse'.  Each `struct sparse'
contains two 12-character strings which represent an offset into the
file and a number of bytes to be written at that offset.  The offset is
absolute, and not relative to the offset in preceding array element.

   The header can hold four of these `struct sparse' at the moment; if
more are needed, they are not stored in the header.

   The `isextended' flag is set when an `extended_header' is needed to
deal with a file.  Note that this means that this flag can only be set
when dealing with a sparse file, and it is only set in the event that
the description of the file will not fit in the allotted room for
sparse structures in the header.  In other words, an extended_header is
needed.

   The `extended_header' structure is used for sparse files which need
more sparse structures than can fit in the header.  The header can fit
4 such structures; if more are needed, the flag `isextended' gets set
and the next block is an `extended_header'.

   Each `extended_header' structure contains an array of 21 sparse
structures, along with a similar `isextended' flag that the header had.
There can be an indeterminate number of such `extended_header's to
describe a sparse file.

`REGTYPE'
`AREGTYPE'
     These flags represent a regular file.  In order to be compatible
     with older versions of `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'
     This flag represents a file linked to another file, of any type,
     previously archived.  Such files are identified in Unix by each
     file having the same device and inode number.  The linked-to name
     is specified in the `linkname' field with a trailing null.

`SYMTYPE'
     This represents a symbolic link to another file.  The linked-to
     name is specified in the `linkname' field with a trailing null.

`CHRTYPE'
`BLKTYPE'
     These represent character special files and block special files
     respectively.  In this case the `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'
     This flag specifies a directory or sub-directory.  The directory
     name in the `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'
     This specifies a FIFO special file.  Note that the archiving of a
     FIFO file archives the existence of this file and not its contents.

`CONTTYPE'
     This specifies a contiguous file, which is the same as a normal
     file except that, in operating systems which support it, all its
     space is allocated contiguously on the disk.  Operating systems
     which do not allow contiguous allocation should silently treat this
     type as a normal file.

`A' ... `Z'
     These are reserved for custom implementations.  Some of these are
     used in the GNU modified format, as described below.


   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'.


File: tar.info,  Node: Extensions,  Next: Sparse Formats,  Prev: Standard,  Up: Tar Internals

GNU Extensions to the Archive Format
====================================

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

The GNU format uses additional file types to describe new types of
files in an archive.  These are listed below.

`GNUTYPE_DUMPDIR'
`'D''
     This represents a directory and a list of files created by the
     `--incremental' (`-G') option.  The `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''
     This represents a file continued from another volume of a
     multi-volume archive created with the `--multi-volume' (`-M')
     option.  The original type of the file is not given here.  The
     `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''
     This flag indicates that we are dealing with a sparse file.  Note
     that archiving a sparse file requires special operations to find
     holes in the file, which mark the positions of these holes, along
     with the number of bytes of data to be found after the hole.

`GNUTYPE_VOLHDR'
`'V''
     This file type is used to mark the volume header that was given
     with the `--label=ARCHIVE-LABEL' (`-V ARCHIVE-LABEL') option when
     the archive was created.  The `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.


File: tar.info,  Node: Sparse Formats,  Next: Snapshot Files,  Prev: Extensions,  Up: Tar Internals

Storing Sparse Files
====================

The notion of sparse file, and the ways of handling it from the point
of view of GNU `tar' user have been described in detail in *note
sparse::.  This chapter describes the internal format GNU `tar' uses to
store such files.

   The support for sparse files in GNU `tar' has a long history.  The
earliest version featuring this support that I was able to find was
1.09, released in November, 1990.  The format introduced back then is
called "old GNU" sparse format and in spite of the fact that its design
contained many flaws, it was the only format GNU `tar' supported until
version 1.14 (May, 2004), which introduced initial support for sparse
archives in PAX archives (*note posix::).  This format was not free
from design flows, either and it was subsequently improved in versions
1.15.2 (November, 2005) and 1.15.92 (June, 2006).

   In addition to GNU sparse format, GNU `tar' is able to read and
extract sparse files archived by `star'.

   The following subsections describe each format in detail.

* Menu:

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


File: tar.info,  Node: Old GNU Format,  Next: PAX 0,  Up: Sparse Formats

C.0.1 Old GNU Format
--------------------

The format introduced some time around 1990 (v. 1.09).  It was designed
on top of standard `ustar' headers in such an unfortunate way that some
of its fields overwrote fields required by POSIX.

   An old GNU sparse header is designated by type `S'
(`GNUTYPE_SPARSE') and has the following layout:

Offset  Size    Name           Data type      Contents
---------------------------------------------------------------------------- 
0       345                    N/A            Not used.
345     12      atime          Number         `atime' of the file.
357     12      ctime          Number         `ctime' of the file .
369     12      offset         Number         For multivolume archives:
                                              the offset of the start of
                                              this volume.
381     4                      N/A            Not used.
385     1                      N/A            Not used.
386     96      sp             `sparse_header'(4 entries) File map.
482     1       isextended     Bool           `1' if an extension sparse
                                              header follows, `0'
                                              otherwise.
483     12      realsize       Number         Real size of the file.

   Each of `sparse_header' object at offset 386 describes a single data
chunk. It has the following structure:

Offset  Size    Data type      Contents
--------------------------------------------------------------------------- 
0       12      Number         Offset of the beginning of the chunk.
12      12      Number         Size of the chunk.

   If the member contains more than four chunks, the `isextended' field
of the header has the value `1' and the main header is followed by one
or more "extension headers".  Each such header has the following
structure:

Offset  Size    Name           Data type      Contents
---------------------------------------------------------------------------- 
0       21      sp             `sparse_header' (21 entires) File map.
504     1       isextended     Bool           `1' if an extension sparse
                                              header follows, or `0'
                                              otherwise.

   A header with `isextended=0' ends the map.


File: tar.info,  Node: PAX 0,  Next: PAX 1,  Prev: Old GNU Format,  Up: Sparse Formats

C.0.2 PAX Format, Versions 0.0 and 0.1
--------------------------------------

There are two formats available in this branch.  The version `0.0' is
the initial version of sparse format used by `tar' versions
1.14-1.15.1.  The sparse file map is kept in extended (`x') PAX header
variables:

`GNU.sparse.size'
     Real size of the stored file

`GNU.sparse.numblocks'
     Number of blocks in the sparse map

`GNU.sparse.offset'
     Offset of the data block

`GNU.sparse.numbytes'
     Size of the data block

   The latter two variables repeat for each data block, so the overall
structure is like this:

     GNU.sparse.size=SIZE
     GNU.sparse.numblocks=NUMBLOCKS
     repeat NUMBLOCKS times
       GNU.sparse.offset=OFFSET
       GNU.sparse.numbytes=NUMBYTES
     end repeat

   This format presented the following two problems:

  1. Whereas the POSIX specification allows a variable to appear
     multiple times in a header, it requires that only the last
     occurrence be meaningful.  Thus, multiple occurrences of
     `GNU.sparse.offset' and `GNU.sparse.numbytes' are conflicting with
     the POSIX specs.

  2. Attempting to extract such archives using a third-party `tar's
     results in extraction of sparse files in _compressed form_.  If
     the `tar' implementation in question does not support POSIX
     format, it will also extract a file containing extension header
     attributes.  This file can be used to expand the file to its
     original state.  However, posix-aware `tar's will usually ignore
     the unknown variables, which makes restoring the file more
     difficult.  *Note Extraction of sparse members in v.0.0 format:
     extracting sparse v.0.x, for the detailed description of how to
     restore such members using non-GNU `tar's.

   GNU `tar' 1.15.2 introduced sparse format version `0.1', which
attempted to solve these problems.  As its predecessor, this format
stores sparse map in the extended POSIX header.  It retains
`GNU.sparse.size' and `GNU.sparse.numblocks' variables, but instead of
`GNU.sparse.offset'/`GNU.sparse.numbytes' pairs it uses a single
variable:

`GNU.sparse.map'
     Map of non-null data chunks.  It is a string consisting of
     comma-separated values "OFFSET,SIZE[,OFFSET-1,SIZE-1...]"

   To address the 2nd problem, the `name' field in `ustar' is replaced
with a special name, constructed using the following pattern:

     %d/GNUSparseFile.%p/%f

   The real name of the sparse file is stored in the variable
`GNU.sparse.name'.  Thus, those `tar' implementations that are not
aware of GNU extensions will at least extract the files into separate
directories, giving the user a possibility to expand it afterwards.
*Note Extraction of sparse members in v.0.1 format: extracting sparse
v.0.x, for the detailed description of how to restore such members
using non-GNU `tar's.

   The resulting `GNU.sparse.map' string can be _very_ long.  Although
POSIX does not impose any limit on the length of a `x' header variable,
this possibly can confuse some tars.


File: tar.info,  Node: PAX 1,  Prev: PAX 0,  Up: Sparse Formats

C.0.3 PAX Format, Version 1.0
-----------------------------

The version `1.0' of sparse format was introduced with GNU `tar'
1.15.92.  Its main objective was to make the resulting file extractable
with little effort even by non-posix aware `tar' implementations.
Starting from this version, the extended header preceding a sparse
member always contains the following variables that identify the format
being used:

`GNU.sparse.major'
     Major version

`GNU.sparse.minor'
     Minor version

   The `name' field in `ustar' header contains a special name,
constructed using the following pattern:

     %d/GNUSparseFile.%p/%f

   The real name of the sparse file is stored in the variable
`GNU.sparse.name'.  The real size of the file is stored in the variable
`GNU.sparse.realsize'.

   The sparse map itself is stored in the file data block, preceding
the actual file data.  It consists of a series of octal numbers of
arbitrary length, delimited by newlines. The map is padded with nulls
to the nearest block boundary.

   The first number gives the number of entries in the map. Following
are map entries, each one consisting of two numbers giving the offset
and size of the data block it describes.

   The format is designed in such a way that non-posix aware tars and
tars not supporting `GNU.sparse.*' keywords will extract each sparse
file in its condensed form with the file map prepended and will place it
into a separate directory.  Then, using a simple program it would be
possible to expand the file to its original form even without GNU `tar'.
*Note Sparse Recovery::, for the detailed information on how to extract
sparse members without GNU `tar'.


File: tar.info,  Node: Snapshot Files,  Next: Dumpdir,  Prev: Sparse Formats,  Up: Tar Internals

Format of the Incremental Snapshot Files
========================================

A "snapshot file" (or "directory file") is created during incremental
backups (*note Incremental Dumps::).  It contains the status of the
file system at the time of the dump and is used to determine which
files were modified since the last backup.

   GNU `tar' version 1.17 supports two snapshot file formats.  The
first format, called "format 0", is the one used by GNU `tar' versions
up to 1.15.1. The second format, called "format 1" is an extended
version of this format, that contains more metadata and allows for
further extensions.

   `Format 0' snapshot file begins with a line containing a decimal
number that represents the UNIX timestamp of the beginning of the last
archivation. This line is followed by directory metadata descriptions,
one per line. Each description has the following format:

     [NFS]DEV INODE NAME

where optional NFS is a single plus character (`+') if this directory
is located on an NFS-mounted partition, DEV and INODE are the device
and inode numbers of the directory, and NAME is the directory name.

   `Format 1' snapshot file begins with a line specifying the format of
the file. This line has the following structure:

     `GNU tar-'TAR-VERSION`-'INCR-FORMAT-VERSION

where TAR-VERSION is the version of GNU `tar' implementation that
created this snapshot, and INCR-FORMAT-VERSION is the version number of
the snapshot format (in this case `1').

   The following line contains two decimal numbers, representing the
time of the last backup. First number is the number of seconds, the
second one is the number of nanoseconds, since the beginning of the
epoch.

   Following lines contain directory metadata, one line per directory.
The line format is:

     [NFS]MTIME-SEC MTIME-NSEC DEV INODE NAME

where MTIME-SEC and MTIME-NSEC represent the last modification time of
this directory with nanosecond precision; NFS, DEV, INODE and NAME have
the same meaning as with `format 0'.


File: tar.info,  Node: Dumpdir,  Prev: Snapshot Files,  Up: Tar Internals

Dumpdir
=======

Incremental archives keep information about contents of each dumped
directory in special data blocks called "dumpdirs".

   Dumpdir is a sequence of entries of the following form:

     C FILENAME \0

where C is one of the "control codes" described below, FILENAME is the
name of the file C operates upon, and `\0' represents a nul character
(ASCII 0).  The white space characters were added for readability, real
dumpdirs do not contain them.

   Each dumpdir ends with a single nul character.

   The following table describes control codes and their meanings:

`Y'
     FILENAME is contained in the archive.

`N'
     FILENAME was present in the directory at the time the archive was
     made, yet it was not dumped to the archive, because it had not
     changed since the last backup.

`D'
     FILENAME is a directory.

`R'
     This code requests renaming of the FILENAME to the name specified
     with the following `T' command.

`T'
     Specify target file name for `R' command (see below).

`X'
     Specify "temporary directory" name for a rename operation (see
     below).

   Codes `Y', `N' and `D' require FILENAME argument to be a relative
file name to the directory this dumpdir describes, whereas codes `R',
`T' and `X' require their argument to be an absolute file name.

   The three codes `R', `T' and `X' specify a "renaming operation".  In
the simplest case it is:

     R`source'\0T`dest'\0

which means "rename file `source' to file `dest'".

   However, there are cases that require using a "temporary directory".
For example, consider the following scenario:

  1. Previous run dumped a directory `foo' which contained the
     following three directories:

          a
          b
          c

  2. They were renamed _cyclically_, so that:

          `a' became `b'
          `b' became `c'
          `c' became `a'

  3. New incremental dump was made.

   This case cannot be handled by three successive renames, since
renaming `a' to `b' will destroy existing directory.  To handle such
case a temporary directory is required. GNU `tar' will create the
following dumpdir (newlines have been added for readability):

     Xfoo\0
     Rfoo/a\0T\0
     Rfoo/b\0Tfoo/c\0
     Rfoo/c\0Tfoo/a\0
     R\0Tfoo/a\0

   The first command, `Xfoo\0', instructs the extractor to create a
temporary directory in the directory `foo'.  Second command,
`Rfoo/aT\0', says "rename file `foo/a' to the temporary directory that
has just been created" (empty file name after a command means use
temporary directory).  Third and fourth commands work as usual, and,
finally, the last command, `R\0Tfoo/a\0' tells tar to rename the
temporary directory to `foo/a'.

   The exact placement of a dumpdir in the archive depends on the
archive format (*note Formats::):

   * PAX archives

     In PAX archives, dumpdir is stored in the extended header of the
     corresponding directory, in variable `GNU.dumpdir'.

   * GNU and old GNU archives

     These formats implement special header type `D', which is similar
     to ustar header `5' (directory), except that it precedes a data
     block containing the dumpdir.


File: tar.info,  Node: Genfile,  Next: Free Software Needs Free Documentation,  Prev: Tar Internals,  Up: Top

Appendix D Genfile
******************

This appendix describes `genfile', an auxiliary program used in the GNU
tar testsuite. If you are not interested in developing GNU tar, skip
this appendix.

   Initially, `genfile' was used to generate data files for the
testsuite, hence its name. However, new operation modes were being
implemented as the testsuite grew more sophisticated, and now `genfile'
is a multi-purpose instrument.

   There are three basic operation modes:

File Generation
     This is the default mode. In this mode, `genfile' generates data
     files.

File Status
     In this mode `genfile' displays status of specified files.

Synchronous Execution.
     In this mode `genfile' executes the given program with
     `--checkpoint' option and executes a set of actions when specified
     checkpoints are reached.

* Menu:

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


File: tar.info,  Node: Generate Mode,  Next: Status Mode,  Up: Genfile

D.1 Generate Mode
=================

In this mode `genfile' creates a data file for the test suite. The size
of the file is given with the `--length' (`-l') option. By default the
file contents is written to the standard output, this can be changed
using `--file' (`-f') command line option. Thus, the following two
commands are equivalent:

     genfile --length 100 > outfile
     genfile --length 100 --file outfile

   If `--length' is not given, `genfile' will generate an empty
(zero-length) file.

   The command line option `--seek=N' istructs `genfile' to skip the
given number of bytes (N) in the output file before writing to it.  It
is similar to the `seek=N' of the `dd' utility.

   You can instruct `genfile' to create several files at one go, by
giving it `--files-from' (`-T') option followed by a name of file
containing a list of file names. Using dash (`-') instead of the file
name causes `genfile' to read file list from the standard input. For
example:

     # Read file names from file `file.list'
     genfile --files-from file.list
     # Read file names from standard input
     genfile --files-from -

   The list file is supposed to contain one file name per line. To use
file lists separated by ASCII NUL character, use `--null' (`-0')
command line option:

     genfile --null --files-from file.list

   The default data pattern for filling the generated file consists of
first 256 letters of ASCII code, repeated enough times to fill the
entire file. This behavior can be changed with `--pattern' option. This
option takes a mandatory argument, specifying pattern name to use.
Currently two patterns are implemented:

`--pattern=default'
     The default pattern as described above.

`--pattern=zero'
     Fills the file with zeroes.

   If no file name was given, the program exits with the code `0'.
Otherwise, it exits with `0' only if it was able to create a file of
the specified length.

   Special option `--sparse' (`-s') instructs `genfile' to create a
sparse file. Sparse files consist of "data fragments", separated by
"holes" or blocks of zeros. On many operating systems, actual disk
storage is not allocated for holes, but they are counted in the length
of the file. To create a sparse file, `genfile' should know where to
put data fragments, and what data to use to fill them. So, when
`--sparse' is given the rest of the command line specifies a so-called
"file map".

   The file map consists of any number of "fragment descriptors". Each
descriptor is composed of two values: a number, specifying fragment
offset from the end of the previous fragment or, for the very first
fragment, from the beginning of the file, and "contents string", i.e.,
a string of characters, specifying the pattern to fill the fragment
with. File offset can be suffixed with the following quantifiers:

`k'
`K'
     The number is expressed in kilobytes.

`m'
`M'
     The number is expressed in megabytes.

`g'
`G'
     The number is expressed in gigabytes.

   For each letter in contents string `genfile' will generate a "block"
of data, filled with this letter and will write it to the fragment. The
size of block is given by `--block-size' option. It defaults to 512.
Thus, if the string consists of N characters, the resulting file
fragment will contain `N*BLOCK-SIZE' of data.

   Last fragment descriptor can have only file offset part. In this
case `genfile' will create a hole at the end of the file up to the
given offset.

   For example, consider the following invocation:

     genfile --sparse --file sparsefile 0 ABCD 1M EFGHI 2000K

It will create 3101184-bytes long file of the following structure:

Offset                    Length         Contents
0                         4*512=2048     Four 512-byte blocks, filled
                                         with letters `A', `B', `C' and
                                         `D'.
2048                      1046528        Zero bytes
1050624                   5*512=2560     Five blocks, filled with letters
                                         `E', `F', `G', `H', `I'.
1053184                   2048000        Zero bytes

   The exit code of `genfile --status' command is `0' only if created
file is actually sparse.


File: tar.info,  Node: Status Mode,  Next: Exec Mode,  Prev: Generate Mode,  Up: Genfile

D.2 Status Mode
===============

In status mode, `genfile' prints file system status for each file
specified in the command line. This mode is toggled by `--stat' (`-S')
command line option. An optional argument to this option specifies
output "format": a comma-separated list of `struct stat' fields to be
displayed. This list can contain following identifiers :

name
     The file name.

dev
st_dev
     Device number in decimal.

ino
st_ino
     Inode number.

mode[.NUMBER]
st_mode[.NUMBER]
     File mode in octal.  Optional NUMBER specifies octal mask to be
     applied to the mode before outputting.  For example, `--stat
     mode.777' will preserve lower nine bits of it.  Notice, that you
     can use any punctuation character in place of `.'.

nlink
st_nlink
     Number of hard links.

uid
st_uid
     User ID of owner.

gid
st_gid
     Group ID of owner.

size
st_size
     File size in decimal.

blksize
st_blksize
     The size in bytes of each file block.

blocks
st_blocks
     Number of blocks allocated.

atime
st_atime
     Time of last access.

mtime
st_mtime
     Time of last modification

ctime
st_ctime
     Time of last status change

sparse
     A boolean value indicating whether the file is `sparse'.

   Modification times are displayed in UTC as UNIX timestamps, unless
suffixed with `H' (for "human-readable"), as in `ctimeH', in which case
usual `tar tv' output format is used.

   The default output format is: `name,dev,ino,mode,
nlink,uid,gid,size,blksize,blocks,atime,mtime,ctime'.

   For example, the following command will display file names and
corresponding times of last access for each file in the current working
directory:

     genfile --stat=name,atime *


File: tar.info,  Node: Exec Mode,  Prev: Status Mode,  Up: Genfile

D.3 Exec Mode
=============

This mode is designed for testing the behavior of `paxutils' commands
when some of the files change during archiving. It is an experimental
mode.

   The `Exec Mode' is toggled by `--run' command line option (or its
alias `-r'). The argument to this option gives the command line to be
executed. The actual command line is constructed by inserting
`--checkpoint' option between the command name and its first argument
(if any). Due to this, the argument to `--run' may not use traditional
`tar' option syntax, i.e., the following is wrong:

     # Wrong!
     genfile --run 'tar cf foo bar'

Use the following syntax instead:

     genfile --run 'tar -cf foo bar'

   The rest of command line after `--run' or its equivalent specifies
checkpoint values and actions to be executed upon reaching them.
Checkpoint values are introduced with `--checkpoint' command line
option. Argument to this option is the number of checkpoint in decimal.

   Any number of "actions" may be specified after a checkpoint.
Available actions are

`--cut FILE'
`--truncate FILE'
     Truncate FILE to the size specified by previous `--length' option
     (or 0, if it is not given).

`--append FILE'
     Append data to FILE. The size of data and its pattern are given by
     previous `--length' and `pattern' options.

`--touch FILE'
     Update the access and modification times of FILE. These timestamps
     are changed to the current time, unless `--date' option was given,
     in which case they are changed to the specified time. Argument to
     `--date' option is a date specification in an almost arbitrary
     format (*note Date input formats::).

`--exec COMMAND'
     Execute given shell command.


   Option `--verbose' instructs `genfile' to print on standard output
notifications about checkpoints being executed and to verbosely
describe exit status of the command.

   While the command is being executed its standard output remains
connected to descriptor 1. All messages it prints to file descriptor 2,
except checkpoint notifications, are forwarded to standard error.

   `Genfile' exits with the exit status of the executed command.


File: tar.info,  Node: Free Software Needs Free Documentation,  Next: Copying This Manual,  Prev: Genfile,  Up: Top

Appendix E Free Software Needs Free Documentation
*************************************************

The biggest deficiency in the free software community today is not in
the software--it is the lack of good free documentation that we can
include with the free software.  Many of our most important programs do
not come with free reference manuals and free introductory texts.
Documentation is an essential part of any software package; when an
important free software package does not come with a free manual and a
free tutorial, that is a major gap.  We have many such gaps today.

   Consider Perl, for instance.  The tutorial manuals that people
normally use are non-free.  How did this come about?  Because the
authors of those manuals published them with restrictive terms--no
copying, no modification, source files not available--which exclude
them from the free software world.

   That wasn't the first time this sort of thing happened, and it was
far from the last.  Many times we have heard a GNU user eagerly
describe a manual that he is writing, his intended contribution to the
community, only to learn that he had ruined everything by signing a
publication contract to make it non-free.

   Free documentation, like free software, is a matter of freedom, not
price.  The problem with the non-free manual is not that publishers
charge a price for printed copies--that in itself is fine.  (The Free
Software Foundation sells printed copies of manuals, too.)  The problem
is the restrictions on the use of the manual.  Free manuals are
available in source code form, and give you permission to copy and
modify.  Non-free manuals do not allow this.

   The criteria of freedom for a free manual are roughly the same as for
free software.  Redistribution (including the normal kinds of
commercial redistribution) must be permitted, so that the manual can
accompany every copy of the program, both on-line and on paper.

   Permission for modification of the technical content is crucial too.
When people modify the software, adding or changing features, if they
are conscientious they will change the manual too--so they can provide
accurate and clear documentation for the modified program.  A manual
that leaves you no choice but to write a new manual to document a
changed version of the program is not really available to our community.

   Some kinds of limits on the way modification is handled are
acceptable.  For example, requirements to preserve the original
author's copyright notice, the distribution terms, or the list of
authors, are ok.  It is also no problem to require modified versions to
include notice that they were modified.  Even entire sections that may
not be deleted or changed are acceptable, as long as they deal with
nontechnical topics (like this one).  These kinds of restrictions are
acceptable because they don't obstruct the community's normal use of
the manual.

   However, it must be possible to modify all the _technical_ content
of the manual, and then distribute the result in all the usual media,
through all the usual channels.  Otherwise, the restrictions obstruct
the use of the manual, it is not free, and we need another manual to
replace it.

   Please spread the word about this issue.  Our community continues to
lose manuals to proprietary publishing.  If we spread the word that
free software needs free reference manuals and free tutorials, perhaps
the next person who wants to contribute by writing documentation will
realize, before it is too late, that only free manuals contribute to
the free software community.

   If you are writing documentation, please insist on publishing it
under the GNU Free Documentation License or another free documentation
license.  Remember that this decision requires your approval--you don't
have to let the publisher decide.  Some commercial publishers will use
a free license if you insist, but they will not propose the option; it
is up to you to raise the issue and say firmly that this is what you
want.  If the publisher you are dealing with refuses, please try other
publishers.  If you're not sure whether a proposed license is free,
write to <licensing@gnu.org>.

   You can encourage commercial publishers to sell more free, copylefted
manuals and tutorials by buying them, and particularly by buying copies
from the publishers that paid for their writing or for major
improvements.  Meanwhile, try to avoid buying non-free documentation at
all.  Check the distribution terms of a manual before you buy it, and
insist that whoever seeks your business must respect your freedom.
Check the history of the book, and try reward the publishers that have
paid or pay the authors to work on it.

   The Free Software Foundation maintains a list of free documentation
published by other publishers, at
`http://www.fsf.org/doc/other-free-books.html'.


File: tar.info,  Node: Copying This Manual,  Next: Index of Command Line Options,  Prev: Free Software Needs Free Documentation,  Up: Top

Appendix F Copying This Manual
******************************

* Menu:

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


File: tar.info,  Node: GNU Free Documentation License,  Up: Copying This Manual

F.1 GNU Free Documentation License
==================================

                      Version 1.2, November 2002

     Copyright (C) 2000,2001,2002 Free Software Foundation, Inc.
     51 Franklin St, Fifth Floor, Boston, MA  02110-1301, USA

     Everyone is permitted to copy and distribute verbatim copies
     of this license document, but changing it is not allowed.

  0. PREAMBLE

     The purpose of this License is to make a manual, textbook, or other
     functional and useful document "free" in the sense of freedom: to
     assure everyone the effective freedom to copy and redistribute it,
     with or without modifying it, either commercially or
     noncommercially.  Secondarily, this License preserves for the
     author and publisher a way to get credit for their work, while not
     being considered responsible for modifications made by others.

     This License is a kind of "copyleft", which means that derivative
     works of the document must themselves be free in the same sense.
     It complements the GNU General Public License, which is a copyleft
     license designed for free software.

     We have designed this License in order to use it for manuals for
     free software, because free software needs free documentation: a
     free program should come with manuals providing the same freedoms
     that the software does.  But this License is not limited to
     software manuals; it can be used for any textual work, regardless
     of subject matter or whether it is published as a printed book.
     We recommend this License principally for works whose purpose is
     instruction or reference.

  1. APPLICABILITY AND DEFINITIONS

     This License applies to any manual or other work, in any medium,
     that contains a notice placed by the copyright holder saying it
     can be distributed under the terms of this License.  Such a notice
     grants a world-wide, royalty-free license, unlimited in duration,
     to use that work under the conditions stated herein.  The
     "Document", below, refers to any such manual or work.  Any member
     of the public is a licensee, and is addressed as "you".  You
     accept the license if you copy, modify or distribute the work in a
     way requiring permission under copyright law.

     A "Modified Version" of the Document means any work containing the
     Document or a portion of it, either copied verbatim, or with
     modifications and/or translated into another language.

     A "Secondary Section" is a named appendix or a front-matter section
     of the Document that deals exclusively with the relationship of the
     publishers or authors of the Document to the Document's overall
     subject (or to related matters) and contains nothing that could
     fall directly within that overall subject.  (Thus, if the Document
     is in part a textbook of mathematics, a Secondary Section may not
     explain any mathematics.)  The relationship could be a matter of
     historical connection with the subject or with related matters, or
     of legal, commercial, philosophical, ethical or political position
     regarding them.

     The "Invariant Sections" are certain Secondary Sections whose
     titles are designated, as being those of Invariant Sections, in
     the notice that says that the Document is released under this
     License.  If a section does not fit the above definition of
     Secondary then it is not allowed to be designated as Invariant.
     The Document may contain zero Invariant Sections.  If the Document
     does not identify any Invariant Sections then there are none.

     The "Cover Texts" are certain short passages of text that are
     listed, as Front-Cover Texts or Back-Cover Texts, in the notice
     that says that the Document is released under this License.  A
     Front-Cover Text may be at most 5 words, and a Back-Cover Text may
     be at most 25 words.

     A "Transparent" copy of the Document means a machine-readable copy,
     represented in a format whose specification is available to the
     general public, that is suitable for revising the document
     straightforwardly with generic text editors or (for images
     composed of pixels) generic paint programs or (for drawings) some
     widely available drawing editor, and that is suitable for input to
     text formatters or for automatic translation to a variety of
     formats suitable for input to text formatters.  A copy made in an
     otherwise Transparent file format whose markup, or absence of
     markup, has been arranged to thwart or discourage subsequent
     modification by readers is not Transparent.  An image format is
     not Transparent if used for any substantial amount of text.  A
     copy that is not "Transparent" is called "Opaque".

     Examples of suitable formats for Transparent copies include plain
     ASCII without markup, Texinfo input format, LaTeX input format,
     SGML or XML using a publicly available DTD, and
     standard-conforming simple HTML, PostScript or PDF designed for
     human modification.  Examples of transparent image formats include
     PNG, XCF and JPG.  Opaque formats include proprietary formats that
     can be read and edited only by proprietary word processors, SGML or
     XML for which the DTD and/or processing tools are not generally
     available, and the machine-generated HTML, PostScript or PDF
     produced by some word processors for output purposes only.

     The "Title Page" means, for a printed book, the title page itself,
     plus such following pages as are needed to hold, legibly, the
     material this License requires to appear in the title page.  For
     works in formats which do not have any title page as such, "Title
     Page" means the text near the most prominent appearance of the
     work's title, preceding the beginning of the body of the text.

     A section "Entitled XYZ" means a named subunit of the Document
     whose title either is precisely XYZ or contains XYZ in parentheses
     following text that translates XYZ in another language.  (Here XYZ
     stands for a specific section name mentioned below, such as
     "Acknowledgements", "Dedications", "Endorsements", or "History".)
     To "Preserve the Title" of such a section when you modify the
     Document means that it remains a section "Entitled XYZ" according
     to this definition.

     The Document may include Warranty Disclaimers next to the notice
     which states that this License applies to the Document.  These
     Warranty Disclaimers are considered to be included by reference in
     this License, but only as regards disclaiming warranties: any other
     implication that these Warranty Disclaimers may have is void and
     has no effect on the meaning of this License.

  2. VERBATIM COPYING

     You may copy and distribute the Document in any medium, either
     commercially or noncommercially, provided that this License, the
     copyright notices, and the license notice saying this License
     applies to the Document are reproduced in all copies, and that you
     add no other conditions whatsoever to those of this License.  You
     may not use technical measures to obstruct or control the reading
     or further copying of the copies you make or distribute.  However,
     you may accept compensation in exchange for copies.  If you
     distribute a large enough number of copies you must also follow
     the conditions in section 3.

     You may also lend copies, under the same conditions stated above,
     and you may publicly display copies.

  3. COPYING IN QUANTITY

     If you publish printed copies (or copies in media that commonly
     have printed covers) of the Document, numbering more than 100, and
     the Document's license notice requires Cover Texts, you must
     enclose the copies in covers that carry, clearly and legibly, all
     these Cover Texts: Front-Cover Texts on the front cover, and
     Back-Cover Texts on the back cover.  Both covers must also clearly
     and legibly identify you as the publisher of these copies.  The
     front cover must present the full title with all words of the
     title equally prominent and visible.  You may add other material
     on the covers in addition.  Copying with changes limited to the
     covers, as long as they preserve the title of the Document and
     satisfy these conditions, can be treated as verbatim copying in
     other respects.

     If the required texts for either cover are too voluminous to fit
     legibly, you should put the first ones listed (as many as fit
     reasonably) on the actual cover, and continue the rest onto
     adjacent pages.

     If you publish or distribute Opaque copies of the Document
     numbering more than 100, you must either include a
     machine-readable Transparent copy along with each Opaque copy, or
     state in or with each Opaque copy a computer-network location from
     which the general network-using public has access to download
     using public-standard network protocols a complete Transparent
     copy of the Document, free of added material.  If you use the
     latter option, you must take reasonably prudent steps, when you
     begin distribution of Opaque copies in quantity, to ensure that
     this Transparent copy will remain thus accessible at the stated
     location until at least one year after the last time you
     distribute an Opaque copy (directly or through your agents or
     retailers) of that edition to the public.

     It is requested, but not required, that you contact the authors of
     the Document well before redistributing any large number of
     copies, to give them a chance to provide you with an updated
     version of the Document.

  4. MODIFICATIONS

     You may copy and distribute a Modified Version of the Document
     under the conditions of sections 2 and 3 above, provided that you
     release the Modified Version under precisely this License, with
     the Modified Version filling the role of the Document, thus
     licensing distribution and modification of the Modified Version to
     whoever possesses a copy of it.  In addition, you must do these
     things in the Modified Version:

       A. Use in the Title Page (and on the covers, if any) a title
          distinct from that of the Document, and from those of
          previous versions (which should, if there were any, be listed
          in the History section of the Document).  You may use the
          same title as a previous version if the original publisher of
          that version gives permission.

       B. List on the Title Page, as authors, one or more persons or
          entities responsible for authorship of the modifications in
          the Modified Version, together with at least five of the
          principal authors of the Document (all of its principal
          authors, if it has fewer than five), unless they release you
          from this requirement.

       C. State on the Title page the name of the publisher of the
          Modified Version, as the publisher.

       D. Preserve all the copyright notices of the Document.

       E. Add an appropriate copyright notice for your modifications
          adjacent to the other copyright notices.

       F. Include, immediately after the copyright notices, a license
          notice giving the public permission to use the Modified
          Version under the terms of this License, in the form shown in
          the Addendum below.

       G. Preserve in that license notice the full lists of Invariant
          Sections and required Cover Texts given in the Document's
          license notice.

       H. Include an unaltered copy of this License.

       I. Preserve the section Entitled "History", Preserve its Title,
          and add to it an item stating at least the title, year, new
          authors, and publisher of the Modified Version as given on
          the Title Page.  If there is no section Entitled "History" in
          the Document, create one stating the title, year, authors,
          and publisher of the Document as given on its Title Page,
          then add an item describing the Modified Version as stated in
          the previous sentence.

       J. Preserve the network location, if any, given in the Document
          for public access to a Transparent copy of the Document, and
          likewise the network locations given in the Document for
          previous versions it was based on.  These may be placed in
          the "History" section.  You may omit a network location for a
          work that was published at least four years before the
          Document itself, or if the original publisher of the version
          it refers to gives permission.

       K. For any section Entitled "Acknowledgements" or "Dedications",
          Preserve the Title of the section, and preserve in the
          section all the substance and tone of each of the contributor
          acknowledgements and/or dedications given therein.

       L. Preserve all the Invariant Sections of the Document,
          unaltered in their text and in their titles.  Section numbers
          or the equivalent are not considered part of the section
          titles.

       M. Delete any section Entitled "Endorsements".  Such a section
          may not be included in the Modified Version.

       N. Do not retitle any existing section to be Entitled
          "Endorsements" or to conflict in title with any Invariant
          Section.

       O. Preserve any Warranty Disclaimers.

     If the Modified Version includes new front-matter sections or
     appendices that qualify as Secondary Sections and contain no
     material copied from the Document, you may at your option
     designate some or all of these sections as invariant.  To do this,
     add their titles to the list of Invariant Sections in the Modified
     Version's license notice.  These titles must be distinct from any
     other section titles.

     You may add a section Entitled "Endorsements", provided it contains
     nothing but endorsements of your Modified Version by various
     parties--for example, statements of peer review or that the text
     has been approved by an organization as the authoritative
     definition of a standard.

     You may add a passage of up to five words as a Front-Cover Text,
     and a passage of up to 25 words as a Back-Cover Text, to the end
     of the list of Cover Texts in the Modified Version.  Only one
     passage of Front-Cover Text and one of Back-Cover Text may be
     added by (or through arrangements made by) any one entity.  If the
     Document already includes a cover text for the same cover,
     previously added by you or by arrangement made by the same entity
     you are acting on behalf of, you may not add another; but you may
     replace the old one, on explicit permission from the previous
     publisher that added the old one.

     The author(s) and publisher(s) of the Document do not by this
     License give permission to use their names for publicity for or to
     assert or imply endorsement of any Modified Version.

  5. COMBINING DOCUMENTS

     You may combine the Document with other documents released under
     this License, under the terms defined in section 4 above for
     modified versions, provided that you include in the combination
     all of the Invariant Sections of all of the original documents,
     unmodified, and list them all as Invariant Sections of your
     combined work in its license notice, and that you preserve all
     their Warranty Disclaimers.

     The combined work need only contain one copy of this License, and
     multiple identical Invariant Sections may be replaced with a single
     copy.  If there are multiple Invariant Sections with the same name
     but different contents, make the title of each such section unique
     by adding at the end of it, in parentheses, the name of the
     original author or publisher of that section if known, or else a
     unique number.  Make the same adjustment to the section titles in
     the list of Invariant Sections in the license notice of the
     combined work.

     In the combination, you must combine any sections Entitled
     "History" in the various original documents, forming one section
     Entitled "History"; likewise combine any sections Entitled
     "Acknowledgements", and any sections Entitled "Dedications".  You
     must delete all sections Entitled "Endorsements."

  6. COLLECTIONS OF DOCUMENTS

     You may make a collection consisting of the Document and other
     documents released under this License, and replace the individual
     copies of this License in the various documents with a single copy
     that is included in the collection, provided that you follow the
     rules of this License for verbatim copying of each of the
     documents in all other respects.

     You may extract a single document from such a collection, and
     distribute it individually under this License, provided you insert
     a copy of this License into the extracted document, and follow
     this License in all other respects regarding verbatim copying of
     that document.

  7. AGGREGATION WITH INDEPENDENT WORKS

     A compilation of the Document or its derivatives with other
     separate and independent documents or works, in or on a volume of
     a storage or distribution medium, is called an "aggregate" if the
     copyright resulting from the compilation is not used to limit the
     legal rights of the compilation's users beyond what the individual
     works permit.  When the Document is included in an aggregate, this
     License does not apply to the other works in the aggregate which
     are not themselves derivative works of the Document.

     If the Cover Text requirement of section 3 is applicable to these
     copies of the Document, then if the Document is less than one half
     of the entire aggregate, the Document's Cover Texts may be placed
     on covers that bracket the Document within the aggregate, or the
     electronic equivalent of covers if the Document is in electronic
     form.  Otherwise they must appear on printed covers that bracket
     the whole aggregate.

  8. TRANSLATION

     Translation is considered a kind of modification, so you may
     distribute translations of the Document under the terms of section
     4.  Replacing Invariant Sections with translations requires special
     permission from their copyright holders, but you may include
     translations of some or all Invariant Sections in addition to the
     original versions of these Invariant Sections.  You may include a
     translation of this License, and all the license notices in the
     Document, and any Warranty Disclaimers, provided that you also
     include the original English version of this License and the
     original versions of those notices and disclaimers.  In case of a
     disagreement between the translation and the original version of
     this License or a notice or disclaimer, the original version will
     prevail.

     If a section in the Document is Entitled "Acknowledgements",
     "Dedications", or "History", the requirement (section 4) to
     Preserve its Title (section 1) will typically require changing the
     actual title.

  9. TERMINATION

     You may not copy, modify, sublicense, or distribute the Document
     except as expressly provided for under this License.  Any other
     attempt to copy, modify, sublicense or distribute the Document is
     void, and will automatically terminate your rights under this
     License.  However, parties who have received copies, or rights,
     from you under this License will not have their licenses
     terminated so long as such parties remain in full compliance.

 10. FUTURE REVISIONS OF THIS LICENSE

     The Free Software Foundation may publish new, revised versions of
     the GNU Free Documentation License from time to time.  Such new
     versions will be similar in spirit to the present version, but may
     differ in detail to address new problems or concerns.  See
     `http://www.gnu.org/copyleft/'.

     Each version of the License is given a distinguishing version
     number.  If the Document specifies that a particular numbered
     version of this License "or any later version" applies to it, you
     have the option of following the terms and conditions either of
     that specified version or of any later version that has been
     published (not as a draft) by the Free Software Foundation.  If
     the Document does not specify a version number of this License,
     you may choose any version ever published (not as a draft) by the
     Free Software Foundation.

F.1.1 ADDENDUM: How to use this License for your documents
----------------------------------------------------------

To use this License in a document you have written, include a copy of
the License in the document and put the following copyright and license
notices just after the title page:

       Copyright (C)  YEAR  YOUR NAME.
       Permission is granted to copy, distribute and/or modify this document
       under the terms of the GNU Free Documentation License, Version 1.2
       or any later version published by the Free Software Foundation;
       with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
       Texts.  A copy of the license is included in the section entitled ``GNU
       Free Documentation License''.

   If you have Invariant Sections, Front-Cover Texts and Back-Cover
Texts, replace the "with...Texts." line with this:

         with the Invariant Sections being LIST THEIR TITLES, with
         the Front-Cover Texts being LIST, and with the Back-Cover Texts
         being LIST.

   If you have Invariant Sections without Cover Texts, or some other
combination of the three, merge those two alternatives to suit the
situation.

   If your document contains nontrivial examples of program code, we
recommend releasing these examples in parallel under your choice of
free software license, such as the GNU General Public License, to
permit their use in free software.


File: tar.info,  Node: Index of Command Line Options,  Next: Index,  Prev: Copying This Manual,  Up: Top

Appendix G Index of Command Line Options
****************************************

This appendix contains an index of all GNU `tar' long command line
options. The options are listed without the preceding double-dash.  For
a cross-reference of short command line options, *note Short Option
Summary::.