This is gnutar.info, produced by makeinfo version 7.2 from tar.texi.

This manual is for GNU ‘tar’ (version 1.35, 11 October 2025), which
creates and extracts files from archives.

   Copyright © 1992, 1994-1997, 1999-2001, 2003-2017, 2021-2023 Free
Software Foundation, Inc.

     Permission is granted to copy, distribute and/or modify this
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     the Back-Cover Texts as in (a) below.  A copy of the license is
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     (a) The FSF's Back-Cover Text is: "You have the freedom to copy and
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INFO-DIR-SECTION Archiving
START-INFO-DIR-ENTRY
* Tar: (gnutar).                   Making tape (or disk) archives.
END-INFO-DIR-ENTRY

INFO-DIR-SECTION Individual utilities
START-INFO-DIR-ENTRY
* tar: (gnutar)tar invocation.                     Invoking GNU ‘tar’.
END-INFO-DIR-ENTRY


File: gnutar.info,  Node: transform,  Next: after,  Prev: quoting styles,  Up: Choosing

6.7 Modifying File and Member Names
===================================

‘Tar’ archives contain detailed information about files stored in them
and full file names are part of that information.  When storing a file
to an archive, its file name is recorded in it, along with the actual
file contents.  When restoring from an archive, a file is created on
disk with exactly the same name as that stored in the archive.  In the
majority of cases this is the desired behavior of a file archiver.
However, there are some cases when it is not.

   First of all, it is often unsafe to extract archive members with
absolute file names or those that begin with a ‘../’.  GNU ‘tar’ takes
special precautions when extracting such names and provides a special
option for handling them, which is described in *note absolute::.

   Secondly, you may wish to extract file names without some leading
directory components, or with otherwise modified names.  In other cases
it is desirable to store files under differing names in the archive.

   GNU ‘tar’ provides several options for these needs.

‘--strip-components=NUMBER’
     Strip given NUMBER of leading components from file names before
     extraction.

   For example, suppose you have archived whole ‘/usr’ hierarchy to a
tar archive named ‘usr.tar’.  Among other files, this archive contains
‘usr/include/stdlib.h’, which you wish to extract to the current working
directory.  To do so, you type:

     $ tar -xf usr.tar --strip=2 usr/include/stdlib.h

   The option ‘--strip=2’ instructs ‘tar’ to strip the two leading
components (‘usr/’ and ‘include/’) off the file name.

   If you add the ‘--verbose’ (‘-v’) option to the invocation above, you
will note that the verbose listing still contains the full file name,
with the two removed components still in place.  This can be
inconvenient, so ‘tar’ provides a special option for altering this
behavior:

‘--show-transformed-names’
     Display file or member names with all requested transformations
     applied.

For example:

     $ tar -xf usr.tar -v --strip=2 usr/include/stdlib.h
     usr/include/stdlib.h
     $ tar -xf usr.tar -v --strip=2 --show-transformed usr/include/stdlib.h
     stdlib.h

   Notice that in both cases the file ‘stdlib.h’ is extracted to the
current working directory, ‘--show-transformed-names’ affects only the
way its name is displayed.

   This option is especially useful for verifying whether the invocation
will have the desired effect.  Thus, before running

     $ tar -x --strip=N

it is often advisable to run

     $ tar -t -v --show-transformed --strip=N

to make sure the command will produce the intended results.

   In case you need to apply more complex modifications to the file
name, GNU ‘tar’ provides a general-purpose transformation option:

‘--transform=EXPRESSION’
‘--xform=EXPRESSION’
     Modify file names using supplied EXPRESSION.

The EXPRESSION is a ‘sed’-like replace expression of the form:

     s/REGEXP/REPLACE/[FLAGS]

where REGEXP is a “regular expression”, REPLACE is a replacement for
each file name part that matches REGEXP.  Both REGEXP and REPLACE are
described in detail in *note The "s" Command: (sed)The "s" Command.

   Any delimiter can be used in lieu of ‘/’, the only requirement being
that it be used consistently throughout the expression.  For example,
the following two expressions are equivalent:

     s/one/two/
     s,one,two,

   Changing delimiters is often useful when the REGEX contains slashes.
For example, it is more convenient to write ‘s,/,-,’ than ‘s/\//-/’.

   As in ‘sed’, you can give several replace expressions, separated by a
semicolon.

   Supported FLAGS are:

‘g’
     Apply the replacement to _all_ matches to the REGEXP, not just the
     first.

‘i’
     Use case-insensitive matching.

‘x’
     REGEXP is an “extended regular expression” (*note Extended regular
     expressions: (sed)Extended regexps.).

‘NUMBER’
     Only replace the NUMBERth match of the REGEXP.

     Note: the POSIX standard does not specify what should happen when
     you mix the ‘g’ and NUMBER modifiers.  GNU ‘tar’ follows the GNU
     ‘sed’ implementation in this regard, so the interaction is defined
     to be: ignore matches before the NUMBERth, and then match and
     replace all matches from the NUMBERth on.

   In addition, several “transformation scope” flags are supported, that
control to what files transformations apply.  These are:

‘r’
     Apply transformation to regular archive members.

‘R’
     Do not apply transformation to regular archive members.

‘s’
     Apply transformation to symbolic link targets.

‘S’
     Do not apply transformation to symbolic link targets.

‘h’
     Apply transformation to hard link targets.

‘H’
     Do not apply transformation to hard link targets.

   Default is ‘rsh’, which means to apply transformations to both
archive members and targets of symbolic and hard links.

   Default scope flags can also be changed using ‘flags=’ statement in
the transform expression.  The flags set this way remain in force until
next ‘flags=’ statement or end of expression, whichever occurs first.
For example:

       --transform 'flags=S;s|^|/usr/local/|'

   Here are several examples of ‘--transform’ usage:

  1. Extract ‘usr/’ hierarchy into ‘usr/local/’:

          $ tar --transform='s,usr/,usr/local/,' -x -f arch.tar

  2. Strip two leading directory components (equivalent to
     ‘--strip-components=2’):

          $ tar --transform='s,/*[^/]*/[^/]*/,,' -x -f arch.tar

  3. Convert each file name to lower case:

          $ tar --transform 's/.*/\L&/' -x -f arch.tar

  4. Prepend ‘/prefix/’ to each file name:

          $ tar --transform 's,^,/prefix/,' -x -f arch.tar

  5. Archive the ‘/lib’ directory, prepending ‘/usr/local’ to each
     archive member:

          $ tar --transform 's,^,/usr/local/,S' -c -f arch.tar /lib

   Notice the use of flags in the last example.  The ‘/lib’ directory
often contains many symbolic links to files within it.  It may look, for
example, like this:

     $ ls -l
     drwxr-xr-x root/root       0 2008-07-08 16:20 /lib/
     -rwxr-xr-x root/root 1250840 2008-05-25 07:44 /lib/libc-2.3.2.so
     lrwxrwxrwx root/root       0 2008-06-24 17:12 /lib/libc.so.6 -> libc-2.3.2.so
     ...

   Using the expression ‘s,^,/usr/local/,’ would mean adding
‘/usr/local’ to both regular archive members and to link targets.  In
this case, ‘/lib/libc.so.6’ would become:

       /usr/local/lib/libc.so.6 -> /usr/local/libc-2.3.2.so

   This is definitely not desired.  To avoid this, the ‘S’ flag is used,
which excludes symbolic link targets from filename transformations.  The
result is:

     $ tar --transform 's,^,/usr/local/,S' -c -v -f arch.tar \
            --show-transformed /lib
     drwxr-xr-x root/root       0 2008-07-08 16:20 /usr/local/lib/
     -rwxr-xr-x root/root 1250840 2008-05-25 07:44 /usr/local/lib/libc-2.3.2.so
     lrwxrwxrwx root/root       0 2008-06-24 17:12 /usr/local/lib/libc.so.6 \
      -> libc-2.3.2.so

   Unlike ‘--strip-components’, ‘--transform’ can be used in any GNU
‘tar’ operation mode.  For example, the following command adds files to
the archive while replacing the leading ‘usr/’ component with ‘var/’:

     $ tar -cf arch.tar --transform='s,^usr/,var/,' /

   To test ‘--transform’ effect we suggest using
‘--show-transformed-names’ option:

     $ tar -cf arch.tar --transform='s,^usr/,var/,' \
            --verbose --show-transformed-names /

   If both ‘--strip-components’ and ‘--transform’ are used together,
then ‘--transform’ is applied first, and the required number of
components is then stripped from its result.

   You can use as many ‘--transform’ options in a single command line as
you want.  The specified expressions will then be applied in order of
their appearance.  For example, the following two invocations are
equivalent:

     $ tar -cf arch.tar --transform='s,/usr/var,/var/' \
                             --transform='s,/usr/local,/usr/,'
     $ tar -cf arch.tar \
                    --transform='s,/usr/var,/var/;s,/usr/local,/usr/,'


File: gnutar.info,  Node: after,  Next: recurse,  Prev: transform,  Up: Choosing

6.8 Operating Only on New Files
===============================

The ‘--after-date=DATE’ (‘--newer=DATE’, ‘-N DATE’) option causes ‘tar’
to only work on files whose data modification or status change times are
newer than the DATE given.  If DATE starts with ‘/’ or ‘.’, it is taken
to be a file name; the data modification time of that file is used as
the date.  If you use this option when creating or appending to an
archive, the archive will only include new files.  If you use
‘--after-date’ when extracting an archive, ‘tar’ will only extract files
newer than the DATE you specify.

   If you only want ‘tar’ to make the date comparison based on
modification of the file's data (rather than status changes), then use
the ‘--newer-mtime=DATE’ option.

   You may use these options with any operation.  Note that these
options differ from the ‘--update’ (‘-u’) operation in that they allow
you to specify a particular date against which ‘tar’ can compare when
deciding whether or not to archive the files.

‘--after-date=DATE’
‘--newer=DATE’
‘-N DATE’
     Only store files newer than DATE.

     Acts on files only if their data modification or status change
     times are later than DATE.  Use in conjunction with any operation.

     If DATE starts with ‘/’ or ‘.’, it is taken to be a file name; the
     data modification time of that file is used as the date.

‘--newer-mtime=DATE’
     Acts like ‘--after-date’, but only looks at data modification
     times.

   These options limit ‘tar’ to operate only on files which have been
modified after the date specified.  A file's status is considered to
have changed if its contents have been modified, or if its owner,
permissions, and so forth, have been changed.  (For more information on
how to specify a date, see *note Date input formats::; remember that the
entire date argument must be quoted if it contains any spaces.)

   Gurus would say that ‘--after-date’ tests both the data modification
time (‘mtime’, the time the contents of the file were last modified) and
the status change time (‘ctime’, the time the file's status was last
changed: owner, permissions, etc.) fields, while ‘--newer-mtime’ tests
only the ‘mtime’ field.

   To be precise, ‘--after-date’ checks _both_ ‘mtime’ and ‘ctime’ and
processes the file if either one is more recent than DATE, while
‘--newer-mtime’ only checks ‘mtime’ and disregards ‘ctime’.  Neither
does it use ‘atime’ (the last time the contents of the file were looked
at).

   Date specifiers can have embedded spaces.  Because of this, you may
need to quote date arguments to keep the shell from parsing them as
separate arguments.  For example, the following command will add to the
archive all the files modified less than two days ago:

     $ tar -cf foo.tar --newer-mtime '2 days ago'

   When any of these options is used with the option ‘--verbose’ (*note
verbose tutorial::) GNU ‘tar’ will try to convert the specified date
back to its textual representation and compare that with the one given
with the option.  If the two dates differ, ‘tar’ will print a warning
saying what date it will use.  This is to help user ensure he is using
the right date.  For example:

     $ tar -c -f archive.tar --after-date='10 days ago' .
     tar: Option --after-date: Treating date '10 days ago' as 2006-06-11
     13:19:37.232434

     *Please Note:* ‘--after-date’ and ‘--newer-mtime’ should not be
     used for incremental backups.  *Note Incremental Dumps::, for
     proper way of creating incremental backups.


File: gnutar.info,  Node: recurse,  Next: one,  Prev: after,  Up: Choosing

6.9 Descending into Directories
===============================

Usually, ‘tar’ will recursively explore all directories (either those
given on the command line or through the ‘--files-from’ option) for the
various files they contain.  However, you may not always want ‘tar’ to
act this way.

   The ‘--no-recursion’ option inhibits ‘tar’'s recursive descent into
specified directories.  If you specify ‘--no-recursion’, you can use the
‘find’ (*note find: (find)Top.) utility for hunting through levels of
directories to construct a list of file names which you could then pass
to ‘tar’.  ‘find’ allows you to be more selective when choosing which
files to archive; see *note files::, for more information on using
‘find’ with ‘tar’.

‘--no-recursion’
     Prevents ‘tar’ from recursively descending directories.

‘--recursion’
     Requires ‘tar’ to recursively descend directories.  This is the
     default.

   When you use ‘--no-recursion’, GNU ‘tar’ grabs directory entries
themselves, but does not descend on them recursively.  Many people use
‘find’ for locating files they want to back up, and since ‘tar’
_usually_ recursively descends on directories, they have to use the
‘-not -type d’ test in their ‘find’ invocation (*note Type:
(find)Type.), as they usually do not want all the files in a directory.
They then use the ‘--files-from’ option to archive the files located via
‘find’.

   The problem when restoring files archived in this manner is that the
directories themselves are not in the archive; so the
‘--same-permissions’ (‘--preserve-permissions’, ‘-p’) option does not
affect them--while users might really like it to.  Specifying
‘--no-recursion’ is a way to tell ‘tar’ to grab only the directory
entries given to it, adding no new files on its own.  To summarize, if
you use ‘find’ to create a list of files to be stored in an archive, use
it as follows:

     $ find DIR TESTS | \
       tar -cf ARCHIVE --no-recursion -T -

   The ‘--no-recursion’ option also applies when extracting: it causes
‘tar’ to extract only the matched directory entries, not the files under
those directories.

   The ‘--no-recursion’ option also affects how globbing patterns are
interpreted (*note controlling pattern-matching::).

   The ‘--no-recursion’ and ‘--recursion’ options apply to later options
and operands, and can be overridden by later occurrences of
‘--no-recursion’ and ‘--recursion’.  For example:

     $ tar -cf jams.tar --no-recursion grape --recursion grape/concord

creates an archive with one entry for ‘grape’, and the recursive
contents of ‘grape/concord’, but no entries under ‘grape’ other than
‘grape/concord’.


File: gnutar.info,  Node: one,  Prev: recurse,  Up: Choosing

6.10 Crossing File System Boundaries
====================================

‘tar’ will normally automatically cross file system boundaries in order
to archive files which are part of a directory tree.  You can change
this behavior by running ‘tar’ and specifying ‘--one-file-system’.  This
option only affects files that are archived because they are in a
directory that is being archived; ‘tar’ will still archive files
explicitly named on the command line or through ‘--files-from’,
regardless of where they reside.

‘--one-file-system’
     Prevents ‘tar’ from crossing file system boundaries when archiving.
     Use in conjunction with any write operation.

   The ‘--one-file-system’ option causes ‘tar’ to modify its normal
behavior in archiving the contents of directories.  If a file in a
directory is not on the same file system as the directory itself, then
‘tar’ will not archive that file.  If the file is a directory itself,
‘tar’ will not archive anything beneath it; in other words, ‘tar’ will
not cross mount points.

   This option is useful for making full or incremental archival backups
of a file system.  If this option is used in conjunction with
‘--verbose’ (‘-v’), files that are excluded are mentioned by name on the
standard error.

* Menu:

* directory::                   Changing Directory
* absolute::                    Absolute File Names


File: gnutar.info,  Node: directory,  Next: absolute,  Up: one

6.10.1 Changing the Working Directory
-------------------------------------

To change the working directory in the middle of a list of file names,
either on the command line or in a file specified using ‘--files-from’
(‘-T’), use ‘--directory’ (‘-C’).  This will change the working
directory to the specified directory after that point in the list.

‘--directory=DIRECTORY’
‘-C DIRECTORY’
     Changes the working directory in the middle of a command line.

   For example,

     $ tar -c -f jams.tar grape prune -C food cherry

will place the files ‘grape’ and ‘prune’ from the current directory into
the archive ‘jams.tar’, followed by the file ‘cherry’ from the directory
‘food’.  This option is especially useful when you have several widely
separated files that you want to store in the same archive.

   Note that the file ‘cherry’ is recorded in the archive under the
precise name ‘cherry’, _not_ ‘food/cherry’.  Thus, the archive will
contain three files that all appear to have come from the same
directory; if the archive is extracted with plain ‘tar --extract’, all
three files will be written in the current directory.

   Contrast this with the command,

     $ tar -c -f jams.tar grape prune -C food red/cherry

which records the third file in the archive under the name ‘red/cherry’
so that, if the archive is extracted using ‘tar --extract’, the third
file will be written in a subdirectory named ‘red’.

   You can use the ‘--directory’ option to make the archive independent
of the original name of the directory holding the files.  The following
command places the files ‘/etc/passwd’, ‘/etc/hosts’, and ‘/lib/libc.a’
into the archive ‘foo.tar’:

     $ tar -c -f foo.tar -C /etc passwd hosts -C /lib libc.a

However, the names of the archive members will be exactly what they were
on the command line: ‘passwd’, ‘hosts’, and ‘libc.a’.  They will not
appear to be related by file name to the original directories where
those files were located.

   Note that ‘--directory’ options are interpreted consecutively.  If
‘--directory’ specifies a relative file name, it is interpreted relative
to the then current directory, which might not be the same as the
original current working directory of ‘tar’, due to a previous
‘--directory’ option.

   When using ‘--files-from’ (*note files::), you can put various ‘tar’
options (including ‘-C’) in the file list.  Notice, however, that in
this case the option and its argument may not be separated by
whitespace.  If you use short option, its argument must either follow
the option letter immediately, without any intervening whitespace, or
occupy the next line.  Otherwise, if you use long option, separate its
argument by an equal sign.

   For instance, the file list for the above example will be:

     -C/etc
     passwd
     hosts
     --directory=/lib
     libc.a

To use it, you would invoke ‘tar’ as follows:

     $ tar -c -f foo.tar --files-from list

   The interpretation of options in file lists is disabled by
‘--verbatim-files-from’ and ‘--null’ options.


File: gnutar.info,  Node: absolute,  Prev: directory,  Up: one

6.10.2 Absolute File Names
--------------------------

By default, GNU ‘tar’ drops a leading ‘/’ on input or output, and
complains about file names containing a ‘..’ component.  There is an
option that turns off this behavior:

‘--absolute-names’
‘-P’
     Do not strip leading slashes from file names, and permit file names
     containing a ‘..’ file name component.

   When ‘tar’ extracts archive members from an archive, it strips any
leading slashes (‘/’) from the member name.  This causes absolute member
names in the archive to be treated as relative file names.  This allows
you to have such members extracted wherever you want, instead of being
restricted to extracting the member in the exact directory named in the
archive.  For example, if the archive member has the name ‘/etc/passwd’,
‘tar’ will extract it as if the name were really ‘etc/passwd’.

   File names containing ‘..’ can cause problems when extracting, so
‘tar’ normally warns you about such files when creating an archive, and
rejects attempts to extracts such files.

   Other ‘tar’ programs do not do this.  As a result, if you create an
archive whose member names start with a slash, they will be difficult
for other people with a non-GNU ‘tar’ program to use.  Therefore, GNU
‘tar’ also strips leading slashes from member names when putting members
into the archive.  For example, if you ask ‘tar’ to add the file
‘/bin/ls’ to an archive, it will do so, but the member name will be
‘bin/ls’(1).

   Symbolic links containing ‘..’ or leading ‘/’ can also cause problems
when extracting, so ‘tar’ normally extracts them last; it may create
empty files as placeholders during extraction.

   If you use the ‘--absolute-names’ (‘-P’) option, ‘tar’ will do none
of these transformations.

   To archive or extract files relative to the root directory, specify
the ‘--absolute-names’ (‘-P’) option.

   Normally, ‘tar’ acts on files relative to the working
directory--ignoring superior directory names when archiving, and
ignoring leading slashes when extracting.

   When you specify ‘--absolute-names’ (‘-P’), ‘tar’ stores file names
including all superior directory names, and preserves leading slashes.
If you only invoked ‘tar’ from the root directory you would never need
the ‘--absolute-names’ option, but using this option may be more
convenient than switching to root.

‘--absolute-names’
     Preserves full file names (including superior directory names) when
     archiving and extracting files.

   ‘tar’ prints out a message about removing the ‘/’ from file names.
This message appears once per GNU ‘tar’ invocation.  It represents
something which ought to be told; ignoring what it means can cause very
serious surprises, later.

   Some people, nevertheless, do not want to see this message.  Wanting
to play really dangerously, one may of course redirect ‘tar’ standard
error to the sink.  For example, under ‘sh’:

     $ tar -c -f archive.tar /home 2> /dev/null

Another solution, both nicer and simpler, would be to change to the ‘/’
directory first, and then avoid absolute notation.  For example:

     $ tar -c -f archive.tar -C / home

   *Note Integrity::, for some of the security-related implications of
using this option.

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

   (1) A side effect of this is that when ‘--create’ is used with
‘--verbose’ the resulting output is not, generally speaking, the same as
the one you'd get running ‘tar --list’ command.  This may be important
if you use some scripts for comparing both outputs.  *Note listing
member and file names::, for the information on how to handle this case.


File: gnutar.info,  Node: Date input formats,  Next: Formats,  Prev: Choosing,  Up: Top

7 Date input formats
********************

First, a quote:

     Our units of temporal measurement, from seconds on up to months,
     are so complicated, asymmetrical and disjunctive so as to make
     coherent mental reckoning in time all but impossible.  Indeed, had
     some tyrannical god contrived to enslave our minds to time, to make
     it all but impossible for us to escape subjection to sodden
     routines and unpleasant surprises, he could hardly have done better
     than handing down our present system.  It is like a set of
     trapezoidal building blocks, with no vertical or horizontal
     surfaces, like a language in which the simplest thought demands
     ornate constructions, useless particles and lengthy
     circumlocutions.  Unlike the more successful patterns of language
     and science, which enable us to face experience boldly or at least
     level-headedly, our system of temporal calculation silently and
     persistently encourages our terror of time.

     ... It is as though architects had to measure length in feet, width
     in meters and height in ells; as though basic instruction manuals
     demanded a knowledge of five different languages.  It is no wonder
     then that we often look into our own immediate past or future, last
     Tuesday or a week from Sunday, with feelings of helpless confusion.
     ...

     --Robert Grudin, ‘Time and the Art of Living’.

   This section describes the textual date representations that GNU
programs accept.  These are the strings you, as a user, can supply as
arguments to the various programs.  The C interface (via the
‘parse_datetime’ function) is not described here.

* Menu:

* General date syntax::          Common rules
* Calendar date items::          ‘14 Nov 2022’
* Time of day items::            ‘9:02pm’
* Time zone items::              ‘UTC’, ‘-0700’, ‘+0900’, ...
* Combined date and time of day items:: ‘2022-11-14T21:02:42,000000-0500’
* Day of week items::            ‘Monday’ and others
* Relative items in date strings:: ‘next tuesday, 2 years ago’
* Pure numbers in date strings:: ‘20221114’, ‘2102’
* Seconds since the Epoch::      ‘@1668477762’
* Specifying time zone rules::   ‘TZ="America/New_York"’, ‘TZ="UTC0"’
* Authors of parse_datetime::    Bellovin, Eggert, Salz, Berets, et al.


File: gnutar.info,  Node: General date syntax,  Next: Calendar date items,  Up: Date input formats

7.1 General date syntax
=======================

A “date” is a string, possibly empty, containing many items separated by
whitespace.  The whitespace may be omitted when no ambiguity arises.
The empty string means the beginning of today (i.e., midnight).  Order
of the items is immaterial.  A date string may contain many flavors of
items:

   • calendar date items
   • time of day items
   • time zone items
   • combined date and time of day items
   • day of the week items
   • relative items
   • pure numbers.

We describe each of these item types in turn, below.

   A few ordinal numbers may be written out in words in some contexts.
This is most useful for specifying day of the week items or relative
items (see below).  Among the most commonly used ordinal numbers, the
word ‘last’ stands for -1, ‘this’ stands for 0, and ‘first’ and ‘next’
both stand for 1.  Because the word ‘second’ stands for the unit of time
there is no way to write the ordinal number 2, but for convenience
‘third’ stands for 3, ‘fourth’ for 4, ‘fifth’ for 5, ‘sixth’ for 6,
‘seventh’ for 7, ‘eighth’ for 8, ‘ninth’ for 9, ‘tenth’ for 10,
‘eleventh’ for 11 and ‘twelfth’ for 12.

   When a month is written this way, it is still considered to be
written numerically, instead of being "spelled in full"; this changes
the allowed strings.

   In the current implementation, only English is supported for words
and abbreviations like ‘AM’, ‘DST’, ‘EST’, ‘first’, ‘January’, ‘Sunday’,
‘tomorrow’, and ‘year’.

   The output of the ‘date’ command is not always acceptable as a date
string, not only because of the language problem, but also because there
is no standard meaning for time zone items like ‘IST’.  When using
‘date’ to generate a date string intended to be parsed later, specify a
date format that is independent of language and that does not use time
zone items other than ‘UTC’ and ‘Z’.  Here are some ways to do this:

     $ LC_ALL=C TZ=UTC0 date
     Tue Nov 15 02:02:42 UTC 2022
     $ TZ=UTC0 date +'%Y-%m-%d %H:%M:%SZ'
     2022-11-15 02:02:42Z
     $ date --rfc-3339=ns  # --rfc-3339 is a GNU extension.
     2022-11-14 21:02:42.000000000-05:00
     $ date --rfc-email  # a GNU extension
     Mon, 14 Nov 2022 21:02:42 -0500
     $ date +'%Y-%m-%d %H:%M:%S %z'  # %z is a GNU extension.
     2022-11-14 21:02:42 -0500
     $ date +'@%s.%N'  # %s and %N are GNU extensions.
     @1668477762.692722128

   Alphabetic case is completely ignored in dates.  Comments may be
introduced between round parentheses, as long as included parentheses
are properly nested.  Hyphens not followed by a digit are currently
ignored.  Leading zeros on numbers are ignored.

   Invalid dates like ‘2022-02-29’ or times like ‘24:00’ are rejected.
In the typical case of a host that does not support leap seconds, a time
like ‘23:59:60’ is rejected even if it corresponds to a valid leap
second.


File: gnutar.info,  Node: Calendar date items,  Next: Time of day items,  Prev: General date syntax,  Up: Date input formats

7.2 Calendar date items
=======================

A “calendar date item” specifies a day of the year.  It is specified
differently, depending on whether the month is specified numerically or
literally.  All these strings specify the same calendar date:

     2022-11-14     # ISO 8601.
     22-11-14       # Assume 19xx for 69 through 99,
                    # 20xx for 00 through 68 (not recommended).
     11/14/2022     # Common U.S. writing.
     14 November 2022
     14 Nov 2022    # Three-letter abbreviations always allowed.
     November 14, 2022
     14-nov-2022
     14nov2022

   The year can also be omitted.  In this case, the last specified year
is used, or the current year if none.  For example:

     11/14
     nov 14

   Here are the rules.

   For numeric months, the ISO 8601 format ‘YEAR-MONTH-DAY’ is allowed,
where YEAR is any positive number, MONTH is a number between 01 and 12,
and DAY is a number between 01 and 31.  A leading zero must be present
if a number is less than ten.  If YEAR is 68 or smaller, then 2000 is
added to it; otherwise, if YEAR is less than 100, then 1900 is added to
it.  The construct ‘MONTH/DAY/YEAR’, popular in the United States, is
accepted.  Also ‘MONTH/DAY’, omitting the year.

   Literal months may be spelled out in full: ‘January’, ‘February’,
‘March’, ‘April’, ‘May’, ‘June’, ‘July’, ‘August’, ‘September’,
‘October’, ‘November’ or ‘December’.  Literal months may be abbreviated
to their first three letters, possibly followed by an abbreviating dot.
It is also permitted to write ‘Sept’ instead of ‘September’.

   When months are written literally, the calendar date may be given as
any of the following:

     DAY MONTH YEAR
     DAY MONTH
     MONTH DAY YEAR
     DAY-MONTH-YEAR

   Or, omitting the year:

     MONTH DAY


File: gnutar.info,  Node: Time of day items,  Next: Time zone items,  Prev: Calendar date items,  Up: Date input formats

7.3 Time of day items
=====================

A “time of day item” in date strings specifies the time on a given day.
Here are some examples, all of which represent the same time:

     20:02:00.000000
     20:02
     8:02pm
     20:02-0500      # In EST (U.S. Eastern Standard Time).

   More generally, the time of day may be given as ‘HOUR:MINUTE:SECOND’,
where HOUR is a number between 0 and 23, MINUTE is a number between 0
and 59, and SECOND is a number between 0 and 59 possibly followed by ‘.’
or ‘,’ and a fraction containing one or more digits.  Alternatively,
‘:SECOND’ can be omitted, in which case it is taken to be zero.  On the
rare hosts that support leap seconds, SECOND may be 60.

   If the time is followed by ‘am’ or ‘pm’ (or ‘a.m.’ or ‘p.m.’), HOUR
is restricted to run from 1 to 12, and ‘:MINUTE’ may be omitted (taken
to be zero).  ‘am’ indicates the first half of the day, ‘pm’ indicates
the second half of the day.  In this notation, 12 is the predecessor of
1: midnight is ‘12am’ while noon is ‘12pm’.  (This is the zero-oriented
interpretation of ‘12am’ and ‘12pm’, as opposed to the old tradition
derived from Latin which uses ‘12m’ for noon and ‘12pm’ for midnight.)

   The time may alternatively be followed by a time zone correction,
expressed as ‘SHHMM’, where S is ‘+’ or ‘-’, HH is a number of zone
hours and MM is a number of zone minutes.  The zone minutes term, MM,
may be omitted, in which case the one- or two-digit correction is
interpreted as a number of hours.  You can also separate HH from MM with
a colon.  When a time zone correction is given this way, it forces
interpretation of the time relative to Coordinated Universal Time (UTC),
overriding any previous specification for the time zone or the local
time zone.  For example, ‘+0530’ and ‘+05:30’ both stand for the time
zone 5.5 hours ahead of UTC (e.g., India).  This is the best way to
specify a time zone correction by fractional parts of an hour.  The
maximum zone correction is 24 hours.

   Either ‘am’/‘pm’ or a time zone correction may be specified, but not
both.


File: gnutar.info,  Node: Time zone items,  Next: Combined date and time of day items,  Prev: Time of day items,  Up: Date input formats

7.4 Time zone items
===================

A “time zone item” specifies an international time zone, indicated by a
small set of letters, e.g., ‘UTC’ or ‘Z’ for Coordinated Universal Time.
Any included periods are ignored.  By following a non-daylight-saving
time zone by the string ‘DST’ in a separate word (that is, separated by
some white space), the corresponding daylight saving time zone may be
specified.  Alternatively, a non-daylight-saving time zone can be
followed by a time zone correction, to add the two values.  This is
normally done only for ‘UTC’; for example, ‘UTC+05:30’ is equivalent to
‘+05:30’.

   Time zone items other than ‘UTC’ and ‘Z’ are obsolescent and are not
recommended, because they are ambiguous; for example, ‘EST’ has a
different meaning in Australia than in the United States, and ‘A’ has
different meaning as a military time zone than as an obsolete RFC 822
time zone.  Instead, it's better to use unambiguous numeric time zone
corrections like ‘-0500’, as described in the previous section.

   If neither a time zone item nor a time zone correction is supplied,
timestamps are interpreted using the rules of the default time zone
(*note Specifying time zone rules::).


File: gnutar.info,  Node: Combined date and time of day items,  Next: Day of week items,  Prev: Time zone items,  Up: Date input formats

7.5 Combined date and time of day items
=======================================

The ISO 8601 date and time of day extended format consists of an ISO
8601 date, a ‘T’ character separator, and an ISO 8601 time of day.  This
format is also recognized if the ‘T’ is replaced by a space.

   In this format, the time of day should use 24-hour notation.
Fractional seconds are allowed, with either comma or period preceding
the fraction.  ISO 8601 fractional minutes and hours are not supported.
Typically, hosts support nanosecond timestamp resolution; excess
precision is silently discarded.

   Here are some examples:

     2022-09-24T20:02:00.052-05:00
     2022-12-31T23:59:59,999999999+11:00
     1970-01-01 00:00Z


File: gnutar.info,  Node: Day of week items,  Next: Relative items in date strings,  Prev: Combined date and time of day items,  Up: Date input formats

7.6 Day of week items
=====================

The explicit mention of a day of the week will forward the date (only if
necessary) to reach that day of the week in the future.

   Days of the week may be spelled out in full: ‘Sunday’, ‘Monday’,
‘Tuesday’, ‘Wednesday’, ‘Thursday’, ‘Friday’ or ‘Saturday’.  Days may be
abbreviated to their first three letters, optionally followed by a
period.  The special abbreviations ‘Tues’ for ‘Tuesday’, ‘Wednes’ for
‘Wednesday’ and ‘Thur’ or ‘Thurs’ for ‘Thursday’ are also allowed.

   A number may precede a day of the week item to move forward
supplementary weeks.  It is best used in expression like ‘third monday’.
In this context, ‘last DAY’ or ‘next DAY’ is also acceptable; they move
one week before or after the day that DAY by itself would represent.

   A comma following a day of the week item is ignored.


File: gnutar.info,  Node: Relative items in date strings,  Next: Pure numbers in date strings,  Prev: Day of week items,  Up: Date input formats

7.7 Relative items in date strings
==================================

“Relative items” adjust a date (or the current date if none) forward or
backward.  The effects of relative items accumulate.  Here are some
examples:

     1 year
     1 year ago
     3 years
     2 days

   The unit of time displacement may be selected by the string ‘year’ or
‘month’ for moving by whole years or months.  These are fuzzy units, as
years and months are not all of equal duration.  More precise units are
‘fortnight’ which is worth 14 days, ‘week’ worth 7 days, ‘day’ worth 24
hours, ‘hour’ worth 60 minutes, ‘minute’ or ‘min’ worth 60 seconds, and
‘second’ or ‘sec’ worth one second.  An ‘s’ suffix on these units is
accepted and ignored.

   The unit of time may be preceded by a multiplier, given as an
optionally signed number.  Unsigned numbers are taken as positively
signed.  No number at all implies 1 for a multiplier.  Following a
relative item by the string ‘ago’ is equivalent to preceding the unit by
a multiplier with value -1.

   The string ‘tomorrow’ is worth one day in the future (equivalent to
‘day’), the string ‘yesterday’ is worth one day in the past (equivalent
to ‘day ago’).

   The strings ‘now’ or ‘today’ are relative items corresponding to
zero-valued time displacement, these strings come from the fact a
zero-valued time displacement represents the current time when not
otherwise changed by previous items.  They may be used to stress other
items, like in ‘12:00 today’.  The string ‘this’ also has the meaning of
a zero-valued time displacement, but is preferred in date strings like
‘this thursday’.

   When a relative item causes the resulting date to cross a boundary
where the clocks were adjusted, typically for daylight saving time, the
resulting date and time are adjusted accordingly.

   The fuzz in units can cause problems with relative items.  For
example, ‘2022-12-31 -1 month’ might evaluate to 2022-12-01, because
2022-11-31 is an invalid date.  To determine the previous month more
reliably, you can ask for the month before the 15th of the current
month.  For example:

     $ date -R
     Thu, 31 Dec 2022 13:02:39 -0400
     $ date --date='-1 month' +'Last month was %B?'
     Last month was December?
     $ date --date="$(date +%Y-%m-15) -1 month" +'Last month was %B!'
     Last month was November!

   Also, take care when manipulating dates around clock changes such as
daylight saving leaps.  In a few cases these have added or subtracted as
much as 24 hours from the clock, so it is often wise to adopt universal
time by setting the ‘TZ’ environment variable to ‘UTC0’ before embarking
on calendrical calculations.


File: gnutar.info,  Node: Pure numbers in date strings,  Next: Seconds since the Epoch,  Prev: Relative items in date strings,  Up: Date input formats

7.8 Pure numbers in date strings
================================

The precise interpretation of a pure decimal number depends on the
context in the date string.

   If the decimal number is of the form YYYYMMDD and no other calendar
date item (*note Calendar date items::) appears before it in the date
string, then YYYY is read as the year, MM as the month number and DD as
the day of the month, for the specified calendar date.

   If the decimal number is of the form HHMM and no other time of day
item appears before it in the date string, then HH is read as the hour
of the day and MM as the minute of the hour, for the specified time of
day.  MM can also be omitted.

   If both a calendar date and a time of day appear to the left of a
number in the date string, but no relative item, then the number
overrides the year.


File: gnutar.info,  Node: Seconds since the Epoch,  Next: Specifying time zone rules,  Prev: Pure numbers in date strings,  Up: Date input formats

7.9 Seconds since the Epoch
===========================

If you precede a number with ‘@’, it represents an internal timestamp as
a count of seconds.  The number can contain an internal decimal point
(either ‘.’ or ‘,’); any excess precision not supported by the internal
representation is truncated toward minus infinity.  Such a number cannot
be combined with any other date item, as it specifies a complete
timestamp.

   Internally, computer times are represented as a count of seconds
since an Epoch--a well-defined point of time.  On GNU and POSIX systems,
the Epoch is 1970-01-01 00:00:00 UTC, so ‘@0’ represents this time, ‘@1’
represents 1970-01-01 00:00:01 UTC, and so forth.  GNU and most other
POSIX-compliant systems support such times as an extension to POSIX,
using negative counts, so that ‘@-1’ represents 1969-12-31 23:59:59 UTC.

   Most modern systems count seconds with 64-bit two's-complement
integers of seconds with nanosecond subcounts, which is a range that
includes the known lifetime of the universe with nanosecond resolution.
Some obsolescent systems count seconds with 32-bit two's-complement
integers and can represent times from 1901-12-13 20:45:52 through
2038-01-19 03:14:07 UTC.  A few systems sport other time ranges.

   On most hosts, these counts ignore the presence of leap seconds.  For
example, on most hosts ‘@1483228799’ represents 2016-12-31 23:59:59 UTC,
‘@1483228800’ represents 2017-01-01 00:00:00 UTC, and there is no way to
represent the intervening leap second 2016-12-31 23:59:60 UTC.


File: gnutar.info,  Node: Specifying time zone rules,  Next: Authors of parse_datetime,  Prev: Seconds since the Epoch,  Up: Date input formats

7.10 Specifying time zone rules
===============================

Normally, dates are interpreted using the rules of the current time
zone, which in turn are specified by the ‘TZ’ environment variable, or
by a system default if ‘TZ’ is not set.  To specify a different set of
default time zone rules that apply just to one date, start the date with
a string of the form ‘TZ="RULE"’.  The two quote characters (‘"’) must
be present in the date, and any quotes or backslashes within RULE must
be escaped by a backslash.

   For example, with the GNU ‘date’ command you can answer the question
"What time is it in New York when a Paris clock shows 6:30am on October
31, 2022?"  by using a date beginning with ‘TZ="Europe/Paris"’ as shown
in the following shell transcript:

     $ export TZ="America/New_York"
     $ date --date='TZ="Europe/Paris" 2022-10-31 06:30'
     Mon Oct 31 01:30:00 EDT 2022

   In this example, the ‘--date’ operand begins with its own ‘TZ’
setting, so the rest of that operand is processed according to
‘Europe/Paris’ rules, treating the string ‘2022-11-14 06:30’ as if it
were in Paris.  However, since the output of the ‘date’ command is
processed according to the overall time zone rules, it uses New York
time.  (Paris was normally six hours ahead of New York in 2022, but this
example refers to a brief Halloween period when the gap was five hours.)

   A ‘TZ’ value is a rule that typically names a location in the ‘tz’
database (https://www.iana.org/time-zones).  A recent catalog of
location names appears in the TWiki Date and Time Gateway
(https://twiki.org/cgi-bin/xtra/tzdatepick.html).  A few non-GNU hosts
require a colon before a location name in a ‘TZ’ setting, e.g.,
‘TZ=":America/New_York"’.

   The ‘tz’ database includes a wide variety of locations ranging from
‘Africa/Abidjan’ to ‘Pacific/Tongatapu’, but if you are at sea and have
your own private time zone, or if you are using a non-GNU host that does
not support the ‘tz’ database, you may need to use a POSIX rule instead.
The previously-mentioned POSIX rule ‘UTC0’ says that the time zone
abbreviation is ‘UTC’, the zone is zero hours away from Greenwich, and
there is no daylight saving time.  POSIX rules can also specify nonzero
Greenwich offsets.  For example, the following shell transcript answers
the question "What time is it five and a half hours east of Greenwich
when a clock seven hours west of Greenwich shows 9:50pm on July 12,
2022?"

     $ TZ="<+0530>-5:30" date --date='TZ="<-07>+7" 2022-07-12 21:50'
     Wed Jul 13 10:20:00 +0530 2022

This example uses the somewhat-confusing POSIX convention for rules.
‘TZ="<-07>+7"’ says that the time zone abbreviation is ‘-07’ and the
time zone is 7 hours west of Greenwich, and ‘TZ="<+0530>-5:30"’ says
that the time zone abbreviation is ‘+0530’ and the time zone is 5 hours
30 minutes east of Greenwich.  (One should never use a setting like
‘TZ="UTC-5"’, since this would incorrectly imply that local time is five
hours east of Greenwich and the time zone is called "UTC".) Although
trickier POSIX ‘TZ’ settings like ‘TZ="<-05>+5<-04>,M3.2.0/2,M11.1.0/2"’
can specify some daylight saving regimes, location-based settings like
‘TZ="America/New_York"’ are typically simpler and more accurate
historically.  *Note Specifying the Time Zone with ‘TZ’: (libc)TZ
Variable.


File: gnutar.info,  Node: Authors of parse_datetime,  Prev: Specifying time zone rules,  Up: Date input formats

7.11 Authors of ‘parse_datetime’
================================

‘parse_datetime’ started life as ‘getdate’, as originally implemented by
Steven M. Bellovin (<smb@research.att.com>) while at the University of
North Carolina at Chapel Hill.  The code was later tweaked by a couple
of people on Usenet, then completely overhauled by Rich $alz
(<rsalz@bbn.com>) and Jim Berets (<jberets@bbn.com>) in August, 1990.
Various revisions for the GNU system were made by David MacKenzie, Jim
Meyering, Paul Eggert and others, including renaming it to ‘get_date’ to
avoid a conflict with the alternative Posix function ‘getdate’, and a
later rename to ‘parse_datetime’.  The Posix function ‘getdate’ can
parse more locale-specific dates using ‘strptime’, but relies on an
environment variable and external file, and lacks the thread-safety of
‘parse_datetime’.

   This chapter was originally produced by François Pinard
(<pinard@iro.umontreal.ca>) from the ‘parse_datetime.y’ source code, and
then edited by K. Berry (<kb@cs.umb.edu>).


File: gnutar.info,  Node: Formats,  Next: Media,  Prev: Date input formats,  Up: Top

8 Controlling the Archive Format
********************************

Due to historical reasons, there are several formats of tar archives.
All of them are based on the same principles, but have some subtle
differences that often make them incompatible with each other.

   GNU tar is able to create and handle archives in a variety of
formats.  The most frequently used formats are (in alphabetical order):

gnu
     Format used by GNU ‘tar’ versions up to 1.13.25.  This format
     derived from an early POSIX standard, adding some improvements such
     as sparse file handling and incremental archives.  Unfortunately
     these features were implemented in a way incompatible with other
     archive formats.

     Archives in ‘gnu’ format are able to hold file names of unlimited
     length.

oldgnu
     Format used by GNU ‘tar’ of versions prior to 1.12.

v7
     Archive format, compatible with the V7 implementation of tar.  This
     format imposes a number of limitations.  The most important of them
     are:

       1. The maximum length of a file name is limited to 99 characters.
       2. The maximum length of a symbolic link is limited to 99
          characters.
       3. It is impossible to store special files (block and character
          devices, fifos etc.)
       4. Maximum value of user or group ID is limited to 2097151
          (7777777 octal)
       5. V7 archives do not contain symbolic ownership information
          (user and group name of the file owner).

     This format has traditionally been used by Automake when producing
     Makefiles.  This practice will change in the future, in the
     meantime, however this means that projects containing file names
     more than 99 characters long will not be able to use GNU ‘tar’ 1.35
     and Automake prior to 1.9.

ustar
     Archive format defined by POSIX.1-1988 specification.  It stores
     symbolic ownership information.  It is also able to store special
     files.  However, it imposes several restrictions as well:

       1. The maximum length of a file name is limited to 256
          characters, provided that the file name can be split at a
          directory separator in two parts, first of them being at most
          155 bytes long.  So, in most cases the maximum file name
          length will be shorter than 256 characters.
       2. The maximum length of a symbolic link name is limited to 100
          characters.
       3. Maximum size of a file the archive is able to accommodate is
          8GB
       4. Maximum value of UID/GID is 2097151.
       5. Maximum number of bits in device major and minor numbers is
          21.

star
     Format used by Jörg Schilling ‘star’ implementation.  GNU ‘tar’ is
     able to read ‘star’ archives but currently does not produce them.

posix
     Archive format defined by POSIX.1-2001 specification.  This is the
     most flexible and feature-rich format.  It does not impose any
     restrictions on file sizes or file name lengths.  This format is
     quite recent, so not all tar implementations are able to handle it
     properly.  However, this format is designed in such a way that any
     tar implementation able to read ‘ustar’ archives will be able to
     read most ‘posix’ archives as well, with the only exception that
     any additional information (such as long file names etc.) will in
     such case be extracted as plain text files along with the files it
     refers to.

     This archive format will be the default format for future versions
     of GNU ‘tar’.

   The following table summarizes the limitations of each of these
formats:

Format  UID            File Size      File Name      Devn
--------------------------------------------------------------------
gnu     1.8e19         Unlimited      Unlimited      63
oldgnu  1.8e19         Unlimited      Unlimited      63
v7      2097151        8GB            99             n/a
ustar   2097151        8GB            256            21
posix   Unlimited      Unlimited      Unlimited      Unlimited

   The default format for GNU ‘tar’ is defined at compilation time.  You
may check it by running ‘tar --help’, and examining the last lines of
its output.  Usually, GNU ‘tar’ is configured to create archives in
‘gnu’ format, however, future version will switch to ‘posix’.

* Menu:

* Compression::                 Using Less Space through Compression
* Attributes::                  Handling File Attributes
* Portability::                 Making ‘tar’ Archives More Portable
* cpio::                        Comparison of ‘tar’ and ‘cpio’


File: gnutar.info,  Node: Compression,  Next: Attributes,  Up: Formats

8.1 Using Less Space through Compression
========================================

* Menu:

* gzip::                        Creating and Reading Compressed Archives
* sparse::                      Archiving Sparse Files


File: gnutar.info,  Node: gzip,  Next: sparse,  Up: Compression

8.1.1 Creating and Reading Compressed Archives
----------------------------------------------

GNU ‘tar’ is able to create and read compressed archives.  It supports a
wide variety of compression programs, namely: ‘gzip’, ‘bzip2’, ‘lzip’,
‘lzma’, ‘lzop’, ‘zstd’, ‘xz’ and traditional ‘compress’.  The latter is
supported mostly for backward compatibility, and we recommend against
using it, because it is by far less effective than the other compression
programs(1).

   Creating a compressed archive is simple: you just specify a
“compression option” along with the usual archive creation commands.
Available compression options are summarized in the table below:

Long                          Short          Archive format
---------------------------------------------------------------------------
‘--gzip’                      ‘-z’           ‘gzip’
‘--bzip2’                     ‘-j’           ‘bzip2’
‘--xz’                        ‘-J’           ‘xz’
‘--lzip’                                     ‘lzip’
‘--lzma’                                     ‘lzma’
‘--lzop’                                     ‘lzop’
‘--zstd’                                     ‘zstd’
‘--compress’                  ‘-Z’           ‘compress’

   For example:

     $ tar czf archive.tar.gz .

   You can also let GNU ‘tar’ select the compression program based on
the suffix of the archive file name.  This is done using
‘--auto-compress’ (‘-a’) command line option.  For example, the
following invocation will use ‘bzip2’ for compression:

     $ tar caf archive.tar.bz2 .

whereas the following one will use ‘lzma’:

     $ tar caf archive.tar.lzma .

   For a complete list of file name suffixes recognized by GNU ‘tar’,
see *note auto-compress::.

   Reading compressed archive is even simpler: you don't need to specify
any additional options as GNU ‘tar’ recognizes its format automatically.
Thus, the following commands will list and extract the archive created
in previous example:

     # List the compressed archive
     $ tar tf archive.tar.gz
     # Extract the compressed archive
     $ tar xf archive.tar.gz

   The format recognition algorithm is based on “signatures”, a special
byte sequences in the beginning of file, that are specific for certain
compression formats.  If this approach fails, ‘tar’ falls back to using
archive name suffix to determine its format (*note auto-compress::, for
a list of recognized suffixes).

   Some compression programs are able to handle different compression
formats.  GNU ‘tar’ uses this, if the principal decompressor for the
given format is not available.  For example, if ‘compress’ is not
installed, ‘tar’ will try to use ‘gzip’.  As of version 1.35 the
following alternatives are tried(2):

Format                 Main decompressor      Alternatives
---------------------------------------------------------------------
compress               compress               gzip
lzma                   lzma                   xz
bzip2                  bzip2                  lbzip2

   The only case when you have to specify a decompression option while
reading the archive is when reading from a pipe or from a tape drive
that does not support random access.  However, in this case GNU ‘tar’
will indicate which option you should use.  For example:

     $ cat archive.tar.gz | tar tf -
     tar: Archive is compressed.  Use -z option
     tar: Error is not recoverable: exiting now

   If you see such diagnostics, just add the suggested option to the
invocation of GNU ‘tar’:

     $ cat archive.tar.gz | tar tzf -

   Notice also, that there are several restrictions on operations on
compressed archives.  First of all, compressed archives cannot be
modified, i.e., you cannot update (‘--update’, alias ‘-u’) them or
delete (‘--delete’) members from them or add (‘--append’, alias ‘-r’)
members to them.  Likewise, you cannot append another ‘tar’ archive to a
compressed archive using ‘--concatenate’ (‘-A’).  Secondly, multi-volume
archives cannot be compressed.

   The following options allow to select a particular compressor
program:

‘-z’
‘--gzip’
‘--ungzip’
     Filter the archive through ‘gzip’.

‘-J’
‘--xz’
     Filter the archive through ‘xz’.

‘-j’
‘--bzip2’
     Filter the archive through ‘bzip2’.

‘--lzip’
     Filter the archive through ‘lzip’.

‘--lzma’
     Filter the archive through ‘lzma’.

‘--lzop’
     Filter the archive through ‘lzop’.

‘--zstd’
     Filter the archive through ‘zstd’.

‘-Z’
‘--compress’
‘--uncompress’
     Filter the archive through ‘compress’.

   When any of these options is given, GNU ‘tar’ searches the compressor
binary in the current path and invokes it.  The name of the compressor
program is specified at compilation time using a corresponding
‘--with-COMPNAME’ option to ‘configure’, e.g.  ‘--with-bzip2’ to select
a specific ‘bzip2’ binary.  *Note lbzip2::, for a detailed discussion.

   The output produced by ‘tar --help’ shows the actual compressor names
along with each of these options.

   You can use any of these options on physical devices (tape drives,
etc.) and remote files as well as on normal files; data to or from such
devices or remote files is reblocked by another copy of the ‘tar’
program to enforce the specified (or default) record size.  The default
compression parameters are used.  You can override them by using the
‘-I’ option (see below), e.g.:

     $ tar -cf archive.tar.gz -I 'gzip -9 -n' subdir

A more traditional way to do this is to use a pipe:

     $ tar cf - subdir | gzip -9 -n > archive.tar.gz

   Compressed archives are easily corrupted, because compressed files
have little redundancy.  The adaptive nature of the compression scheme
means that the compression tables are implicitly spread all over the
archive.  If you lose a few blocks, the dynamic construction of the
compression tables becomes unsynchronized, and there is little chance
that you could recover later in the archive.

   Other compression options provide better control over creating
compressed archives.  These are:

‘--auto-compress’
‘-a’
     Select a compression program to use by the archive file name
     suffix.  The following suffixes are recognized:

     Suffix                 Compression program
     -------------------------------------------------------------------
     ‘.gz’                  ‘gzip’
     ‘.tgz’                 ‘gzip’
     ‘.taz’                 ‘gzip’
     ‘.Z’                   ‘compress’
     ‘.taZ’                 ‘compress’
     ‘.bz2’                 ‘bzip2’
     ‘.tz2’                 ‘bzip2’
     ‘.tbz2’                ‘bzip2’
     ‘.tbz’                 ‘bzip2’
     ‘.lz’                  ‘lzip’
     ‘.lzma’                ‘lzma’
     ‘.tlz’                 ‘lzma’
     ‘.lzo’                 ‘lzop’
     ‘.xz’                  ‘xz’
     ‘.zst’                 ‘zstd’
     ‘.tzst’                ‘zstd’

‘--use-compress-program=COMMAND’
‘-I=COMMAND’
     Use external compression program COMMAND.  Use this option if you
     want to specify options for the compression program, or if you are
     not happy with the compression program associated with the suffix
     at compile time, or if you have a compression program that GNU
     ‘tar’ does not support.  The COMMAND argument is a valid command
     invocation, as you would type it at the command line prompt, with
     any additional options as needed.  Enclose it in quotes if it
     contains white space (*note Running External Commands: external.).

     The COMMAND should follow two conventions:

     First, when invoked without additional options, it should read data
     from standard input, compress it and output it on standard output.

     Secondly, if invoked with the additional ‘-d’ option, it should do
     exactly the opposite, i.e., read the compressed data from the
     standard input and produce uncompressed data on the standard
     output.

     The latter requirement means that you must not use the ‘-d’ option
     as a part of the COMMAND itself.

   The ‘--use-compress-program’ option, in particular, lets you
implement your own filters, not necessarily dealing with
compression/decompression.  For example, suppose you wish to implement
PGP encryption on top of compression, using ‘gpg’ (*note gpg:
(gpg)Top.).  The following script does that:

     #! /bin/sh
     case $1 in
     -d) gpg --decrypt - | gzip -d -c;;
     '') gzip -c | gpg -s;;
     *)  echo "Unknown option $1">&2; exit 1;;
     esac

   Suppose you name it ‘gpgz’ and save it somewhere in your ‘PATH’.
Then the following command will create a compressed archive signed with
your private key:

     $ tar -cf foo.tar.gpgz -Igpgz .

Likewise, the command below will list its contents:

     $ tar -tf foo.tar.gpgz -Igpgz .

* Menu:

* lbzip2::  Using lbzip2 with GNU ‘tar’.

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

   (1) It also had patent problems in the past.

   (2) To verbosely trace the decompressor selection, use the
‘--warning=decompress-program’ option (*note decompress-program:
warnings.).


File: gnutar.info,  Node: lbzip2,  Up: gzip

8.1.1.1 Using lbzip2 with GNU ‘tar’.
....................................

‘Lbzip2’ is a multithreaded utility for handling ‘bzip2’ compression,
written by Laszlo Ersek.  It makes use of multiple processors to speed
up its operation and in general works considerably faster than ‘bzip2’.
For a detailed description of ‘lbzip2’ see
<http://freshmeat.net/projects/lbzip2> and lbzip2: parallel bzip2
utility
(http://www.linuxinsight.com/lbzip2-parallel-bzip2-utility.html).

   Recent versions of ‘lbzip2’ are mostly command line compatible with
‘bzip2’, which makes it possible to automatically invoke it via the
‘--bzip2’ GNU ‘tar’ command line option.  To do so, GNU ‘tar’ must be
configured with the ‘--with-bzip2’ command line option, like this:

     $ ./configure --with-bzip2=lbzip2 [OTHER-OPTIONS]

   Once configured and compiled this way, ‘tar --help’ will show the
following:

     $ tar --help | grep -- --bzip2
       -j, --bzip2                filter the archive through lbzip2

which means that running ‘tar --bzip2’ will invoke ‘lbzip2’.


File: gnutar.info,  Node: sparse,  Prev: gzip,  Up: Compression

8.1.2 Archiving Sparse Files
----------------------------

Files in the file system occasionally have “holes”.  A “hole” in a file
is a section of the file's contents which was never written.  The
contents of a hole reads as all zeros.  On many operating systems,
actual disk storage is not allocated for holes, but they are counted in
the length of the file.  If you archive such a file, ‘tar’ could create
an archive longer than the original.  To have ‘tar’ attempt to recognize
the holes in a file, use ‘--sparse’ (‘-S’).  When you use this option,
then, for any file using less disk space than would be expected from its
length, ‘tar’ searches the file for holes.  It then records in the
archive for the file where the holes (consecutive stretches of zeros)
are, and only archives the "real contents" of the file.  On extraction
(using ‘--sparse’ is not needed on extraction) any such files have also
holes created wherever the holes were found.  Thus, if you use
‘--sparse’, ‘tar’ archives won't take more space than the original.

   GNU ‘tar’ uses two methods for detecting holes in sparse files.
These methods are described later in this subsection.

‘-S’
‘--sparse’
     This option instructs ‘tar’ to test each file for sparseness before
     attempting to archive it.  If the file is found to be sparse it is
     treated specially, thus allowing to decrease the amount of space
     used by its image in the archive.

     This option is meaningful only when creating or updating archives.
     It has no effect on extraction.

   Consider using ‘--sparse’ when performing file system backups, to
avoid archiving the expanded forms of files stored sparsely in the
system.

   Even if your system has no sparse files currently, some may be
created in the future.  If you use ‘--sparse’ while making file system
backups as a matter of course, you can be assured the archive will never
take more space on the media than the files take on disk (otherwise,
archiving a disk filled with sparse files might take hundreds of tapes).
*Note Incremental Dumps::.

   However, be aware that ‘--sparse’ option may present a serious
drawback.  Namely, in order to determine the positions of holes in a
file ‘tar’ may have to read it before trying to archive it, so in total
the file may be read *twice*.  This may happen when your OS or your FS
does not support “SEEK_HOLE/SEEK_DATA” feature in “lseek” (See
‘--hole-detection’, below).

   When using ‘POSIX’ archive format, GNU ‘tar’ is able to store sparse
files using in three distinct ways, called “sparse formats”.  A sparse
format is identified by its “number”, consisting, as usual of two
decimal numbers, delimited by a dot.  By default, format ‘1.0’ is used.
If, for some reason, you wish to use an earlier format, you can select
it using ‘--sparse-version’ option.

‘--sparse-version=VERSION’
     Select the format to store sparse files in.  Valid VERSION values
     are: ‘0.0’, ‘0.1’ and ‘1.0’.  *Note Sparse Formats::, for a
     detailed description of each format.

   Using ‘--sparse-format’ option implies ‘--sparse’.

‘--hole-detection=METHOD’
     Enforce concrete hole detection method.  Before the real contents
     of sparse file are stored, ‘tar’ needs to gather knowledge about
     file sparseness.  This is because it needs to have the file's map
     of holes stored into tar header before it starts archiving the file
     contents.  Currently, two methods of hole detection are
     implemented:

        • ‘--hole-detection=seek’ Seeking the file for data and holes.
          It uses enhancement of the ‘lseek’ system call (‘SEEK_HOLE’
          and ‘SEEK_DATA’) which is able to reuse file system knowledge
          about sparse file contents - so the detection is usually very
          fast.  To use this feature, your file system and operating
          system must support it.  At the time of this writing (2015)
          this feature, in spite of not being accepted by POSIX, is
          fairly widely supported by different operating systems.

        • ‘--hole-detection=raw’ Reading byte-by-byte the whole sparse
          file before the archiving.  This method detects holes like
          consecutive stretches of zeroes.  Comparing to the previous
          method, it is usually much slower, although more portable.

   When no ‘--hole-detection’ option is given, ‘tar’ uses the ‘seek’, if
supported by the operating system.

   Using ‘--hole-detection’ option implies ‘--sparse’.


File: gnutar.info,  Node: Attributes,  Next: Portability,  Prev: Compression,  Up: Formats

8.2 Handling File Attributes
============================

When ‘tar’ reads files, it updates their access times.  To avoid this,
use the ‘--atime-preserve[=METHOD]’ option, which can either reset the
access time retroactively or avoid changing it in the first place.

‘--atime-preserve’
‘--atime-preserve=replace’
‘--atime-preserve=system’
     Preserve the access times of files that are read.  This works only
     for files that you own, unless you have superuser privileges.

     ‘--atime-preserve=replace’ works on most systems, but it also
     restores the data modification time and updates the status change
     time.  Hence it doesn't interact with incremental dumps nicely
     (*note Incremental Dumps::), and it can set access or data
     modification times incorrectly if other programs access the file
     while ‘tar’ is running.

     ‘--atime-preserve=system’ avoids changing the access time in the
     first place, if the operating system supports this.  Unfortunately,
     this may or may not work on any given operating system or file
     system.  If ‘tar’ knows for sure it won't work, it complains right
     away.

     Currently ‘--atime-preserve’ with no operand defaults to
     ‘--atime-preserve=replace’, but this is intended to change to
     ‘--atime-preserve=system’ when the latter is better-supported.

‘-m’
‘--touch’
     Do not extract data modification time.

     When this option is used, ‘tar’ leaves the data modification times
     of the files it extracts as the times when the files were
     extracted, instead of setting it to the times recorded in the
     archive.

     This option is meaningless with ‘--list’ (‘-t’).

‘--same-owner’
     Create extracted files with the same ownership they have in the
     archive.

     This is the default behavior for the superuser, so this option is
     meaningful only for non-root users, when ‘tar’ is executed on those
     systems able to give files away.  This is considered as a security
     flaw by many people, at least because it makes quite difficult to
     correctly account users for the disk space they occupy.  Also, the
     ‘suid’ or ‘sgid’ attributes of files are easily and silently lost
     when files are given away.

     When writing an archive, ‘tar’ writes the user ID and user name
     separately.  If it can't find a user name (because the user ID is
     not in ‘/etc/passwd’), then it does not write one.  When restoring,
     it tries to look the name (if one was written) up in ‘/etc/passwd’.
     If it fails, then it uses the user ID stored in the archive
     instead.

‘--no-same-owner’
‘-o’
     Do not attempt to restore ownership when extracting.  This is the
     default behavior for ordinary users, so this option has an effect
     only for the superuser.

‘--numeric-owner’
     The ‘--numeric-owner’ option allows (ANSI) archives to be written
     without user/group name information or such information to be
     ignored when extracting.  It effectively disables the generation
     and/or use of user/group name information.  This option forces
     extraction using the numeric ids from the archive, ignoring the
     names.

     This is useful in certain circumstances, when restoring a backup
     from an emergency floppy with different passwd/group files for
     example.  It is otherwise impossible to extract files with the
     right ownerships if the password file in use during the extraction
     does not match the one belonging to the file system(s) being
     extracted.  This occurs, for example, if you are restoring your
     files after a major crash and had booted from an emergency floppy
     with no password file or put your disk into another machine to do
     the restore.

     The numeric ids are _always_ saved into ‘tar’ archives.  The
     identifying names are added at create time when provided by the
     system, unless ‘--format=oldgnu’ is used.  Numeric ids could be
     used when moving archives between a collection of machines using a
     centralized management for attribution of numeric ids to users and
     groups.  This is often made through using the NIS capabilities.

     When making a ‘tar’ file for distribution to other sites, it is
     sometimes cleaner to use a single owner for all files in the
     distribution, and nicer to specify the write permission bits of the
     files as stored in the archive independently of their actual value
     on the file system.  The way to prepare a clean distribution is
     usually to have some Makefile rule creating a directory, copying
     all needed files in that directory, then setting ownership and
     permissions as wanted (there are a lot of possible schemes), and
     only then making a ‘tar’ archive out of this directory, before
     cleaning everything out.  Of course, we could add a lot of options
     to GNU ‘tar’ for fine tuning permissions and ownership.  This is
     not the good way, I think.  GNU ‘tar’ is already crowded with
     options and moreover, the approach just explained gives you a great
     deal of control already.

‘-p’
‘--same-permissions’
‘--preserve-permissions’
     Extract all protection information.

     This option causes ‘tar’ to set the modes (access permissions) of
     extracted files exactly as recorded in the archive.  If this option
     is not used, the current ‘umask’ setting limits the permissions on
     extracted files.  This option is by default enabled when ‘tar’ is
     executed by a superuser.

     This option is meaningless with ‘--list’ (‘-t’).


File: gnutar.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
* hard links::                  Hard 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: gnutar.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 on
case-insensitive file systems like FAT32, you should not rely on case
distinction for file names.


File: gnutar.info,  Node: dereference,  Next: hard links,  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.  When ‘--dereference’
(‘-h’) is used with ‘--create’ (‘-c’), ‘tar’ archives the files symbolic
links point to, instead of the links themselves.

   When creating portable archives, use ‘--dereference’ (‘-h’): some
systems do not support symbolic links, and moreover, your distribution
might be unusable if it contains unresolved symbolic links.

   When reading from an archive, the ‘--dereference’ (‘-h’) option
causes ‘tar’ to follow an already-existing symbolic link when ‘tar’
writes or reads a file named in the archive.  Ordinarily, ‘tar’ does not
follow such a link, though it may remove the link before writing a new
file.  *Note Dealing with Old Files::.

   The ‘--dereference’ option is unsafe if an untrusted user can modify
directories while ‘tar’ is running.  *Note Security::.


File: gnutar.info,  Node: hard links,  Next: old,  Prev: dereference,  Up: Portability

8.3.3 Hard Links
----------------

Normally, when ‘tar’ archives a hard link, it writes a block to the
archive naming the target of the link (a ‘1’ type block).  In that way,
the actual file contents is stored in file only once.  For example,
consider the following two files:

     $ ls -l
     -rw-r--r-- 2 gray staff 4 2007-10-30 15:11 one
     -rw-r--r-- 2 gray staff 4 2007-10-30 15:11 jeden

   Here, ‘jeden’ is a link to ‘one’.  When archiving this directory with
a verbose level 2, you will get an output similar to the following:

     $ tar cvvf ../archive.tar .
     drwxr-xr-x gray/staff        0 2007-10-30 15:13 ./
     -rw-r--r-- gray/staff        4 2007-10-30 15:11 ./jeden
     hrw-r--r-- gray/staff        0 2007-10-30 15:11 ./one link to ./jeden

   The last line shows that, instead of storing two copies of the file,
‘tar’ stored it only once, under the name ‘jeden’, and stored file ‘one’
as a hard link to this file.

   It may be important to know that all hard links to the given file are
stored in the archive.  For example, this may be necessary for exact
reproduction of the file system.  The following option does that:

‘--check-links’
‘-l’
     Check the number of links dumped for each processed file.  If this
     number does not match the total number of hard links for the file,
     print a warning message.

   For example, trying to archive only file ‘jeden’ with this option
produces the following diagnostics:

     $ tar -c -f ../archive.tar -l jeden
     tar: Missing links to 'jeden'.

   Although creating special records for hard links helps keep a
faithful record of the file system contents and makes archives more
compact, it may present some difficulties when extracting individual
members from the archive.  For example, trying to extract file ‘one’
from the archive created in previous examples produces, in the absence
of file ‘jeden’:

     $ tar xf archive.tar ./one
     tar: ./one: Cannot hard link to './jeden': No such file or directory
     tar: Error exit delayed from previous errors

   The reason for this behavior is that ‘tar’ cannot seek back in the
archive to the previous member (in this case, ‘one’), to extract it(1).
If you wish to avoid such problems at the cost of a bigger archive, use
the following option:

‘--hard-dereference’
     Dereference hard links and store the files they refer to.

   For example, trying this option on our two sample files, we get two
copies in the archive, each of which can then be extracted independently
of the other:

     $ tar -c -vv -f ../archive.tar --hard-dereference .
     drwxr-xr-x gray/staff        0 2007-10-30 15:13 ./
     -rw-r--r-- gray/staff        4 2007-10-30 15:11 ./jeden
     -rw-r--r-- gray/staff        4 2007-10-30 15:11 ./one

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

   (1) There are plans to fix this in future releases.


File: gnutar.info,  Node: old,  Next: ustar,  Prev: hard links,  Up: Portability

8.3.4 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: gnutar.info,  Node: ustar,  Next: gnu,  Prev: old,  Up: Portability

8.3.5 Ustar Archive Format
--------------------------

The archive format defined by the 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 an archive in ‘ustar’ format, use the ‘--format=ustar’
option in conjunction with ‘--create’ (‘-c’).


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

8.3.6 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: gnutar.info,  Node: posix,  Next: Checksumming,  Prev: gnu,  Up: Portability

8.3.7 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: gnutar.info,  Node: PAX keywords,  Up: posix

8.3.7.1 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.  If the pattern
     contains shell metacharacters like ‘*’, it should be quoted to
     prevent the shell from expanding the pattern before ‘tar’ sees it.

     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/%f

     This default is selected to ensure the reproducibility of the
     archive.  POSIX standard recommends to use ‘%d/PaxHeaders.%p/%f’
     instead, which means the two archives created with the same set of
     options and containing the same set of files will be byte-to-byte
     different.  This default will be used if the environment variable
     ‘POSIXLY_CORRECT’ is set.

‘exthdr.mtime=VALUE’

     This keyword defines the value of the ‘mtime’ field that is written
     into the ustar header blocks for the extended headers.  By default,
     the ‘mtime’ field is set to the modification time of the archive
     member described by that extended header (or to the value of the
     ‘--mtime’ option, if supplied).

‘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.%n

     If the environment variable ‘POSIXLY_CORRECT’ is set, the following
     value is used instead:

          $TMPDIR/GlobalHead.%p.%n

     In both cases, ‘$TMPDIR’ stands for the value of the TMPDIR
     environment variable.  If TMPDIR is not set, ‘tar’ uses ‘/tmp’.

‘globexthdr.mtime=VALUE’

     This keyword defines the value of the ‘mtime’ field that is written
     into the ustar header blocks for the global extended headers.  By
     default, the ‘mtime’ field is set to the time when ‘tar’ was
     invoked.

‘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.

   In any of the forms described above, the VALUE may be a string
enclosed in curly braces.  In that case, the string between the braces
is understood either as a textual time representation, as described in
*note Date input formats::, or a name of the existing file, starting
with ‘/’ or ‘.’.  In the latter case, the modification time of that file
is used.

   For example, to set all modification times to the current date, you
use the following option:

     --pax-option 'mtime:={now}'

   As another example, here is the option that ensures that any two
archives created using it, will be binary equivalent if they have the
same contents:

     --pax-option delete=atime

If you extract files from such an archive and recreate the archive from
them, you will also need to eliminate changes due to ctime:

     --pax-option 'delete=atime,delete=ctime'

   Normally ‘tar’ saves an mtime value with subsecond resolution in an
extended header for any file with a timestamp that is not on a
one-second boundary.  This is in addition to the traditional mtime
timestamp in the header block.  Although you can suppress subsecond
timestamp resolution with ‘--pax-option delete=mtime’, this hack will
not work for timestamps before 1970 or after 2242-03-16 12:56:31 UTC.

   If the environment variable ‘POSIXLY_CORRECT’ is set, two POSIX
archives created using the same options on the same set of files might
not be byte-to-byte equivalent even with the above options.  This is
because the POSIX default for extended header names includes the ‘tar’
process ID, which typically differs at each run.  To produce
byte-to-byte equivalent archives in this case, either unset
‘POSIXLY_CORRECT’, or use the following option, which can be combined
with the above options:

     --pax-option exthdr.name=%d/PaxHeaders/%f


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

8.3.8 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 eighth bit set, because they use signed checksums,
while GNU ‘tar’ uses unsigned checksums while creating archives, as per
POSIX standards.  On reading, GNU ‘tar’ computes both checksums and
accepts either of them.  It is somewhat worrying that a lot of people
may go around doing backup of their files using faulty (or at least
non-standard) software, not learning about it until it's time to restore
their missing files with an incompatible file extractor, or vice versa.

   GNU ‘tar’ computes checksums both ways, and accepts either of them 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 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: gnutar.info,  Node: Large or Negative Values,  Next: Other Tars,  Prev: Checksumming,  Up: Portability

8.3.9 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: gnutar.info,  Node: Other Tars,  Prev: Large or Negative Values,  Up: Portability

8.3.10 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: gnutar.info,  Node: Split Recovery,  Next: Sparse Recovery,  Up: Other Tars

8.3.10.1 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/%f.%n

where symbols preceded 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, then the member names will be:

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

   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/longfile: Unknown file type 'x', extracted as
     normal file
     Unexpected EOF in archive
     $ tar xf vol-2.tar
     tmp/GlobalHead.1: Unknown file type 'g', extracted as normal file
     GNUFileParts/PaxHeaders/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: gnutar.info,  Node: Sparse Recovery,  Prev: Split Recovery,  Up: Other Tars

8.3.10.2 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 something 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/sparsefile \
       /home/gray/GNUSparseFile/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/NAME’.

   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: gnutar.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 i-numbers (e.g., the BSD file system);
‘cpio’s way requires you to play some games (in its "binary" format,
i-numbers are only 16 bits, and in its "portable ASCII" format, they're
18 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: gnutar.info,  Node: Media,  Next: Reliability and security,  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: gnutar.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 the
‘--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=SIZE[SUF]’
     Change tape after writing SIZE units of data.  Unless SUF is given,
     SIZE is treated as kilobytes, i.e.  ‘SIZE x 1024’ bytes.  The
     following suffixes alter this behavior:

     Suffix         Units                  Byte Equivalent
     -------------------------------------------------------------
     b              Blocks                 SIZE x 512
     B              Kilobytes              SIZE x 1024
     c              Bytes                  SIZE
     G              Gigabytes              SIZE x 1024^3
     K              Kilobytes              SIZE x 1024
     k              Kilobytes              SIZE x 1024
     M              Megabytes              SIZE x 1024^2
     P              Petabytes              SIZE x 1024^5
     T              Terabytes              SIZE x 1024^4
     w              Words                  SIZE x 2

     Table 9.1: Size Suffixes

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

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


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

9.2 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.  Its
source code 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: gnutar.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: gnutar.info,  Node: Blocking,  Next: Many,  Prev: Common Problems and Solutions,  Up: Media

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

“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 François has migrated that terminology back into the source
     code too.

   The term “physical block” means the basic transfer chunk from or to a
device, after which reading or writing may stop without anything being
lost.  In this manual, the term “block” usually refers to a disk
physical block, _assuming_ that each disk block is 512 bytes in length.
It is true that some disk devices have different physical blocks, but
‘tar’ 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(1).  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

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

   (1) If this message is not needed, you can turn it off using the
‘--warning=no-record-size’ option.


File: gnutar.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: gnutar.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 losing
information.

   Using higher blocking (putting more disk blocks per tape block) will
use the tape more efficiently as there will be less tape gaps.  But
reading such tapes may be more difficult for the system, as more memory
will be required to receive at once the whole record.  Further, if there
is a reading error on a huge record, this is less likely that the system
will succeed in recovering the information.  So, blocking should not be
too low, nor it should be too high.  ‘tar’ uses by default a blocking of
20 for historical reasons, and it does not really matter when reading or
writing to disk.  Current tape technology would easily accommodate
higher blockings.  Sun recommends a blocking of 126 for Exabytes and 96
for DATs.  We were told that for some DLT drives, the blocking should be
a multiple of 4Kb, preferably 64Kb (‘-b 128’) or 256 for decent
performance.  Other manufacturers may use different recommendations for
the same tapes.  This might also depends of the buffering techniques
used inside modern tape controllers.  Some imposes a minimum blocking,
or a maximum blocking.  Others request blocking to be some exponent of
two.

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

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

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


File: gnutar.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: gnutar.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: gnutar.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: gnutar.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: gnutar.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 file.

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

‘--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[SUF]’
‘-L SIZE[SUF]’
     Set maximum length of a volume.  The SUF, if given, specifies units
     in which SIZE is expressed, e.g.  ‘2M’ mean 2 megabytes (*note
     Table 9.1: size-suffixes, for a list of allowed size suffixes).
     Without SUF, units of 1024 bytes (kilobyte) are assumed.

     This option selects ‘--multi-volume’ automatically.  For example:

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

     or, which is equivalent:

          $ tar --create --tape-length=4G --file=/dev/tape FILES

   When GNU ‘tar’ comes to the end of a storage media, it asks you to
change the volume.  The built-in prompt for POSIX locale is(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=COMMAND’
‘--new-volume-script=COMMAND’
‘-F COMMAND’
     Specify the command to invoke when switching volumes.  The COMMAND
     can be used to eject cassettes, or to broadcast messages such as
     ‘Someone please come change my tape’ when performing unattended
     backups.

   The COMMAND can contain additional options, if such are needed.
*Note Running External Commands: external, for a detailed discussion of
the way GNU ‘tar’ runs external commands.  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_BLOCKING_FACTOR’
     Current blocking factor (*note Blocking::).

‘TAR_VOLUME’
     Ordinal number of the volume ‘tar’ is about to start.

‘TAR_SUBCOMMAND’
     A 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’.

   These variables can be used in the COMMAND itself, provided that they
are properly quoted to prevent them from being expanded by the shell
that invokes ‘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 -cM -f /dev/tape0 -f /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/bash
     # For this script it's advisable to use a shell, such as Bash,
     # that supports a TAR_FD value greater than 9.

     echo Preparing volume $TAR_VOLUME of $TAR_ARCHIVE.

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

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

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

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

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

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

   If an archive member is split across volumes (i.e., its entry begins
on one volume of the media and ends on another), you need to specify
‘--multi-volume’ to extract it successfully.  In this case, you should
load the volume where the archive member starts, and use ‘tar --extract
--multi-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: gnutar.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 gave.  *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: gnutar.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: gnutar.info,  Node: label,  Next: verify,  Prev: Using Multiple Tapes,  Up: Media

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

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(1) 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 label by
specifying ‘--test-label’ option.  This option reads only the first
block of an archive, so it can be used with slow storage devices.  For
example:

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

   If ‘--test-label’ is used with one or more command line arguments,
‘tar’ compares the volume label with each argument.  It exits with code
0 if a match is found, and with code 1 otherwise(2).  No output is
displayed, unless you also used the ‘--verbose’ option.  For example:

     $ tar --test-label --file=iamanarchive 'iamalabel'
     ⇒ 0
     $ tar --test-label --file=iamanarchive 'alabel'
     ⇒ 1

   When used with the ‘--verbose’ option, ‘tar’ prints the actual volume
label (if any), and a verbose diagnostics in case of a mismatch:

     $ tar --test-label --verbose --file=iamanarchive 'iamalabel'
     iamalabel
     ⇒ 0
     $ tar --test-label --verbose --file=iamanarchive 'alabel'
     iamalabel
     tar: Archive label mismatch
     ⇒ 1

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

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

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

   Similarly, ‘tar’ will refuse to list or extract the archive if its
label doesn't match the ARCHIVE-LABEL specified.  In those cases,
ARCHIVE-LABEL argument is interpreted as a globbing-style pattern which
must match the actual magnetic volume label.  *Note exclude::, for a
precise description of how match is attempted(3).  If the switch
‘--multi-volume’ (‘-M’) is being used, the volume label matcher will
also suffix ARCHIVE-LABEL by ‘ Volume [1-9]*’ if the initial match
fails, before giving up.  Since the volume numbering is automatically
added in labels at creation time, it sounded logical to equally help the
user taking care of it when the archive is being read.

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

     $ tar -cM -f /dev/tape -V "Daily backup for `date +%Y-%m-%d`"
     $ tar --create --file=/dev/tape --multi-volume \
          --label="Daily backup for `date +%Y-%m-%d`"

   Some more notes about volume labels:

   • Each label has its own date and time, which corresponds to the time
     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 to get an idea of tape throughput is
     unreliable.  It gives correct results only if the delays for
     rewinding tapes and the operator switching them were negligible,
     which is usually not the case.

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

   (1) Until version 1.10, that option was called ‘--volume’, but is not
available under that name anymore.

   (2) Note that GNU ‘tar’ versions up to 1.23 indicated mismatch with
an exit code 2 and printed a spurious diagnostics on stderr.

   (3) 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: gnutar.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: gnutar.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: gnutar.info,  Node: Reliability and security,  Next: Changes,  Prev: Media,  Up: Top

10 Reliability and Security
***************************

The ‘tar’ command reads and writes files as any other application does,
and is subject to the usual caveats about reliability and security.
This section contains some commonsense advice on the topic.

* Menu:

* Reliability::
* Security::


File: gnutar.info,  Node: Reliability,  Next: Security,  Up: Reliability and security

10.1 Reliability
================

Ideally, when ‘tar’ is creating an archive, it reads from a file system
that is not being modified, and encounters no errors or inconsistencies
while reading and writing.  If this is the case, the archive should
faithfully reflect what was read.  Similarly, when extracting from an
archive, ideally ‘tar’ ideally encounters no errors and the extracted
files faithfully reflect what was in the archive.

   However, when reading or writing real-world file systems, several
things can go wrong; these include permissions problems, corruption of
data, and race conditions.

* Menu:

* Permissions problems::
* Data corruption and repair::
* Race conditions::


File: gnutar.info,  Node: Permissions problems,  Next: Data corruption and repair,  Up: Reliability

10.1.1 Permissions Problems
---------------------------

If ‘tar’ encounters errors while reading or writing files, it normally
reports an error and exits with nonzero status.  The work it does may
therefore be incomplete.  For example, when creating an archive, if
‘tar’ cannot read a file then it cannot copy the file into the archive.


File: gnutar.info,  Node: Data corruption and repair,  Next: Race conditions,  Prev: Permissions problems,  Up: Reliability

10.1.2 Data Corruption and Repair
---------------------------------

If an archive becomes corrupted by an I/O error, this may corrupt the
data in an extracted file.  Worse, it may corrupt the file's metadata,
which may cause later parts of the archive to become misinterpreted.  An
tar-format archive contains a checksum that most likely will detect
errors in the metadata, but it will not detect errors in the data.

   If data corruption is a concern, you can compute and check your own
checksums of an archive by using other programs, such as ‘cksum’.

   When attempting to recover from a read error or data corruption in an
archive, you may need to skip past the questionable data and read the
rest of the archive.  This requires some expertise in the archive format
and in other software tools.


File: gnutar.info,  Node: Race conditions,  Prev: Data corruption and repair,  Up: Reliability

10.1.3 Race conditions
----------------------

If some other process is modifying the file system while ‘tar’ is
reading or writing files, the result may well be inconsistent due to
race conditions.  For example, if another process creates some files in
a directory while ‘tar’ is creating an archive containing the
directory's files, ‘tar’ may see some of the files but not others, or it
may see a file that is in the process of being created.  The resulting
archive may not be a snapshot of the file system at any point in time.
If an application such as a database system depends on an accurate
snapshot, restoring from the ‘tar’ archive of a live file system may
therefore break that consistency and may break the application.  The
simplest way to avoid the consistency issues is to avoid making other
changes to the file system while tar is reading it or writing it.

   When creating an archive, several options are available to avoid race
conditions.  Some hosts have a way of snapshotting a file system, or of
temporarily suspending all changes to a file system, by (say) suspending
the only virtual machine that can modify a file system; if you use these
facilities and have ‘tar -c’ read from a snapshot when creating an
archive, you can avoid inconsistency problems.  More drastically, before
starting ‘tar’ you could suspend or shut down all processes other than
‘tar’ that have access to the file system, or you could unmount the file
system and then mount it read-only.

   When extracting from an archive, one approach to avoid race
conditions is to create a directory that no other process can write to,
and extract into that.


File: gnutar.info,  Node: Security,  Prev: Reliability,  Up: Reliability and security

10.2 Security
=============

In some cases ‘tar’ may be used in an adversarial situation, where an
untrusted user is attempting to gain information about or modify
otherwise-inaccessible files.  Dealing with untrusted data (that is,
data generated by an untrusted user) typically requires extra care,
because even the smallest mistake in the use of ‘tar’ is more likely to
be exploited by an adversary than by a race condition.

* Menu:

* Privacy::
* Integrity::
* Live untrusted data::
* Security rules of thumb::


File: gnutar.info,  Node: Privacy,  Next: Integrity,  Up: Security

10.2.1 Privacy
--------------

Standard privacy concerns apply when using ‘tar’.  For example, suppose
you are archiving your home directory into a file ‘/archive/myhome.tar’.
Any secret information in your home directory, such as your SSH secret
keys, are copied faithfully into the archive.  Therefore, if your home
directory contains any file that should not be read by some other user,
the archive itself should be not be readable by that user.  And even if
the archive's data are inaccessible to untrusted users, its metadata
(such as size or last-modified date) may reveal some information about
your home directory; if the metadata are intended to be private, the
archive's parent directory should also be inaccessible to untrusted
users.

   One precaution is to create ‘/archive’ so that it is not accessible
to any user, unless that user also has permission to access all the
files in your home directory.

   Similarly, when extracting from an archive, take care that the
permissions of the extracted files are not more generous than what you
want.  Even if the archive itself is readable only to you, files
extracted from it have their own permissions that may differ.


File: gnutar.info,  Node: Integrity,  Next: Live untrusted data,  Prev: Privacy,  Up: Security

10.2.2 Integrity
----------------

When creating archives, take care that they are not writable by a
untrusted user; otherwise, that user could modify the archive, and when
you later extract from the archive you will get incorrect data.

   When ‘tar’ extracts from an archive, by default it writes into files
relative to the working directory.  If the archive was generated by an
untrusted user, that user therefore can write into any file under the
working directory.  If the working directory contains a symbolic link to
another directory, the untrusted user can also write into any file under
the referenced directory.  When extracting from an untrusted archive, it
is therefore good practice to create an empty directory and run ‘tar’ in
that directory.

   When extracting from two or more untrusted archives, each one should
be extracted independently, into different empty directories.
Otherwise, the first archive could create a symbolic link into an area
outside the working directory, and the second one could follow the link
and overwrite data that is not under the working directory.  For
example, when restoring from a series of incremental dumps, the archives
should have been created by a trusted process, as otherwise the
incremental restores might alter data outside the working directory.

   If you use the ‘--absolute-names’ (‘-P’) option when extracting,
‘tar’ respects any file names in the archive, even file names that begin
with ‘/’ or contain ‘..’.  As this lets the archive overwrite any file
in your system that you can write, the ‘--absolute-names’ (‘-P’) option
should be used only for trusted archives.

   Conversely, with the ‘--keep-old-files’ (‘-k’) and ‘--skip-old-files’
options, ‘tar’ refuses to replace existing files when extracting.  The
difference between the two options is that the former treats existing
files as errors whereas the latter just silently ignores them.

   Finally, with the ‘--no-overwrite-dir’ option, ‘tar’ refuses to
replace the permissions or ownership of already-existing directories.
These options may help when extracting from untrusted archives.


File: gnutar.info,  Node: Live untrusted data,  Next: Security rules of thumb,  Prev: Integrity,  Up: Security

10.2.3 Dealing with Live Untrusted Data
---------------------------------------

Extra care is required when creating from or extracting into a file
system that is accessible to untrusted users.  For example, superusers
who invoke ‘tar’ must be wary about its actions being hijacked by an
adversary who is reading or writing the file system at the same time
that ‘tar’ is operating.

   When creating an archive from a live file system, ‘tar’ is vulnerable
to denial-of-service attacks.  For example, an adversarial user could
create the illusion of an indefinitely-deep directory hierarchy
‘d/e/f/g/...’ by creating directories one step ahead of ‘tar’, or the
illusion of an indefinitely-long file by creating a sparse file but
arranging for blocks to be allocated just before ‘tar’ reads them.
There is no easy way for ‘tar’ to distinguish these scenarios from
legitimate uses, so you may need to monitor ‘tar’, just as you'd need to
monitor any other system service, to detect such attacks.

   While a superuser is extracting from an archive into a live file
system, an untrusted user might replace a directory with a symbolic
link, in hopes that ‘tar’ will follow the symbolic link and extract data
into files that the untrusted user does not have access to.  Even if the
archive was generated by the superuser, it may contain a file such as
‘d/etc/passwd’ that the untrusted user earlier created in order to break
in; if the untrusted user replaces the directory ‘d/etc’ with a symbolic
link to ‘/etc’ while ‘tar’ is running, ‘tar’ will overwrite
‘/etc/passwd’.  This attack can be prevented by extracting into a
directory that is inaccessible to untrusted users.

   Similar attacks via symbolic links are also possible when creating an
archive, if the untrusted user can modify an ancestor of a top-level
argument of ‘tar’.  For example, an untrusted user that can modify
‘/home/eve’ can hijack a running instance of ‘tar -cf -
/home/eve/Documents/yesterday’ by replacing ‘/home/eve/Documents’ with a
symbolic link to some other location.  Attacks like these can be
prevented by making sure that untrusted users cannot modify any files
that are top-level arguments to ‘tar’, or any ancestor directories of
these files.


File: gnutar.info,  Node: Security rules of thumb,  Prev: Live untrusted data,  Up: Security

10.2.4 Security Rules of Thumb
------------------------------

This section briefly summarizes rules of thumb for avoiding security
pitfalls.

   • Protect archives at least as much as you protect any of the files
     being archived.

   • Extract from an untrusted archive only into an otherwise-empty
     directory.  This directory and its parent should be accessible only
     to trusted users.  For example:

          $ chmod go-rwx .
          $ mkdir -m go-rwx dir
          $ cd dir
          $ tar -xvf /archives/got-it-off-the-net.tar.gz

     As a corollary, do not do an incremental restore from an untrusted
     archive.

   • Do not let untrusted users access files extracted from untrusted
     archives without checking first for problems such as setuid
     programs.

   • Do not let untrusted users modify directories that are ancestors of
     top-level arguments of ‘tar’.  For example, while you are executing
     ‘tar -cf /archive/u-home.tar /u/home’, do not let an untrusted user
     modify ‘/’, ‘/archive’, or ‘/u’.

   • Pay attention to the diagnostics and exit status of ‘tar’.

   • When archiving live file systems, monitor running instances of
     ‘tar’ to detect denial-of-service attacks.

   • Avoid unusual options such as ‘--absolute-names’ (‘-P’),
     ‘--dereference’ (‘-h’), ‘--overwrite’, ‘--recursive-unlink’, and
     ‘--remove-files’ unless you understand their security implications.


File: gnutar.info,  Node: Changes,  Next: Recipes,  Prev: Reliability and security,  Up: Top

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

This appendix lists some important user-visible changes between various
versions of GNU ‘tar’.  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 the ‘--wildcards’
     option.  If you want to tar to mimic the behavior of versions prior
     to 1.15.91, add this option to your ‘TAR_OPTIONS’ variable.

     *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-formats: (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 implementations,
     it was declared deprecated in version 1.14.  However, to facilitate
     transition to its new semantics, it was supported by versions 1.15
     and 1.15.90.  The present use of ‘-l’ as a short variant of
     ‘--check-links’ was introduced in version 1.15.91.

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: gnutar.info,  Node: Recipes,  Next: Configuring Help Summary,  Prev: Changes,  Up: Top

Appendix B Recipes
******************

This appendix provides several recipes for performing common tasks using
GNU ‘tar’.

* Menu:

* copy directory hierarchy::
* intermediate directories::


File: gnutar.info,  Node: copy directory hierarchy,  Next: intermediate directories,  Up: Recipes

B.1 Copying directory hierarchies
=================================

This is a traditional way to copy a directory hierarchy preserving the
dates, modes, owners and link-structure of all the files therein.  It
was used back when the ‘cp’ command lacked the ‘-a’ option:

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

You can avoid subshells by using ‘-C’ option:

     $ tar -C sourcedir -cf - . | tar -C targetdir -xf -

The same command using long option forms:

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

or

     $ tar --directory sourcedir --create --file=- . \
            | tar --directory targetdir --extract --file=-


File: gnutar.info,  Node: intermediate directories,  Prev: copy directory hierarchy,  Up: Recipes

B.2 Restoring Intermediate Directories
======================================

A common concern is how to extract permissions and ownerships of
intermediate directories when extracting only selected members from the
archive.  To illustrate this, consider the following archive:

     # tar tvf A.tar
     drwxr-xr-x root/root         0 2017-11-16 14:39 foo/
     dr-xr-x--- gray/user         0 2017-11-16 14:39 foo/bar/
     -rw-r--r-- gray/user        10 2017-11-16 14:40 foo/bar/file

   Suppose you extract only the file ‘foo/bar/file’, while being ‘root’:

     # tar xvf A.tar foo/bar/file
     foo/bar/file

   Now, let's inspect the content of the created directories:

     # find foo -ls
     427257    0 drwxr-xr-x   3 root     root    16 Nov 17 16:10 foo
     427258    0 drwxr-xr-x   2 root     root    17 Nov 17 16:10 foo/bar
     427259    0 -rw-r--r--   1 gray     user    10 Nov  6 14:40 foo/bar/file

   The requested file is restored, including its ownership and
permissions.  The intermediate directories, however, are created with
the default permissions, current timestamp and owned by the current
user.  This is because by the time ‘tar’ has reached the requested file,
it had already skipped the entries for its parent directories, so it has
no iformation about their ownership and modes.

   To restore meta information about the intermediate directories,
you'll need to specify them explicitly in the command line and use the
‘--no-recursive’ option (*note recurse::) to avoid extracting their
content.

   To automate this process, ‘Neal P. Murphy’ proposed the following
shell script(1):

     #! /bin/sh
     (while read path
      do
        path=`dirname $path`
        while [ -n "$path" -a "$path" != "." ]
        do
          echo $path
          path=`dirname $path`
        done
      done < $2 | sort | uniq) |
      tar -x --no-recursion -v -f $1 -T - -T $2

   The script takes two arguments: the name of the archive file, and the
name of the file list file.

   To complete our example, the file list will contain single line:

     foo/bar/file

   Supposing its name is ‘file.list’ and the script is named
‘restore.sh’, you can invoke it as follows:

     # sh restore.sh A.tar file.list

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

   (1) The original version of the script can be seen at
<http://lists.gnu.org/archive/html/bug-tar/2016-11/msg00024.html>


File: gnutar.info,  Node: Configuring Help Summary,  Next: Fixing Snapshot Files,  Prev: Recipes,  Up: Top

Appendix C 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: gnutar.info,  Node: Fixing Snapshot Files,  Next: Tar Internals,  Prev: Configuring Help Summary,  Up: Top

Appendix D Fixing Snapshot Files
********************************

Various situations can cause device numbers to change: upgrading your
kernel version, reconfiguring your hardware, loading kernel modules in a
different order, using virtual volumes that are assembled dynamically
(such as with LVM or RAID), hot-plugging drives (e.g.  external USB or
Firewire drives), etc.  In the majority of cases this change is
unnoticed by the users.  However, it influences ‘tar’ incremental
backups: the device number is stored in tar snapshot files (*note
Snapshot Files::) and is used to determine whether the file has changed
since the last backup.  If the device numbers change for some reason, by
default the next backup you run will be a full backup.

   To minimize the impact in these cases, GNU ‘tar’ comes with the
‘tar-snapshot-edit’ utility for inspecting and updating device numbers
in snapshot files.  (The utility, written by Dustin J. Mitchell, is also
available from the GNU ‘tar’ home page
(http://www.gnu.org/software/tar/utils/tar-snapshot-edit.html).)

   To obtain a summary of the device numbers found in the snapshot file,
run

     $ tar-snapshot-edit SNAPFILE

where SNAPFILE is the name of the snapshot file (you can supply as many
files as you wish in a single command line).  You can then compare the
numbers across snapshot files, or against those currently in use on the
live filesystem (using ‘ls -l’ or ‘stat’).

   Assuming the device numbers have indeed changed, it's often possible
to simply tell GNU ‘tar’ to ignore the device number when processing the
incremental snapshot files for these backups, using the
‘--no-check-device’ option (*note device numbers::).

   Alternatively, you can use the ‘tar-edit-snapshot’ script's ‘-r’
option to update all occurrences of the given device number in the
snapshot file(s).  It takes a single argument of the form
‘OLDDEV-NEWDEV’, where OLDDEV is the device number used in the snapshot
file, and NEWDEV is the corresponding new device number.  Both numbers
may be specified in hex (e.g., ‘0xfe01’), decimal (e.g., ‘65025’), or as
a major:minor number pair (e.g., ‘254:1’).  To change several device
numbers at once, specify them in a single comma-separated list, as in
‘-r 0x3060-0x4500,0x307-0x4600’.

   Before updating the snapshot file, it is a good idea to create a
backup copy of it.  This is accomplished by ‘-b’ option.  The name of
the backup file is obtained by appending ‘~’ to the original file name.

   An example session:
     $ tar-snapshot-edit root_snap.0 boot_snap.0
     File: root_snap.0
       Detected snapshot file version: 2

       Device 0x0000 occurs 1 times.
       Device 0x0003 occurs 1 times.
       Device 0x0005 occurs 1 times.
       Device 0x0013 occurs 1 times.
       Device 0x6801 occurs 1 times.
       Device 0x6803 occurs 6626 times.
       Device 0xfb00 occurs 1 times.

     File: boot_snap.0
       Detected snapshot file version: 2

       Device 0x6801 occurs 3 times.
     $ tar-snapshot-edit -b -r 0x6801-0x6901,0x6803-0x6903 root_snap.0 boot_snap.0
     File: root_snap.0
       Detected snapshot file version: 2

       Updated 6627 records.

     File: boot_snap.0
       Detected snapshot file version: 2

       Updated 3 records.


File: gnutar.info,  Node: Tar Internals,  Next: Genfile,  Prev: Fixing Snapshot Files,  Up: Top

Appendix E Tar Internals
************************

* Menu:

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


File: gnutar.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’ does not treat missing end-of-file marker as an
error and silently ignores the fact.  You can instruct it to issue a
warning, however, by using the ‘--warning=missing-zero-blocks’ option
(*note missing-zero-blocks: General Warnings.).

   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'

     /* Jörg 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.  (In the extended GNU format, the numeric fields can
take other forms.)

   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; for archive
members that are symbolic or hard links to another file, this field is
specified as zero.

   The ‘mtime’ field represents the data modification time of the file
at the time it was archived.  It represents the integer number of
seconds since January 1, 1970, 00:00 Coordinated Universal Time.

   The ‘chksum’ field represents 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 filled with spaces (ASCII 32).

   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: gnutar.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.

   For fields containing numbers or timestamps that are out of range for
the basic format, the GNU format uses a base-256 representation instead
of an ASCII octal number.  If the leading byte is 0xff (255), all the
bytes of the field (including the leading byte) are concatenated in
big-endian order, with the result being a negative number expressed in
two's complement form.  If the leading byte is 0x80 (128), the
non-leading bytes of the field are concatenated in big-endian order,
with the result being a positive number expressed in binary form.
Leading bytes other than 0xff, 0x80 and ASCII octal digits are reserved
for future use, as are base-256 representations of values that would be
in range for the basic format.

   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: gnutar.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 flaws, either and it was subsequently improved in versions 1.15.2
(November, 2005) and 1.15.92 (June, 2006).

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

   The following subsections describe each format in detail.

* Menu:

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


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

Old GNU Format
--------------

The format introduced in November 1990 (v.  1.09) was designed on top of
standard ‘ustar’ headers in such an unfortunate way that some of its
fields overwrote fields required by POSIX.

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

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

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

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

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

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

   A header with ‘isextended=0’ ends the map.


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

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's ‘tar’
     results in extraction of sparse files in _condensed form_.  If the
     ‘tar’ implementation in question does not support POSIX format, it
     will also extract a file containing extension header attributes.
     This file can be used to expand the file to its original state.
     However, posix-aware ‘tar’s will usually ignore the unknown
     variables, which makes restoring the file more difficult.  *Note
     Extraction of sparse members in v.0.0 format: extracting sparse
     v0x, 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
v0x, for the detailed description of how to restore such members using
non-GNU ‘tar’s.

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


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

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 decimal numbers delimited
by newlines.  The map is padded with nulls to the nearest block
boundary.

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

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


File: gnutar.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 1.35 supports three snapshot file formats.  The
first format, called “format 0”, is the one used by GNU ‘tar’ versions
up to and including 1.15.1.  The second format, called “format 1” is an
extended version of this format, that contains more metadata and allows
for further extensions.  It was used by alpha release version 1.15.90.
For alpha version 1.15.91 and stable releases version 1.16 up through
1.35, the “format 2” is used.

   GNU ‘tar’ is able to read all three formats, but will create
snapshots only in format 2.

   This appendix describes all three formats in detail.

  0. ‘Format 0’ snapshot file begins with a line containing a decimal
     number that represents a 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:

     NFS
          A single plus character (‘+’), if this directory is located on
          an NFS-mounted partition, otherwise empty.

          (That is, for non-NFS directories, the first character on the
          description line contains the start of the DEV field.)

     DEV
          Device number of the directory;

     INODE
          I-node number of the directory;

     NAME
          Name of the directory.  Any special characters (white-space,
          backslashes, etc.)  are quoted.

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

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

     Lines that follow contain directory metadata, one line per
     directory.  Each line is formatted as follows:

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

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

  2. ‘Format 2’ snapshot file begins with a format identifier, as
     described for version 1, e.g.:

          GNU tar-1.35-2

     This line is followed by newline.  Rest of file consists of
     records, separated by null (ASCII 0) characters.  Thus, in contrast
     to the previous formats, format 2 snapshot is a binary file.

     First two records are decimal integers, representing the time of
     the last backup.  First number is the number of seconds, the second
     one is the number of nanoseconds, since the beginning of the epoch.
     These are followed by arbitrary number of directory records.

     Each “directory record” contains a set of metadata describing a
     particular directory.  Parts of a directory record are delimited
     with ASCII 0 characters.  The following table describes each part.
     The “Number” type in this table stands for a decimal integer in
     ASCII notation.  (Negative values are preceded with a "-"
     character, while positive values have no leading punctuation.)

     Field              Type        Description
     ---------------------------------------------------------------------------
     nfs                Character   ‘1’ if the directory is located on an
                                    NFS-mounted partition, or ‘0’ otherwise;
     timestamp_sec      Number      Modification time, seconds;
     timestamp_nsec     Number      Modification time, nanoseconds;
     dev                Number      Device number;
     ino                Number      I-node number;
     name               String      Directory name; in contrast to the
                                    previous versions it is not quoted;
     contents           Dumpdir     Contents of the directory;
                                    *Note Dumpdir::, for a description of its
                                    format.

     Dumpdirs stored in snapshot files contain only records of types
     ‘Y’, ‘N’ and ‘D’.

     The specific range of values allowed in each of the “Number” fields
     depends on the underlying C datatypes as determined when ‘tar’ is
     compiled.  To see the specific ranges allowed for a particular
     ‘tar’ binary, you can use the ‘--show-snapshot-field-ranges’
     option:

          $ tar --show-snapshot-field-ranges
          This tar's snapshot file field ranges are
             (field name      => [ min, max ]):

              nfs             => [ 0, 1 ],
              timestamp_sec   => [ -9223372036854775808, 9223372036854775807 ],
              timestamp_nsec  => [ 0, 999999999 ],
              dev             => [ 0, 18446744073709551615 ],
              ino             => [ 0, 18446744073709551615 ],

     (This example is from a GNU/Linux x86_64 system.)


File: gnutar.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 ‘T’ command, that immediately follows it.

‘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:

     Rsource\0Tdest\0

which means "rename file ‘source’ to file ‘dest’".

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

  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 the existing directory.  To correctly
process it, GNU ‘tar’ needs a temporary directory, so it creates the
following dumpdir (newlines have been added for readability):

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

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

   The exact placement of a dumpdir in the archive depends on the
archive format (*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: gnutar.info,  Node: Genfile,  Next: GNU Free Documentation License,  Prev: Tar Internals,  Up: Top

Appendix F 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 four 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.

Set File Time
     Set last access and modification times of files given in the
     command line.

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.
* Set File Time::       Set File Time Mode.
* Exec Mode::           Synchronous Execution Mode.


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

F.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”, that
specifies 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.

   Contents string can be either a fragment size or a pattern.  Fragment
size is a decimal number, prefixed with an equals sign.  It can be
suffixed with a quantifier, as discussed above.  If fragment size is
given, the fragment of that size will be filled with the currently
selected pattern (*note -pattern: Generate Mode.) and written to the
file.

   A pattern is a string of arbitrary ASCII characters.  For each of
them, ‘genfile’ will generate a “block” of data, filled with that
character and will write it to the fragment.  The size of block is given
by ‘--block-size’ option.  It defaults to 512.  Thus, if pattern
consists of N characters, the resulting file fragment will contain
‘N*BLOCK-SIZE’ bytes of data.

   The 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.

   A dash appearing as a fragment descriptor instructs ‘genfile’ to read
file map from the standard input.  Each line of input should consist of
fragment offset and contents string, separated by any amount of
whitespace.

   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 --sparse’ command is ‘0’ only if created
file is actually sparse.  If it is not, the appropriate error message is
displayed and the command exists with code ‘1’.  The ‘--quite’ (‘-q’)
option suppresses this behavior.  If ‘--quite’ is given, ‘genfile
--sparse’ exits with code ‘0’ if it was able to create the file, whether
the resulting file is sparse or not.


File: gnutar.info,  Node: Status Mode,  Next: Set File Time,  Prev: Generate Mode,  Up: Genfile

F.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 *

   By default, ‘genfile’ follows symbolic links and returns information
about files pointed to by them.  To get information about the symlink
files themselves, use the ‘--no-dereference’ (‘-h’) option.


File: gnutar.info,  Node: Set File Time,  Next: Exec Mode,  Prev: Status Mode,  Up: Genfile

F.3 Set File Time
=================

This mode is requested by the ‘--set-time’ (‘-t’) command line option.
In this mode ‘genfile’ operates similarly to the ‘touch’ command: for
each file listed in the command line, it sets its access and
modification times to the current timestamp or to the value given with
the ‘--date’ option.  The ‘--date’ option takes a date specification in
an almost arbitrary format as its argument (*note Date input formats::),
e.g.:

     genfile --set-time --date='2 days ago' a b c

   By default, ‘genfile’ follows symbolic links and sets times of the
file they point to.  This can be changed by supplying the
‘--no-dereference’ (‘-h’) option: if it is given, ‘genfile’ will change
access and modification times of the symbolic link itself.  Notice, that
not all operating systems allow this.


File: gnutar.info,  Node: Exec Mode,  Prev: Set File Time,  Up: Genfile

F.4 Exec Mode
=============

This mode is designed for testing the behavior of ‘paxutils’ commands
when some of the files change during archiving.  It supposes that the
command being executed supports ‘--checkpoint’ and ‘--checkpoint-action’
options (*note Checkpoints: (gnutar)checkpoints.).

   The ‘Exec Mode’ is enabled by ‘--run’ command line option (or its
alias ‘-r’).  The non-optional arguments supply the command line to be
executed.  ‘Genfile’ modifies this command line by inserting the
following options between the command name and first argument:

     --checkpoint=N
     --checkpoint-action "echo=genfile checkpoint %u"
     --checkpoint-action "wait=SIGUSR1"

   Here, N stands for the checkpoint granularity (for GNU ‘tar’, it is
the number of archive records read or written between each pair of
checkpoints).  The default value is 1.  This value can be changed using
the optional argument to the ‘--run’ option.  For example, to run
actions on each 10th checkpoint:

     genfile --run=10 ...

   If the command line contains options, it must be preceded by a
double-dash (‘--’), which will prevent these options from being
interpreted by ‘genfile’ itself.  For example:

     genfile --run --checkpoint=2 --truncate foo -- tar -c -f a.tar .

   Notice also, that when running ‘tar’, its command line may not
contain traditional options (cluster of letters without dash).

   A set of options is provided for defining checkpoint values and
actions to be executed upon reaching them.  Checkpoint values are
introduced with the ‘--checkpoint’ command line option.  Argument to
this option is the number of checkpoint in decimal.

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

‘--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::).

‘-h’
‘--no-dereference’
     Modifies the action of the ‘--touch’ option.  If both options are
     given and FILE argument to the ‘--touch’ names a symbolic link,
     ‘genfile’ will modify access and modification times of the symbolic
     link file itself, instead the file the symlink points to.

‘--exec COMMAND’
     Execute given shell command.

‘--delete FILE’
‘--unlink FILE’
     Delete the named file or directory.  If deleting the directory, it
     must be empty.

   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.

   In exec mode, ‘genfile’ exits with the exit status of the executed
command.


File: gnutar.info,  Node: GNU Free Documentation License,  Next: Index of Command Line Options,  Prev: Genfile,  Up: Top

Appendix G GNU Free Documentation License
*****************************************

                     Version 1.3, 3 November 2008

     Copyright © 2000-2002, 2007-2008, 2022 Free Software
     Foundation, Inc.
     <https://fsf.org/>

     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
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     assure everyone the effective freedom to copy and redistribute it,
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     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
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====================================================

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File: gnutar.info,  Node: Index of Command Line Options,  Next: Index,  Prev: GNU Free Documentation License,  Up: Top

Appendix H 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, see *note Short Option
Summary::.

[index]
* Menu:

* absolute-names:                        absolute.           (line   10)
* absolute-names, summary:               Option Summary.     (line    6)
* acls, summary:                         Option Summary.     (line   14)
* add-file:                              files.              (line   83)
* after-date:                            after.              (line   24)
* after-date, summary:                   Option Summary.     (line   17)
* anchored:                              controlling pattern-matching.
                                                             (line   78)
* anchored, summary:                     Option Summary.     (line   21)
* append:                                append.             (line    6)
* append <1>:                            appending files.    (line    6)
* append, summary:                       Operation Summary.  (line    6)
* atime-preserve:                        Attributes.         (line   10)
* atime-preserve, summary:               Option Summary.     (line   25)
* auto-compress:                         gzip.               (line  161)
* auto-compress, summary:                Option Summary.     (line   71)
* backup:                                backup.             (line   41)
* backup, summary:                       Option Summary.     (line   78)
* block-number:                          verbose.            (line  117)
* block-number, summary:                 Option Summary.     (line   84)
* blocking-factor:                       Blocking Factor.    (line    8)
* blocking-factor, summary:              Option Summary.     (line   91)
* bzip2, summary:                        Option Summary.     (line   97)
* catenate:                              concatenate.        (line    6)
* catenate, summary:                     Operation Summary.  (line   11)
* check-device, described:               Incremental Dumps.  (line  107)
* check-device, summary:                 Option Summary.     (line  103)
* check-links, described:                hard links.         (line   31)
* check-links, summary:                  Option Summary.     (line  161)
* checkpoint:                            checkpoints.        (line    6)
* checkpoint-action:                     checkpoints.        (line    6)
* checkpoint-action, defined:            checkpoints.        (line   22)
* checkpoint-action, summary:            Option Summary.     (line  117)
* checkpoint, defined:                   checkpoints.        (line   13)
* checkpoint, summary:                   Option Summary.     (line  108)
* clamp-mtime, summary:                  Option Summary.     (line  178)
* compare:                               compare.            (line    6)
* compare, summary:                      Operation Summary.  (line   16)
* compress:                              gzip.               (line  124)
* compress, summary:                     Option Summary.     (line  170)
* concatenate:                           concatenate.        (line    6)
* concatenate, summary:                  Operation Summary.  (line   23)
* confirmation, summary:                 Option Summary.     (line  182)
* create, additional options:            create options.     (line    6)
* create, complementary notes:           Basic tar.          (line   11)
* create, introduced:                    Creating the archive.
                                                             (line    6)
* create, summary:                       Operation Summary.  (line   29)
* create, using with --verbose:          create verbose.     (line    6)
* create, using with --verify:           verify.             (line   24)
* delay-directory-restore:               Directory Modification Times and Permissions.
                                                             (line   62)
* delay-directory-restore, summary:      Option Summary.     (line  186)
* delete:                                delete.             (line    6)
* delete, summary:                       Operation Summary.  (line   34)
* delete, using before -append:          append.             (line   47)
* dereference:                           dereference.        (line    6)
* dereference, summary:                  Option Summary.     (line  192)
* diff, summary:                         Operation Summary.  (line   39)
* directory:                             directory.          (line   11)
* directory, summary:                    Option Summary.     (line  199)
* exclude:                               exclude.            (line    6)
* exclude <1>:                           exclude.            (line    9)
* exclude-backups:                       exclude.            (line  114)
* exclude-backups, summary:              Option Summary.     (line  212)
* exclude-caches:                        exclude.            (line  134)
* exclude-caches-all:                    exclude.            (line  142)
* exclude-caches-all, summary:           Option Summary.     (line  236)
* exclude-caches-under:                  exclude.            (line  138)
* exclude-caches-under, summary:         Option Summary.     (line  229)
* exclude-caches, summary:               Option Summary.     (line  221)
* exclude-from:                          exclude.            (line    6)
* exclude-from <1>:                      exclude.            (line   20)
* exclude-from, summary:                 Option Summary.     (line  215)
* exclude-ignore:                        exclude.            (line   76)
* exclude-ignore-recursive:              exclude.            (line   81)
* exclude-ignore-recursive, summary:     Option Summary.     (line  246)
* exclude-ignore, summary:               Option Summary.     (line  241)
* exclude-tag:                           exclude.            (line  151)
* exclude-tag-all:                       exclude.            (line  159)
* exclude-tag-all, summary:              Option Summary.     (line  263)
* exclude-tag-under:                     exclude.            (line  155)
* exclude-tag-under, summary:            Option Summary.     (line  257)
* exclude-tag, summary:                  Option Summary.     (line  251)
* exclude-vcs:                           exclude.            (line   85)
* exclude-vcs-ignores:                   exclude.            (line   42)
* exclude-vcs-ignores, summary:          Option Summary.     (line  275)
* exclude-vcs, summary:                  Option Summary.     (line  268)
* exclude, potential problems with:      problems with exclude.
                                                             (line    6)
* exclude, summary:                      Option Summary.     (line  207)
* extract:                               extract.            (line    6)
* extract, additional options:           extract options.    (line    6)
* extract, complementary notes:          Basic tar.          (line   49)
* extract, summary:                      Operation Summary.  (line   44)
* extract, using with --listed-incremental: Incremental Dumps.
                                                             (line  120)
* file:                                  file.               (line    6)
* file, short description:               file.               (line   15)
* file, summary:                         Option Summary.     (line  283)
* file, tutorial:                        file tutorial.      (line    6)
* files-from:                            files.              (line   14)
* files-from, summary:                   Option Summary.     (line  290)
* force-local, short description:        Device.             (line   70)
* force-local, summary:                  Option Summary.     (line  297)
* format, summary:                       Option Summary.     (line  303)
* full-time, summary:                    Option Summary.     (line  328)
* get, summary:                          Operation Summary.  (line   50)
* group:                                 override.           (line   99)
* group-map, summary:                    Option Summary.     (line  356)
* group, summary:                        Option Summary.     (line  346)
* gunzip, summary:                       Option Summary.     (line  366)
* gzip:                                  gzip.               (line   99)
* gzip, summary:                         Option Summary.     (line  366)
* hard-dereference, described:           hard links.         (line   59)
* hard-dereference, summary:             Option Summary.     (line  375)
* help:                                  help tutorial.      (line    6)
* help, introduction:                    help.               (line   26)
* help, summary:                         Option Summary.     (line  381)
* hole-detection:                        sparse.             (line   66)
* hole-detection, summary:               Option Summary.     (line  387)
* ignore-case:                           controlling pattern-matching.
                                                             (line   85)
* ignore-case, summary:                  Option Summary.     (line  392)
* ignore-command-error:                  Writing to an External Program.
                                                             (line  111)
* ignore-command-error, summary:         Option Summary.     (line  396)
* ignore-failed-read:                    Ignore Failed Read. (line    7)
* ignore-failed-read, summary:           Option Summary.     (line  400)
* ignore-zeros:                          Ignore Zeros.       (line    6)
* ignore-zeros, short description:       Blocking Factor.    (line  152)
* ignore-zeros, summary:                 Option Summary.     (line  405)
* incremental, summary:                  Option Summary.     (line  413)
* incremental, using with --list:        Incremental Dumps.  (line  185)
* index-file, summary:                   Option Summary.     (line  421)
* info-script:                           Multi-Volume Archives.
                                                             (line   83)
* info-script, short description:        Device.             (line  121)
* info-script, summary:                  Option Summary.     (line  425)
* interactive:                           interactive.        (line   14)
* interactive, summary:                  Option Summary.     (line  434)
* keep-directory-symlink, summary:       Option Summary.     (line  442)
* keep-newer-files:                      Keep Newer Files.   (line    6)
* keep-newer-files, summary:             Option Summary.     (line  456)
* keep-old-files:                        Keep Old Files.     (line    9)
* keep-old-files, introduced:            Dealing with Old Files.
                                                             (line   16)
* keep-old-files, summary:               Option Summary.     (line  461)
* label:                                 Tape Files.         (line    6)
* label <1>:                             label.              (line    6)
* label, summary:                        Option Summary.     (line  470)
* level, described:                      Incremental Dumps.  (line   75)
* level, summary:                        Option Summary.     (line  478)
* list:                                  list.               (line    6)
* list, summary:                         Operation Summary.  (line   55)
* list, using with --incremental:        Incremental Dumps.  (line  185)
* list, using with --listed-incremental: Incremental Dumps.  (line  185)
* list, using with --verbose:            list.               (line   33)
* list, using with file name arguments:  list.               (line   25)
* listed-incremental, described:         Incremental Dumps.  (line   14)
* listed-incremental, summary:           Option Summary.     (line  488)
* listed-incremental, using with --extract: Incremental Dumps.
                                                             (line  120)
* listed-incremental, using with --list: Incremental Dumps.  (line  185)
* lzip:                                  gzip.               (line  112)
* lzip, summary:                         Option Summary.     (line  497)
* lzma:                                  gzip.               (line  115)
* lzma, summary:                         Option Summary.     (line  502)
* lzop:                                  gzip.               (line  118)
* mode:                                  override.           (line   14)
* mode, summary:                         Option Summary.     (line  512)
* mtime:                                 override.           (line   30)
* mtime, summary:                        Option Summary.     (line  519)
* multi-volume:                          Multi-Volume Archives.
                                                             (line    6)
* multi-volume, short description:       Device.             (line   88)
* multi-volume, summary:                 Option Summary.     (line  534)
* new-volume-script:                     Multi-Volume Archives.
                                                             (line   83)
* new-volume-script, short description:  Device.             (line  121)
* new-volume-script, summary:            Option Summary.     (line  425)
* new-volume-script, summary <1>:        Option Summary.     (line  540)
* newer:                                 after.              (line   24)
* newer-mtime:                           after.              (line   35)
* newer-mtime, summary:                  Option Summary.     (line  553)
* newer, summary:                        Option Summary.     (line  544)
* no-acls, summary:                      Option Summary.     (line  559)
* no-anchored:                           controlling pattern-matching.
                                                             (line   78)
* no-anchored, summary:                  Option Summary.     (line  563)
* no-auto-compress, summary:             Option Summary.     (line  567)
* no-check-device, described:            Incremental Dumps.  (line  103)
* no-check-device, summary:              Option Summary.     (line  572)
* no-delay-directory-restore:            Directory Modification Times and Permissions.
                                                             (line   68)
* no-delay-directory-restore, summary:   Option Summary.     (line  577)
* no-ignore-case:                        controlling pattern-matching.
                                                             (line   85)
* no-ignore-case, summary:               Option Summary.     (line  583)
* no-ignore-command-error:               Writing to an External Program.
                                                             (line  116)
* no-ignore-command-error, summary:      Option Summary.     (line  586)
* no-null, described:                    nul.                (line   15)
* no-null, summary:                      Option Summary.     (line  590)
* no-overwrite-dir, summary:             Option Summary.     (line  596)
* no-quote-chars, summary:               Option Summary.     (line  601)
* no-recursion:                          recurse.            (line   11)
* no-recursion, summary:                 Option Summary.     (line  606)
* no-same-owner:                         Attributes.         (line   63)
* no-same-owner, summary:                Option Summary.     (line  611)
* no-same-permissions, summary:          Option Summary.     (line  618)
* no-seek, summary:                      Option Summary.     (line  624)
* no-selinux, summary:                   Option Summary.     (line  630)
* no-unquote:                            Selecting Archive Members.
                                                             (line   42)
* no-unquote, summary:                   Option Summary.     (line  634)
* no-verbatim-files-from:                files.              (line   79)
* no-verbatim-files-from, summary:       Option Summary.     (line  638)
* no-wildcards:                          controlling pattern-matching.
                                                             (line   41)
* no-wildcards-match-slash:              controlling pattern-matching.
                                                             (line   91)
* no-wildcards-match-slash, summary:     Option Summary.     (line  655)
* no-wildcards, summary:                 Option Summary.     (line  652)
* no-xattrs, summary:                    Option Summary.     (line  658)
* null, described:                       nul.                (line   11)
* null, summary:                         Option Summary.     (line  662)
* numeric-owner:                         Attributes.         (line   69)
* numeric-owner, summary:                Option Summary.     (line  675)
* occurrence, described:                 append.             (line   34)
* occurrence, summary:                   Option Summary.     (line  693)
* old-archive, summary:                  Option Summary.     (line  708)
* one-file-system:                       one.                (line   14)
* one-file-system, summary:              Option Summary.     (line  711)
* one-top-level, summary:                Option Summary.     (line  716)
* overwrite:                             Overwrite Old Files.
                                                             (line    6)
* overwrite-dir:                         Overwrite Old Files.
                                                             (line   28)
* overwrite-dir, introduced:             Dealing with Old Files.
                                                             (line    6)
* overwrite-dir, summary:                Option Summary.     (line  732)
* overwrite, introduced:                 Dealing with Old Files.
                                                             (line   32)
* overwrite, summary:                    Option Summary.     (line  727)
* owner:                                 override.           (line   67)
* owner-map, summary:                    Option Summary.     (line  747)
* owner, summary:                        Option Summary.     (line  737)
* pax-option:                            PAX keywords.       (line    6)
* pax-option, summary:                   Option Summary.     (line  757)
* portability, summary:                  Option Summary.     (line  763)
* posix, summary:                        Option Summary.     (line  767)
* preserve-order:                        Same Order.         (line    6)
* preserve-order, summary:               Option Summary.     (line  770)
* preserve-permissions:                  Setting Access Permissions.
                                                             (line   10)
* preserve-permissions, short description: Attributes.       (line  109)
* preserve-permissions, summary:         Option Summary.     (line  774)
* quote-chars, summary:                  Option Summary.     (line  785)
* quoting-style:                         quoting styles.     (line   36)
* quoting-style, summary:                Option Summary.     (line  789)
* read-full-records:                     Reading.            (line    6)
* read-full-records <1>:                 read full records.  (line    6)
* read-full-records, short description:  Blocking Factor.    (line  168)
* read-full-records, summary:            Option Summary.     (line  796)
* record-size, summary:                  Option Summary.     (line  802)
* recursion:                             recurse.            (line   22)
* recursion, summary:                    Option Summary.     (line  810)
* recursive-unlink:                      Recursive Unlink.   (line    6)
* recursive-unlink, summary:             Option Summary.     (line  815)
* remove-files:                          remove files.       (line    6)
* remove-files, summary:                 Option Summary.     (line  820)
* restrict, summary:                     Option Summary.     (line  825)
* rmt-command, summary:                  Option Summary.     (line  831)
* rsh-command:                           Device.             (line   73)
* rsh-command, summary:                  Option Summary.     (line  836)
* same-order:                            Same Order.         (line    6)
* same-order, summary:                   Option Summary.     (line  841)
* same-owner:                            Attributes.         (line   44)
* same-owner, summary:                   Option Summary.     (line  850)
* same-permissions:                      Setting Access Permissions.
                                                             (line   10)
* same-permissions, short description:   Attributes.         (line  109)
* same-permissions, summary:             Option Summary.     (line  774)
* same-permissions, summary <1>:         Option Summary.     (line  857)
* seek, summary:                         Option Summary.     (line  861)
* selinux, summary:                      Option Summary.     (line  871)
* show-defaults:                         defaults.           (line    6)
* show-defaults, summary:                Option Summary.     (line  875)
* show-omitted-dirs:                     verbose.            (line  110)
* show-omitted-dirs, summary:            Option Summary.     (line  888)
* show-snapshot-field-ranges:            Snapshot Files.     (line  111)
* show-snapshot-field-ranges, summary:   Option Summary.     (line  893)
* show-stored-names:                     list.               (line   67)
* show-stored-names, summary:            Option Summary.     (line  899)
* show-transformed-names:                transform.          (line   45)
* show-transformed-names, summary:       Option Summary.     (line  899)
* skip-old-files, introduced:            Dealing with Old Files.
                                                             (line   28)
* skip-old-files, summary:               Option Summary.     (line  908)
* sort, summary:                         Option Summary.     (line  921)
* sparse:                                sparse.             (line   24)
* sparse-version:                        sparse.             (line   59)
* sparse-version, summary:               Option Summary.     (line  944)
* sparse, summary:                       Option Summary.     (line  938)
* starting-file:                         Starting File.      (line    6)
* starting-file, summary:                Option Summary.     (line  950)
* strip-components:                      transform.          (line   25)
* strip-components, summary:             Option Summary.     (line  957)
* suffix:                                backup.             (line   67)
* suffix, summary:                       Option Summary.     (line  968)
* tape-length:                           Multi-Volume Archives.
                                                             (line   33)
* tape-length, short description:        Device.             (line   96)
* tape-length, summary:                  Option Summary.     (line  973)
* test-label:                            label.              (line   35)
* test-label, summary:                   Option Summary.     (line  983)
* to-command:                            Writing to an External Program.
                                                             (line    9)
* to-command, summary:                   Option Summary.     (line  988)
* to-stdout:                             Writing to Standard Output.
                                                             (line   14)
* to-stdout, summary:                    Option Summary.     (line  993)
* totals:                                verbose.            (line   45)
* totals, summary:                       Option Summary.     (line  999)
* touch:                                 Data Modification Times.
                                                             (line   15)
* touch <1>:                             Attributes.         (line   33)
* touch, summary:                        Option Summary.     (line 1005)
* transform:                             transform.          (line   74)
* transform, summary:                    Option Summary.     (line 1012)
* uncompress:                            gzip.               (line  124)
* uncompress, summary:                   Option Summary.     (line  170)
* uncompress, summary <1>:               Option Summary.     (line 1026)
* ungzip:                                gzip.               (line   99)
* ungzip, summary:                       Option Summary.     (line  366)
* ungzip, summary <1>:                   Option Summary.     (line 1030)
* unlink-first:                          Unlink First.       (line    6)
* unlink-first, introduced:              Dealing with Old Files.
                                                             (line   51)
* unlink-first, summary:                 Option Summary.     (line 1034)
* unquote:                               Selecting Archive Members.
                                                             (line   39)
* unquote, summary:                      Option Summary.     (line 1040)
* update:                                update.             (line    6)
* update <1>:                            how to update.      (line    6)
* update, summary:                       Operation Summary.  (line   60)
* usage:                                 help.               (line   53)
* use-compress-program:                  gzip.               (line  185)
* use-compress-program, summary:         Option Summary.     (line 1044)
* utc, summary:                          Option Summary.     (line 1050)
* verbatim-files-from:                   files.              (line   74)
* verbatim-files-from, summary:          Option Summary.     (line 1055)
* verbose:                               verbose.            (line   18)
* verbose, introduced:                   verbose tutorial.   (line    6)
* verbose, summary:                      Option Summary.     (line 1077)
* verbose, using with --create:          create verbose.     (line    6)
* verbose, using with --list:            list.               (line   33)
* verify, short description:             verify.             (line    8)
* verify, summary:                       Option Summary.     (line 1085)
* verify, using with --create:           verify.             (line   24)
* version:                               help.               (line    6)
* version, summary:                      Option Summary.     (line 1091)
* volno-file:                            Multi-Volume Archives.
                                                             (line   74)
* volno-file, summary:                   Option Summary.     (line 1097)
* warning, explained:                    warnings.           (line   12)
* warning, summary:                      Option Summary.     (line 1103)
* wildcards:                             controlling pattern-matching.
                                                             (line   38)
* wildcards-match-slash:                 controlling pattern-matching.
                                                             (line   91)
* wildcards-match-slash, summary:        Option Summary.     (line 1113)
* wildcards, summary:                    Option Summary.     (line 1109)
* xattrs-exclude, summary:               Option Summary.     (line 1120)
* xattrs-include, summary:               Option Summary.     (line 1124)
* xattrs, summary:                       Option Summary.     (line 1116)
* xform:                                 transform.          (line   74)
* xform, summary:                        Option Summary.     (line 1012)
* xz:                                    gzip.               (line  104)
* xz, summary:                           Option Summary.     (line 1130)
* zstd:                                  gzip.               (line  121)

