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This is paxutils.info, produced by makeinfo version 3.12i from
paxutils.texi.
START-INFO-DIR-ENTRY
* pax utilities: (paxutils). pax and other archiving utilities.
* cpio: (paxutils)cpio invocation. Handling cpio archives.
* pax: (paxutils)pax invocation. The POSIX archiver.
* tar: (paxutils)tar invocation. Making tape (or disk) archives.
* mt: (paxutils)mt invocation. Basic tape positioning.
* rmt: (paxutils)rmt invocation. The remote tape facility.
END-INFO-DIR-ENTRY
This file documents `paxutils' 2.4i.
Copyright (C) 1992, 1994, 1995, 1996, 1997, 1998 Free Software
Foundation, Inc.
Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.
Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, provided that
the entire resulting derived work is distributed under the terms of a
permission notice identical to this one.
Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions, except that this permission notice may be stated in a
translation approved by the Foundation.
File: paxutils.info, Node: backup, Next: suffix, Prev: to-stdout, Up: Preventing overwrite
Backup options
..............
`tar' offers options for making backups of files before writing new
versions. These options control the details of these backups. They may
apply to the archive itself before it is created or rewritten, as well
as to individual extracted members. Other free programs (GNU `cp',
`install', `ln', and `mv', for example) offer similar options.
Backup options may prove unexpectedly useful when extracting archives
containing many members having identical names, or when extracting
archives on systems having file name limitations, making different
members appear as having similar names through the side-effect of name
truncation. (This is true only if we have a good scheme for truncated
backup names, of which we are not sure at all: we suspect work is
needed in this area. The MS-DOS/MS-Windows version works with numbered
backups even when file names are truncated.) When any existing file is
backed up before being overwritten by extraction, clashing files are
automatically renamed to be unique, and the true name is kept for only
the last file of a series of clashing files. By using verbose mode,
users may track exactly what happens. We recommend that you *always*
use numbered backups when unpacking archives on MS-DOS and on other
systems that have file name limitations.
When `tar' is given the `--backup'[=METHOD] option, it makes backups
of files that are about to be overwritten or removed. Without this
option, the original versions are destroyed. However, *Note
keep-old-files::.
At the detail level, some decisions are still experimental, and may
change in the future. We are awaiting comments from our users. So
please do not get in the habit of depending blindly on the details of
the backup features. For example, currently, directories themselves
are never renamed through using these options, so extracting a file
over a directory still has good chances to fail. Also, backup options
apply to created archives, not only to extracted members. For created
archives, backups will not be attempted when the archive is a block or
character device, or when it refers to a remote file.
For the sake of simplicity and efficiency, backups are made by
renaming old files prior to creation or extraction, and not by copying.
The original name is restored if the file creation fails. If a
failure occurs after a partial extraction of a file, both the backup
and the partially extracted file are kept.
Some people express the desire to _always_ use the
`--backup'[=METHOD] option by defining some kind of alias or script.
This is not as easy as one may think, due to the fact that old-style
options should appear first and consume arguments a bit unpredictably
for an alias or script. But if you are ready to give up using
old-style options, you may resort to using something like a (Bourne
shell) function here:
$ tar () { /usr/local/bin/tar --backup $*; }
The `--backup'[=METHOD] option also accepts an optional value,
METHOD, which determines the type of backups that will be made. If
this value is not given, the value of the `VERSION_CONTROL' environment
variable is used. And if `VERSION_CONTROL' is not set, the default
backup method is `existing'.
This value corresponds to the Emacs variable `version-control'; the
same values for METHOD are accepted as in Emacs. The values also have
more descriptive names. Here are the valid METHODs (unique
abbreviations are accepted):
`t'
`numbered'
Always make numbered backups.
`nil'
`existing'
Make numbered backups of files that already have them, simple
backups of the others.
`never'
`simple'
Always make simple backups.
File: paxutils.info, Node: suffix, Prev: backup, Up: Preventing overwrite
Deciding the backup file suffixes
.................................
The `--suffix=SUFFIX' option informs `tar' about the SUFFIX which
should be appended to each backup file made when `--backup'[=METHOD] is
used. If the `--suffix=SUFFIX' option is not specified, the value of
the `SIMPLE_BACKUP_SUFFIX' environment variable is used. And if
`SIMPLE_BACKUP_SUFFIX' is not set, the default is `~', just as in Emacs.
File: paxutils.info, Node: Unlinking, Next: Attributes, Prev: Preventing overwrite, Up: extract
Options to destroy files
------------------------
Some people argue that `tar' should not hesitate to overwrite files
with other files when extracting. When extracting a `tar' archive, they
expect to see a faithful copy of the state of the filesystem when the
archive was created. It is debatable that this would always be proper
behavior. For example, suppose one has an archive in which `usr/local'
is a link to `usr/local2'. Since the archive was created, maybe the
site removed the link and renamed the whole hierarchy from `/usr/local2'
to `/usr/local'. Such things happen all the time. We guess it would
not be welcome at all, in the typical case, for `tar' to remove the
whole hierarchy just to make room for the link to be reinstated, unless
it _also_ simultaneously restores the full `/usr/local2', of course!
By default, `tar' errs on the side of caution. Some options allow
users to change its behavior. When no special options are given for
this, `tar' removes single files when extracting over them, usually
follows symbolic links when found, and does not destroy directories.
* Menu:
* unlink-first:: Removing files prior to extraction
* recursive-unlink:: Removing whole directories
File: paxutils.info, Node: unlink-first, Next: recursive-unlink, Prev: Unlinking, Up: Unlinking
Removing files prior to extraction
..................................
The `--unlink-first' (`-U') option tells `tar' to try removing files
before extracting over them, instead of trying to overwrite them.
File: paxutils.info, Node: recursive-unlink, Prev: unlink-first, Up: Unlinking
Removing whole directories
..........................
When the `--recursive-unlink' option is specified, `tar' tries to
remove files and directory hierarchies before extracting over them.
*This is a dangerous option!*
So, `tar' is indeed able to remove a whole hierarchy to reestablish a
symbolic link, for example, but _only if_ `--recursive-unlink' is
specified to allow this behavior.
File: paxutils.info, Node: Attributes, Next: Scarce, Prev: Unlinking, Up: extract
Handling file attributes
------------------------
* Menu:
* touch:: Setting modification times
* same-owner:: Restoring file ownership
* same-permissions:: Setting access permissions
* no-attributes:: Leaving file attributes alone
File: paxutils.info, Node: touch, Next: same-owner, Prev: Attributes, Up: Attributes
Setting modification times
..........................
Normally, `tar' sets the modification times of extracted files to
the modification times recorded for the files in the archive, but
limits the permissions of extracted files by the current `umask'
setting.
To set the modification times of extracted files to the time when
the files were extracted, use the `--touch' (`-m') option in
conjunction with `--extract' (`--get', `-x').
In fact, `--touch' (`-m') sets the modification time of extracted
archive members to the time they were extracted, not the time recorded
for them in the archive. Use in conjunction with `--extract' (`--get',
`-x').
The `--touch' (`-m') option does not extract file modification time.
When this option is used, `tar' leaves the modification time of the
files it extracts as the time when the files were extracted, instead of
setting it to the time recorded in the archive.
This option is meaningless with `--list' (`-t').
File: paxutils.info, Node: same-owner, Next: same-permissions, Prev: touch, Up: Attributes
Restoring file ownership
........................
The `--same-owner' option invites `tar' to create extracted files
with the same ownership they have in the archive, if this is possible.
When using `tar' as super-user at extraction time, ownership is
always restored. So this option is meaningful only for non-root users,
when `tar' is executed on those systems able to give files away. This
is considered as a security flaw by many people, at least because it
makes quite difficult to correctly charge 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'), it does not write one. When restoring, and doing a
`chmod', as when you use `--same-permissions'
(`--preserve-permissions', `-p'), () it tries to look up the name (if
one was written) in `/etc/passwd'. If it fails, then it uses the user
ID stored in the archive instead.
File: paxutils.info, Node: same-permissions, Next: no-attributes, Prev: same-owner, Up: Attributes
Setting access permissions
..........................
The `--same-permissions' (`--preserve-permissions', `-p') option
requests the extraction of 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.
To set the modes (access permissions) of extracted files to those
recorded for those files in the archive, use `--same-permissions'
(`--preserve-permissions', `-p') in conjunction with the `--extract'
(`--get', `-x') subcommand.
The {Aucune Valeur Pour «same-permission»} option asks that the
modes of extracted archive members be those recorded in the archive,
ignoring current umask settings. Use in conjunction with `--extract'
(`--get', `-x').
This option is meaningless with `--list' (`-t').
File: paxutils.info, Node: no-attributes, Prev: same-permissions, Up: Attributes
Leaving file attributes alone
.............................
The experimental `--no-attributes' option has the effect of
inhibiting all attempts at restoring file attributes, like owner, group,
mode, or time stamps. In this case, `tar' lets the underlying system
handle such things, and does not try to intervene in any way.
File: paxutils.info, Node: Scarce, Prev: Attributes, Up: extract
Coping with scarce resources
----------------------------
* Menu:
* starting-file:: Starting file
* same-order:: Same order
* preserve:: Preserving permissions and order
File: paxutils.info, Node: starting-file, Next: same-order, Prev: Scarce, Up: Scarce
Starting file
.............
Part of an archive may be skipped through the usage of the
`--starting-file=NAME' (`-K NAME') option. It forces the chosen
subcommand to start in the middle of an archive. Use in conjunction
with `--extract' (`--get', `-x') or `--list' (`-t').
If a previous attempt to extract files failed due to lack of disk
space, you can use `--starting-file=NAME' (`-K NAME') to start
extracting only after member NAME of the archive. This assumes, of
course, that there is now free space, or that you are now extracting
into a different file system. (You could also choose to suspend `tar',
remove unnecessary files from the file system, and then restart the
same `tar' subcommand. In this case, `--starting-file=NAME' (`-K
NAME') is not necessary. *Note incremental::, *Note interactive::, and
*Note exclude::.)
File: paxutils.info, Node: same-order, Next: preserve, Prev: starting-file, Up: Scarce
Same order
..........
The `--same-order' (`--preserve-order', `-s') option allows `tar' to
process large lists of file names on machines with small amounts of
memory. Use in conjunction with `--compare' (`--diff', `-d'), `--list'
(`-t'), or `--extract' (`--get', `-x').
The `--same-order' (`--preserve-order', `-s') option tells `tar'
that the list of file names to be listed or extracted is sorted in the
same order as the files in the archive. This allows a large list of
names to be used, even on a small machine that would not otherwise be
able to hold all the names in memory at the same time. Such a sorted
list can easily be created by running `tar -t' on the archive and
editing its output.
This option is probably never needed on modern computer systems.
File: paxutils.info, Node: preserve, Prev: same-order, Up: Scarce
Preserving permissions and order
................................
Bizarrely, the `--preserve' option is the same as using both
`--same-permissions' (`--preserve-permissions', `-p') and
`--same-order' (`--preserve-order', `-s') simultaneously.
The `--preserve' option has no equivalent short option name. It is
really equivalent to `--same-permissions' (`--preserve-permissions',
`-p') plus `--same-order' (`--preserve-order', `-s').
File: paxutils.info, Node: create, Next: Applications, Prev: extract, Up: Subcommands
How to create archives
======================
One of the basic subcommands of `tar' is `--create' (`-c'), which
you use to create a `tar' archive. We will explain `--create' (`-c')
first because, in order to learn about the other subcommands, you will
find it useful to have an archive available to practice on.
To make this easier, in this section you will first create a
directory containing three files. Then we will show you how to create
an _archive_ inside the new directory. Both the directory and the
archive are specifically for you to practice on. The rest of this
chapter and the next chapter will show many examples using this
directory and the files you will create: some of those files may be
other directories and other archives.
The three files you will archive in this example are called `blues',
`folk', and `jazz'. The archive is called `collection.tar'.
This section will proceed slowly, detailing how to use `--create'
(`-c') in `verbose' mode, and showing examples using both short and long
forms. In the rest of the tutorial, and in the examples in the next
chapter, we will proceed at a slightly quicker pace. This section
moves more slowly to allow beginning users to understand how `tar'
works.
Creating an empty archive would have a kind of elegance. One can
initialize an empty archive and later use `--append' (`-r') for adding
all members. Some applications would not welcome making an exception
in the way of adding the first archive member.
There is little or no reason to create an empty archive, but you can
if you wish. (In itself, an empty archive is useless, but later you can
use `--append' (`-r') to add members. There are no actions using an
empty archive that you cannot do as well or better with a regular
archive.) Moreover, there have been reports that some versions of
`tar' have trouble with an empty archive on magnetic tapes.
Consequently, `tar' will not create an empty archive directly;
specifically, `tar' will not create an archive if you specify the
`--create' (`-c') option but fail to specify
`--files-from=FILE-OF-NAMES' (`-T FILE-OF-NAMES') and do not provide
any arguments besides options.
If, after considering all this, you should want to create an empty
archive, you must specify a value for the `--files-from=FILE-OF-NAMES'
(`-T FILE-OF-NAMES') option that has no names in it, as in the
following example:
$ tar --create --file=empty-archive.tar --files-from=/dev/null
$ tar cfT empty-archive.tar /dev/null
On the other hand, many people reported that it is dangerously easy
for `tar' to destroy a magnetic tape with an empty archive.(1) The two
most common errors are:
1. Mistakingly using `create' instead of `extract', when the intent
was to extract the full contents of an archive. This error is
likely: keys `c' and `x' are right next ot each other on the
QWERTY keyboard. Instead of being unpacked, the archive then gets
wholly destroyed. When users speak about "exploding" an archive,
they usually mean something else ... Ahem!
2. Forgetting the argument to `file', when the intent was to create
an archive with a single file in it. This error is likely because
a tired user can easily add the `f' key to the cluster of option
letters, by the mere force of habit, without realizing the full
consequences of doing so. The usual consequence is that the single
file, which was meant to be archived, is instead destroyed.
So, recognizing the likelihood and the catastrophic nature of these
errors, `tar' now distances itself from elegance, and cravenly refuses
to create an archive when `--create' (`-c') option is given, there are
no arguments besides options, and `--files-from=FILE-OF-NAMES' (`-T
FILE-OF-NAMES') option is _not_ used. Observe:
$ tar c
tar: Cravenly refusing to create an empty archive
Try `tar --help' for more information.
$ tar cf foo
tar: Cravenly refusing to create an empty archive
Try `tar --help' for more information.
To get around the cautiousness of `tar' and nevertheless create an
archive with nothing in it, one may still use, as the value for the
`--files-from=FILE-OF-NAMES' (`-T FILE-OF-NAMES') option, a file with
no names in it, as shown in the following commands:
$ tar --create --file=empty-archive.tar --files-from=/dev/null
$ tar cfT empty-archive.tar /dev/null
* Menu:
* name-prefix:: Using name prefixes
* atime-preserve:: Preserving last access time
* remove-files:: Removing files
* append:: How to add files to existing archives
* update:: Updating an archive
--------- Notes en bas de page ---------
(1) This is well described in `The Unix-Hater's Handbook' by Simson
Garfinkel, Daniel Weise, and Steven Strassmann, IDG Books, ISBN
1-56884-203-1.
File: paxutils.info, Node: name-prefix, Next: atime-preserve, Prev: create, Up: create
Using name prefixes
-------------------
Sometimes the directory structure you want an archive to create when
you unpack it is different from the directory structure you built it in.
This is often true when you are using a `tar' production in a
makefile, to build distribution archives in a source-code directory.
The most convenient way to do this would be with a command like the
following:
$ tar --create --file=distribution.tar *
But this makes an archive that will unpack as a lot of individual files,
which is usually not what you want. Assuming your build directory is
named `project', you might do something like
$ tar --directory=.. --create --file=distribution.tar project
But this means you will be stuck with the name of your build
directory as the top-level name of the distribution archive. Often you
would like your distribution archive to unpack into a single directory,
but with a synthetic top-level directory name (perhaps including a
version number).
You can get this effect with the `--name-prefix=PREFIX' option. Its
argument is a name prefix which gets prepended to each file name entry
in an archive as it is added to the archive.
For example, if you say
$ tar --name-prefix='project-5.23/' --create --file=collection.tar *
you will make an archive of all the files and directories in the
current directory, but it will extract into the single subdirectory
`project-5.23'.
Note that you will almost always want the argument of
`--name-prefix=PREFIX' to end with a directory delimiter (forward slash
under Unix-like operating systems, forward slash or backslash under
DOS). Otherwise you will make an archive in which each file has a
prefix welded directly to the first segment of its name. While this
capability is there for full generality, it is probably not what you
want.
File: paxutils.info, Node: atime-preserve, Next: remove-files, Prev: name-prefix, Up: create
Preserving last access time
---------------------------
When `tar' reads files, their access time gets updated. To have
`tar' attempt to set the access times back to what they were before
they were read, use the `--atime-preserve' option. This doesn't work
for files that you don't own, unless you're root, and it doesn't
interact nicely with incremental dumps (*note Backups::.), but it is
good enough for some purposes.
This options also preserves the modification time field, which has no
effect unless the file is being modified simultaneously by another
program. As such, it also can eliminate evidence of reading or writing
of the file by another program, if the access happens between the
beginning and ending of `tar' reading the file.
A drawback of the `--atime-preserve' option is that it alters the
`ctime' of the dumped files. Some system administrators value `ctime'
much more than `atime'.(1) They should ponder carefully the advantages
and disadvantages before using this option in system backups.
This option doesn't have any effect on MS-DOS, since `atime' is not
supported by the DOS FAT file system. (It _does_ work on MS-Windows
9X.)
--------- Notes en bas de page ---------
(1) This is at least the case of the current `paxutils' maintainer!
File: paxutils.info, Node: remove-files, Next: append, Prev: atime-preserve, Up: create
Removing files
--------------
The effect of the `--remove-files' option is to remove files once
they have been added to the archive.
File: paxutils.info, Node: append, Next: update, Prev: remove-files, Up: create
How to add files to existing archives
-------------------------------------
If you want to add files to an existing archive, you don't need to
create a new archive; you can use `--append' (`-r'). The archive must
already exist in order to use `--append' (`-r'). (A related subcommand
is the `--update' (`-u') subcommand; you can use this to add newer
versions of archive members to an existing archive. To learn how to do
this with `--update' (`-u'), refer to *Note update::.)
If you use `--append' (`-r') to add a file to an archive which
already contains a member with the same name, then the old member is
not deleted. What does happen, however, is somewhat complex. `tar'
_allows_ you to have infinite numbers of archive members with the same
name. Some subcommands treat these same-named members no differently
than any other set of archive members: for example, if you view an
archive with `--list' (`-t'), you will see all of those members listed,
with their modification times, owners, etc.
Other subcommands don't deal with these members as well as you might
prefer; if you were to use `--extract' (`--get', `-x') to extract the
archive, only the most recently added copy of a member with the same
name as other members would end up in the working directory. This is
because `--extract' (`--get', `-x') extracts an archive in the order
the members appeared in the archive; the most recently archived members
will be extracted last. Additionally, an extracted member will
_overwrite_ a file of the same name which existed in the directory
already, and `tar' will not prompt you about this. Thus, only the most
recently archived member will end up being extracted, as it will
overwrite the one extracted before it, and so on. (One way of
extracting all of the members without overwriting them is to use the
`--backup'[=METHOD] option; see *Note backup::.)
Other than using `--backup'[=METHOD], there are a few additional
ways to get around this.
If you want to replace an archive member, use `--delete' to delete
the member you want to remove from the archive, and then use `--append'
(`-r') to add the member you want to be in the archive. (Replacing one
member with another will not work on certain types of media, such as
tapes; see *Note delete:: and *Note Media::, for more information.)
Note that you cannot change the order of the archive; the most recently
added member will still appear last. In this sense, you cannot truly
"replace" one member with another.
* Menu:
* Appending files:: Appending files to an archive
* Multiple files:: Multiple files with the same name
File: paxutils.info, Node: Appending files, Next: Multiple files, Prev: append, Up: append
Appending files to an archive
.............................
The simplest way to add a file to an already existing archive is the
`--append' (`-r') subcommand, which writes specified files into the
archive whether or not they are already among the archived files. When
you use `--append' (`-r'), you _must_ specify file name arguments, as
there is no default. If you specify a file that already exists in the
archive, another copy of the file will be added to the end of the
archive. As with other subcommands, the member names of the newly added
files will be exactly the same as their names given on the command line
(except that backslashes in MS-DOS file names are mirrored to Unix-style
forward slashes). The `--verbose' (`-v') option will print out the
names of the files as they are written into the archive.
`--append' (`-r') cannot be performed on some tape drives,
unfortunately, due to deficiencies in the formats those tape drives
use. The archive must be a valid `tar' archive, or else the results of
using this subcommand will be unpredictable. *Note Media::.
To demonstrate use of `--append' (`-r') to add a file to an archive,
create a file called `rock' in the `practice' directory. Make sure you
are in the `practice' directory. Then, run the following `tar' command
to add `rock' to `collection.tar':
$ tar --append --file=collection.tar rock
If you now use the `--list' (`-t') subcommand, you will see that `rock'
has been added to the archive:
$ tar --list --file=collection.tar
jazz
blues
folk
rock
File: paxutils.info, Node: Multiple files, Prev: Appending files, Up: append
Multiple files with the same name
.................................
You can use `--append' (`-r') to add copies of files which have been
updated since the archive was created. (However, we do not recommend
doing this, since there is another `tar' option called `--update'
(`-u'); *note update::., for more information. We describe this use of
`--append' (`-r') here for the sake of completeness.)
When you extract the archive, the older version will effectively be
lost. This happens because files are extracted from an archive in the
order in which they were archived. Thus, when the archive is
extracted, a file archived later in time will overwrite a file of the
same name which was archived earlier, even though the older version of
the file will remain in the archive unless you delete all versions of
the file.
Suppose you change the file `blues' and then append the changed
version to `collection.tar'. As you saw above, the original `blues' is
in the archive `collection.tar'. If you change the file and append the
new version of the file to the archive, there will be two copies in the
archive. When you extract the archive, the older version of the file
will be extracted first, and then overwritten by the newer version when
it is extracted.
You can append the new, changed copy of the file `blues' to the
archive in this way:
$ tar --append --verbose --file=collection.tar blues
blues
Because you specified the `--verbose' (`-v') option, `tar' has printed
the name of the file being appended as it was acted on. Now list the
contents of the archive:
$ tar --list --verbose --file=collection.tar
-rw-rw-rw- myself users 28 1996-10-18 16:31 jazz
-rw-rw-rw- myself users 21 1996-09-23 16:44 blues
-rw-rw-rw- myself users 20 1996-09-23 16:44 folk
-rw-rw-rw- myself users 20 1996-09-23 16:44 rock
-rw-rw-rw- myself users 58 1996-10-24 18:30 blues
The newest version of `blues' is now at the end of the archive (note
the different creation dates and file sizes). If you extract the
archive, the older version of the file `blues' will be overwritten by
the newer version. You can confirm this by extracting the archive and
running `ls' on the directory. *Note Writing::, for more information.
(_Please note:_ This is the case unless you employ the
`--backup'[=METHOD] option; .)
File: paxutils.info, Node: update, Prev: append, Up: create
Updating an archive
-------------------
In the previous section, you learned how to use `--append' (`-r') to
add a file to an existing archive. A related subcommand is `--update'
(`-u'). The `--update' (`-u') subcommand updates a `tar' archive by
comparing the date of the specified archive members against the date of
the file with the same name. If the file has been modified more
recently than the archive member, then the newer version of the file is
added to the archive (as with `--append' (`-r')).
Unfortunately, you cannot use `--update' (`-u') with magnetic tape
drives. The command will fail.
Both `--update' (`-u') and `--append' (`-r') work by adding to the
end of the archive. When you extract a file from the archive, only the
version stored last will wind up in the file system, unless you use the
`--backup'[=METHOD] option ().
* Menu:
* How to update:: How to update members of an archive
* Unsuitability of update:: Updating does not always work
File: paxutils.info, Node: How to update, Next: Unsuitability of update, Prev: update, Up: update
How to update members of an archive
...................................
You must use file name arguments with the `--update' (`-u')
subcommand. If you don't specify any files, `tar' won't act on any
files and won't tell you that it didn't do anything (which might end up
confusing you).
To see the `--update' (`-u') option at work, create a new file,
`classical', in your practice directory, and some extra text to the
file `blues', using any text editor. Then invoke `tar' with the
`update' subcommand and the `--verbose' (`-v') option specified, using
the names of all the files in the practice directory as file name
arguments:
$ tar --update -v -f collection.tar blues folk rock classical
blues
classical
$
Because we have specified verbose mode, `tar' prints out the names of
the files it is working on, which in this case are the names of the
files that needed to be updated. If you run `tar --list' and look at
the archive, you will see `blues' and `classical' at its end. There
will be a total of two versions of the member `blues'; the one at the
end will be newer and larger, since you added text before updating it.
(The reason `tar' does not overwrite the older file when updating it
is because writing to the middle of a section of tape is a difficult
process. Tapes are not designed to get data inserted in their middle;
doing so would destroy any data after the place you write. *Note
Media::, for more information about tapes.)
`--update' (`-u') is not suitable for performing backups for two
reasons: it does not change directory content entries, and it lengthens
the archive every time it is used. The `tar' options intended
specifically for backups are more efficient. If you need to run
backups, please consult *Note Backups::.
File: paxutils.info, Node: Unsuitability of update, Prev: How to update, Up: update
Updating does not always work
.............................
Sometimes, you may have a situation for which you would think usign
`--update' (`-u') would be ideal; unfortunately, this is not always the
case. Two situations for which `--update' (`-u') is unsuitable are
updating an archive stored on a magnetic tape, and making a backup.
Other subcommands and options provide more appropriate solutions in
these situations.
`tar' does not overwrite the older file when updating an archive.
This is because writing to the middle of a section of tape is a
difficult process. Tapes are not really designed to go backward
(although some have the capability), and their linear nature makes
inserting data into the middle of a tape impossible, even if there is
room at the end of the tape. In this way, tapes differ from other kinds
of storage media, such as floppy diskettes and hard drives. *Note
Media::, for more information about tapes and other kinds of storage
media.
`--update' (`-u') is not suitable for performing backups for two
reasons: it does not change directory content entries, and it lengthens
the archive every time it is used. The `tar' options intended
specifically for backups are more efficient. If you need to run
backups, please consult *Note Backups::.
File: paxutils.info, Node: Applications, Prev: create, Up: Subcommands
Other uses for `tar'
====================
You can easily use archive files to transport a group of files from
one system to another: put all relevant files into an archive on one
computer system, transfer the archive to another system, and extract
the contents there. The basic transfer medium might be magnetic tape,
Internet FTP, or even electronic mail (though you must encode the
archive with `uuencode' in order to transport it properly by mail).
Both machines do not have to use the same operating system, as long as
they both support the `tar' program.
For example, here is how you might copy a directory's contents from
one disk to another, while preserving the dates, modes, owners, and
link-structure of all the files therein. In this case, the transfer
medium is a "pipe", which is a Unix redirection mechanism:
$ cd sourcedir; tar --create --file=- . | (cd targetdir; tar --extract --file=-)
The command also works using short option forms:
$ cd sourcedir; tar -cf - . | (cd targetdir; tar -xf -)
If you are really confident you are using `tar' and that standard input
and output were not overridden by something else as the default archive
at your installation, you might write the above examples more simply:
$ cd sourcedir; tar --create . | (cd targetdir; tar --extract)
$ cd sourcedir; tar -c . | (cd targetdir; tar -x)
This is one of the easiest methods to transfer a `tar' archive.
* Menu:
* concatenate:: Combining archives with `tar'
* Using cat with archives:: Combining archives with `cat'
* delete:: Removing archive members from an archive
File: paxutils.info, Node: concatenate, Next: Using cat with archives, Prev: Applications, Up: Applications
Combining archives with `tar'
-----------------------------
Sometimes it may be convenient to add a second archive onto the end
of an archive rather than adding individual files to the archive. To
add the contents of one or more archives to the end of another archive,
you should use the `--concatenate' (`--catenate', `-A') subcommand.
To use `--concatenate' (`--catenate', `-A'), name two archives to be
concatenated on the command line. (Nothing happens if you don't list
any.) The members, and their member names, will be copied verbatim
from those archives. If this causes multiple members to have the same
name, it does not delete any members; all the members with the same
name coexist. For information on how this affects reading the archive,
.
To demonstrate how `--concatenate' (`--catenate', `-A') works,
create two small archives called `bluesrock.tar' and `folkjazz.tar',
using the relevant files from `practice':
$ tar -cvf bluesrock.tar blues rock
blues
rock
$ tar -cvf folkjazz.tar folk jazz
folk
jazz
If you like, you can run `tar --list' to make sure the archives contain
what they are supposed to:
$ tar -tvf bluesrock.tar
-rw-rw-rw- myself users 105 1997-01-21 19:42 blues
-rw-rw-rw- myself users 33 1997-01-20 15:34 rock
$ tar -tvf folkjazz.tar
-rw-rw-rw- myself users 20 1996-09-23 16:44 folk
-rw-rw-rw- myself users 65 1997-01-30 14:15 jazz
We can concatenate these two archives with `tar':
$ tar --concatenate --file=bluesrock.tar jazzfolk.tar
If you now list the contents of the `bluesrock.tar', you will see
that now it also contains the archive members of `jazzfolk.tar':
$ tar --list --file=bluesrock.tar
blues
rock
jazz
folk
When you use `--concatenate' (`--catenate', `-A'), the source and
target archives must already exist and must have been created using
compatible format parameters (). The new, concatenated archive will be
called by the same name as the first archive listed on the command
line.
Like `--append' (`-r'), this subcommand cannot be performed on some
tape drives, due to deficiencies in the formats those tape drives use.
It may seem more intuitive to you to try to use `cat' to concatenate
two archives instead of using the `--concatenate' (`--catenate', `-A')
subcommand; after all, `cat' is the utility for combining files.
However, `tar' archives incorporate an end-of-file marker which must
be removed if the concatenated archives are to be read properly as one
archive. `--concatenate' (`--catenate', `-A') removes the
end-of-archive marker from the target archive before each new archive
is appended. If you use `cat' to combine the archives, the result will
not be a valid `tar' format archive. If you need to retrieve files
from an archive that was added to using the `cat' utility, use the
`--ignore-zeros' (`-i') option. *Note ignore-zeros::, for further
information on dealing with archives improperly combined using the
`cat' utility.
You must specify the source archives using `--file=ARCHIVE-NAME'
(`-f ARCHIVE-NAME') (*note file::.). If you do not specify the target
archive, `tar' uses the value of the environment variable `TAPE', or,
if this has not been set, the default archive name. If the default
archive name is the standard output, `tar' will print an error message.
* Menu:
* Using cat with archives:: Combining archives with `cat'
File: paxutils.info, Node: Using cat with archives, Next: delete, Prev: concatenate, Up: Applications
Combining archives with `cat'
-----------------------------
It may seem more intuitive to you to try to use `cat' to concatenate
two archives instead of using the `--concatenate' (`--catenate', `-A')
subcommand; after all, `cat' is the utility for combining files.
However, `tar' archives incorporate an end-of-file marker which must be
removed if the concatenated archives are to be read properly as one
archive. `--concatenate' (`--catenate', `-A') removes the
end-of-archive marker from the target archive before each new archive
is appended.
Therefore, if you use `cat' to combine the archives, the result will
not be a valid `tar' format archive. If you need to retrieve files
from an archive that was added to using the `cat' utility, use the
`--ignore-zeros' (`-i') option. *Note ignore-zeros::, for further
information on dealing with archives improperly combined using the
`cat' shell utility.
You must specify the names of the archives that will be concatenated
using `--file=ARCHIVE-NAME' (`-f ARCHIVE-NAME') (*note file::.). If
you do not specify the target archive (the archive to which you will
concatenate other archives), `tar' uses the value of the environment
variable `TAPE', or, if this has not been set, the default archive name.
File: paxutils.info, Node: delete, Prev: Using cat with archives, Up: Applications
Removing archive members from an archive
----------------------------------------
You can remove members from an archive by using the `--delete'
option. Specify the name of the archive with `--file=ARCHIVE-NAME'
(`-f ARCHIVE-NAME') and then specify the names of the members to be
deleted; if you list no member names, nothing will be deleted. The
`--verbose' (`-v') option will cause `tar' to print the names of the
members as they are deleted. As with `--extract' (`--get', `-x'), you
must give the exact member names when using `--delete'. `--delete'
will remove all versions of the named file from the archive. The
`--delete' subcommand can run very slowly.
Unlike other subcommands, `--delete' has no short form.
The `--delete' subcommand will rewrite the archive. You can only
use `--delete' on an archive if the archive device allows you to write
to any point on the media at any time, that is, if it is a
random-access device like a hard disk. The `--delete' subcommand does
not work on magnetic tapes. Do not try to delete an archive member
from a magnetic tape; the action will not succeed, and you will be
likely to scramble the archive and damage your tape. There is no safe
way (except by completely rewriting the archive) to delete files from
most kinds of magnetic tape. *Note Media::.
To delete all versions of the file `blues' from the archive
`collection.tar' in the `practice' directory, make sure you are in that
directory, and then:
$ tar --list --file=collection.tar
jazz
blues
folk
rock
blues
blues
classical
$ tar --delete --file=collection.tar blues
$ tar --list --file=collection.tar
jazz
folk
rock
classical
$
The `--delete' option has been reported to work properly when `tar'
acts as a filter from `stdin' to `stdout'.
File: paxutils.info, Node: Choosing, Next: Formats, Prev: Subcommands, Up: Top
Choosing files and names for `tar'
**********************************
Certain options to `tar' enable you to specify a name for your
archive. Other options let you decide which files to include in, or
exclude from, the archive, based on when or whether files were