tar Operationstar Operations
The basic tar operations, --create (-c), --list (-t) and
--extract (--get, -x), are currently presented and described in the tutorial
chapter of this manual. This section provides some complementary notes
for these operations.
tar
to destroy a magnetic tape with an empty archive(5). The two most
common errors are:
create instead of extract, when the
intent was to extract the full contents of an archive. This error
is likely: keys c and x are right next ot each other on
the QWERTY keyboard. Instead of being unpacked, the archive then
gets wholly destroyed. When users speak about exploding an
archive, they usually mean something else :-).
file, when the intent was to create
an archive with a single file in it. This error is likely because a
tired user can easily add the f key to the cluster of option
letters, by the mere force of habit, without realizing the full
consequence of doing so. The usual consequence is that the single
file, which was meant to be saved, is rather destroyed.
tar now takes some distance from elegance, and
cowardly refuses to create an archive when --create (-c) option is
given, there are no arguments besides options, and --files-from=file-of-names (-T file-of-names)
option is not used. To get around the cautiousness of GNU
tar and nevertheless create an archive with nothing in it,
one may still use, as the value for the --files-from=file-of-names (-T file-of-names) option,
a file with no names in it, as shown in the following commands:
tar --create --file=empty-archive.tar --files-from=/dev/null tar cfT empty-archive.tar /dev/null
tar archive, as a pipe.
tar now shows dates as `1996-11-09', while it used to
show them as `Nov 11 1996'. (One can revert to the old behavior by
defining USE_OLD_CTIME in `src/list.c' before reinstalling.)
But preferrably, people you should get used to ISO 8601 dates. Local
American dates should be made available again with full date localisation
support, once ready. In the meantime, programs not being localisable
for dates should prefer international dates, that's really the way to go.
Look up http://www.ft.uni-erlangen.de/~mskuhn/iso-time.html if you
are curious, it contains a detailed explanation of the ISO 8601 standard.
tar Operations
Now that you have learned the basics of using GNU tar, you may
want to learn about further ways in which tar can help you.
This chapter presents five, more advanced operations which you probably
won't use on a daily basis, but which serve more specialized functions.
We also explain the different styles of options and why you might want
to use one or another, or a combination of them in your tar
commands. Additionally, this chapter includes options which allow you to
define the output from tar more carefully, and provide help and
error correction in special circumstances.
@FIXME{check this after the chapter is actually revised to make sure it still introduces the info in the chapter correctly : ).}
tar Operations@UNREVISED
In the last chapter, you learned about the first three operations to
tar. This chapter presents the remaining five operations to
tar: `--append', `--update', `--concatenate',
`--delete', and `--compare'.
You are not likely to use these operations as frequently as those covered in the last chapter; however, since they perform specialized functions, they are quite useful when you do need to use them. We will give examples using the same directory and files that you created in the last chapter. As you may recall, the directory is called `practice', the files are `jazz', `blues', `folk', `rock', and the two archive files you created are `collection.tar' and `music.tar'.
We will also use the archive files `afiles.tar' and `bfiles.tar'. `afiles.tar' contains the members `apple', `angst', and `aspic'. `bfiles.tar' contains the members `./birds', `baboon', and `./box'.
Unless we state otherwise, all practicing you do and examples you follow in this chapter will take place in the `practice' directory that you created in the previous chapter; see section Preparing a Practice Directory for Examples. (Below in this section, we will remind you of the state of the examples where the last chapter left them.)
The five operations that we will cover in this chapter are:
Currently, the listing of the directory using ls is as follows:
The archive file `collection.tar' looks like this:
$ tar -tvf collection.tar
The archive file `music.tar' looks like this:
$ tar -tvf music.tar
@FIXME{need to fill in the above!!!}
--append@UNREVISED
If you want to add files to an existing archive, you don't need to create a new archive; you can use --append (-r). The archive must already exist in order to use `--append'. (A related operation is the `--update' operation; you can use this to add newer versions of archive members to an existing archive. To learn how to do this with `--update', see section Updating an Archive.)
@FIXME{Explain in second paragraph whether you can get to the previous version -- explain whole situation somewhat more clearly.}
If you use --append (-r) to add a file that has the same name as an
archive member to an archive containing that archive member, then the
old member is not deleted. What does happen, however, is somewhat
complex. tar allows you to have infinite numbers of files
with the same name. Some operations treat these same-named members no
differently than any other set of archive members: for example, if you
view an archive with --list (-t), you will see all of those members
listed, with their modification times, owners, etc.
Other operations don't deal with these members as perfectly as you might
prefer; if you were to use --extract (--get, -x) to extract the archive,
only the most recently added copy of a member with the same name as four
other members would end up in the working directory. This is because
`--extract' extracts an archive in the order the members appeared
in the archive; the most recently archived members will be extracted
last. Additionally, an extracted member will overwrite a file of
the same name which existed in the directory already, and tar
will not prompt you about this. Thus, only the most recently archived
member will end up being extracted, as it will overwrite the one
extracted before it, and so on.
@FIXME{ hag -- you might want to incorporate some of the above into the MMwtSN node; not sure. i didn't know how to make it simpler...}
There are a few ways to get around this. @FIXME-xref{Multiple Members with the Same Name}.
If you want to replace an archive member, use --delete to delete the member you want to remove from the archive, , and then use `--append' to add the member you want to be in the archive. Note that you can not change the order of the archive; the most recently added member will still appear last. In this sense, you cannot truely "replace" one member with another. (Replacing one member with another will not work on certain types of media, such as tapes; see section Removing Archive Members Using `--delete' and section Tapes and Other Archive Media, for more information.)
The simplest way to add a file to an already existing archive is the --append (-r) operation, which writes specified files into the archive whether or not they are already among the archived files. When you use `--append', you must specify file name arguments, as there is no default. If you specify a file that already exists in the archive, another copy of the file will be added to the end of the archive. As with other operations, the member names of the newly added files will be exactly the same as their names given on the command line. The --verbose (-v) option will print out the names of the files as they are written into the archive.
`--append' cannot be performed on some tape drives, unfortunately,
due to deficiencies in the formats those tape drives use. The archive
must be a valid tar archive, or else the results of using this
operation will be unpredictable. See section Tapes and Other Archive Media.
To demonstrate using `--append' to add a file to an archive,
create a file called `rock' in the `practice' directory.
Make sure you are in the `practice' directory. Then, run the
following tar command to add `rock' to
`collection.tar':
$ tar --append --file=collection.tar rock
If you now use the --list (-t) operation, you will see that `rock' has been added to the archive:
$ tar --list --file=collection.tar -rw-rw-rw- me user 28 1996-10-18 16:31 jazz -rw-rw-rw- me user 21 1996-09-23 16:44 blues -rw-rw-rw- me user 20 1996-09-23 16:44 folk -rw-rw-rw- me user 20 1996-09-23 16:44 rock
@FIXME{in theory, dan will (soon) try to turn this node into what it's title claims it will become...}
You can use --append (-r) to add copies of files which have been
updated since the archive was created. (However, we do not recommend
doing this since there is another tar option called
`--update'; see section Updating an Archive for more information. We describe this
use of `--append' here for the sake of completeness.) @FIXME{is
this really a good idea, to give this whole description for something
which i believe is basically a Stupid way of doing something? certain
aspects of it show ways in which tar is more broken than i'd personally
like to admit to, specifically the last sentence. On the other hand, i
don't think it's a good idea to be saying that re explicitly don't
recommend using something, but i can't see any better way to deal with
the situation.} When you extract the archive, the older version will be
effectively lost. This works because files are extracted from an
archive in the order in which they were archived. Thus, when the
archive is extracted, a file archived later in time will overwrite a
file of the same name which was archived earlier, even though the older
version of the file will remain in the archive unless you delete all
versions of the file.
Supposing you change the file `blues' and then append the changed version to `collection.tar'. As you saw above, the original `blues' is in the archive `collection.tar'. If you change the file and append the new version of the file to the archive, there will be two copies in the archive. When you extract the archive, the older version of the file will be extracted first, and then overwritten by the newer version when it is extracted.
You can append the new, changed copy of the file `blues' to the archive in this way:
$ tar --append --verbose --file=collection.tar blues blues
Because you specified the `--verbose' option, tar has
printed the name of the file being appended as it was acted on. Now
list the contents of the archive:
$ tar --list --verbose --file=collection.tar -rw-rw-rw- me user 28 1996-10-18 16:31 jazz -rw-rw-rw- me user 21 1996-09-23 16:44 blues -rw-rw-rw- me user 20 1996-09-23 16:44 folk -rw-rw-rw- me user 20 1996-09-23 16:44 rock -rw-rw-rw- me user 58 1996-10-24 18:30 blues
The newest version of `blues' is now at the end of the archive
(note the different creation dates and file sizes). If you extract
the archive, the older version of the file `blues' will be
overwritten by the newer version. You can confirm this by extracting
the archive and running `ls' on the directory. See section Changing How tar Writes Files
for more information. (Please note: This is the case unless
you employ the --backup option; @FIXME-ref{Multiple Members
with the Same Name}.)
In the previous section, you learned how to use --append (-r) to add
a file to an existing archive. A related operation is
--update (-u). The `--update' operation updates a tar
archive by comparing the date of the specified archive members against
the date of the file with the same name. If the file has been modified
more recently than the archive member, then the newer version of the
file is added to the archive (as with --append (-r)).
Unfortunately, you cannot use `--update' with magnetic tape drives. The operation will fail.
@FIXME{other examples of media on which --update will fail? need to ask charles and/or mib/thomas/dave shevett..}
Both `--update' and `--append' work by adding to the end of the archive. When you extract a file from the archive, only the version stored last will wind up in the file system, unless you use the --backup option (@FIXME-ref{Multiple Members with the Same Name}).
--update
You must use file name arguments with the --update (-u) operation.
If you don't specify any files, tar won't act on any files and
won't tell you that it didn't do anything (which may end up confusing
you).
@FIXME{note: the above parenthetical added because in fact, this behavior just confused the author. :-) }
To see the `--update' option at work, create a new file,
`classical', in your practice directory, and some extra text to the
file `blues', using any text editor. Then invoke tar with
the `update' operation and the --verbose (-v) option specified,
using the names of all the files in the practice directory as file name
arguments:
$ tar --update -v -f collection.tar blues folk rock classical blues classical $
Because we have specified verbose mode, tar prints out the names
of the files it is working on, which in this case are the names of the
files that needed to be updated. If you run `tar --list' and look
at the archive, you will see `blues' and `classical' at its
end. There will be a total of two versions of the member `blues';
the one at the end will be newer and larger, since you added text before
updating it.
(The reason tar does not overwrite the older file when updating
it is because writing to the middle of a section of tape is a difficult
process. Tapes are not designed to go backward. See section Tapes and Other Archive Media for more
information about tapes.
--update (-u) is not suitable for performing backups for two
reasons: it does not change directory content entries, and it lengthens
the archive every time it is used. The GNU tar options intended
specifically for backups are more efficient. If you need to run
backups, please consult section Performing Backups and Restoring Files.
--concatenateSometimes it may be convenient to add a second archive onto the end of an archive rather than adding individual files to the archive. To add one or more archives to the end of another archive, you should use the --concatenate (--catenate, -A) operation.
To use `--concatenate', name the archives to be concatenated on the command line. (Nothing happens if you don't list any.) The members, and their member names, will be copied verbatim from those archives. If this causes multiple members to have the same name, it does not delete any members; all the members with the same name coexist. For information on how this affects reading the archive, @FIXME-ref{Multiple Members with the Same Name}.
To demonstrate how `--concatenate' works, create two small archives called `bluesrock.tar' and `folkjazz.tar', using the relevant files from `practice':
$ tar -cvf bluesrock.tar blues rock blues classical $ tar -cvf folkjazz.tar folk jazz folk jazz
If you like, You can run `tar --list' to make sure the archives contain what they are supposed to:
$ tar -tvf bluesrock.tar -rw-rw-rw- melissa user 105 1997-01-21 19:42 blues -rw-rw-rw- melissa user 33 1997-01-20 15:34 rock $ tar -tvf folkjazz.tar -rw-rw-rw- melissa user 20 1996-09-23 16:44 folk -rw-rw-rw- melissa user 65 1997-01-30 14:15 jazz
We can concatenate these two archives with tar:
$ cd .. $ tar --concatenate --file=bluesrock.tar jazzfolk.tar
If you now list the contents of the `bluesclass.tar', you will see that now it also contains the archive members of `jazzfolk.tar':
$ tar --list --file=bluesrock.tar blues rock jazz folk
When you use `--concatenate', the source and target archives must already exist and must have been created using compatable format parameters (@FIXME-pxref{Matching Format Parameters}). The new, concatenated archive will be called by the same name as the first archive listed on the command line. @FIXME{is there a way to specify a new name?}
Like --append (-r), this operation cannot be performed on some tape drives, due to deficiencies in the formats those tape drives use.
It may seem more intuitive to you to want or try to use cat to
concatenate two archives instead of using the `--concatenate'
operation; after all, cat is the utility for combining files.
However, tar archives incorporate an end-of-file marker which
must be removed if the concatenated archives are to be read properly as
one archive. `--concatenate' removes the end-of-archive marker
from the target archive before each new archive is appended. If you use
cat to combine the archives, the result will not be a valid
tar format archive. If you need to retrieve files from an
archive that was added to using the cat utility, use the
--ignore-zeros (-i) option. See section Ignoring Blocks of Zeros for further
information on dealing with archives improperly combined using the
cat shell utility.
@FIXME{this shouldn't go here. where should it go?} You must specify
the source archives using --file=archive-name (-f archive-name) (see section Choosing and Naming Archive Files). If you
do not specify the target archive, tar uses the value of the
environment variable TAPE, or, if this has not been set, the
default archive name.
You can remove members from an archive by using the --delete
option. Specify the name of the archive with --file=archive-name (-f archive-name) and then
specify the names of the members to be deleted; if you list no member
names, nothing will be deleted. The --verbose (-v) option will
cause tar to print the names of the members as they are deleted.
As with --extract (--get, -x), you must give the exact member names when
using `tar --delete'. `--delete' will remove all versions of
the named file from the archive. The `--delete' operation can run
very slowly.
Unlike other operations, `--delete' has no short form.
This operation will rewrite the archive. You can only use `--delete' on an archive if the archive device allows you to write to any point on the media, such as a disk; because of this, it does not work on magnetic tapes. Do not try to delete an archive member from a magnetic tape; the action will not succeed, and you will be likely to scramble the archive and damage your tape. There is no safe way (except by completely re-writing the archive) to delete files from most kinds of magnetic tape. See section Tapes and Other Archive Media.
To delete all versions of the file `blues' from the archive `collection.tar' in the `practice' directory, make sure you are in that directory, and then,
$ tar --list --file=collection.tar blues folk jazz rock practice/blues practice/folk practice/jazz practice/rock practice/blues $ tar --delete --file=collection.tar blues $ tar --list --file=collection.tar folk jazz rock $
@FIXME{I changed the order of these nodes around and haven't had a chance to fix the above example's results, yet. I have to play with this and follow it and see what it actually does!}
The --delete option has been reported to work properly when
tar acts as a filter from stdin to stdout.
The `--compare' (`-d'), or `--diff' operation compares
specified archive members against files with the same names, and then
reports differences in file size, mode, owner, modification date and
contents. You should only specify archive member names, not file
names. If you do not name any members, then tar will compare the
entire archive. If a file is represented in the archive but does not
exist in the file system, tar reports a difference.
You have to specify the record size of the archive when modifying an archive with a non-default record size.
tar ignores files in the file system that do not have
corresponding members in the archive.
The following example compares the archive members `rock',
`blues' and `funk' in the archive `bluesrock.tar' with
files of the same name in the file system. (Note that there is no file,
`funk'; tar will report an error message.)
$ tar --compare --file=bluesrock.tar rock blues funk rock blues tar: funk not found in archive
@FIXME{what does this actually depend on? i'm making a guess,
here.}Depending on the system where you are running tar and the
version you are running, tar may have a different error message,
such as:
funk: does not exist
@FIXME-xref{somewhere, for more information about format parameters. Melissa says: such as "format variations"? But why? Clearly I don't get it yet; I'll deal when I get to that section.}
The spirit behind the --compare (--diff, -d) option is to check whether the archive represents the current state of files on disk, more than validating the integrity of the archive media. For this later goal, See section Verifying Data as It is Stored.
--extract@UNREVISED
@FIXME{i need to get dan to go over these options with me and see if there's a better way of organizing them.}
The previous chapter showed how to use --extract (--get, -x) to extract
an archive into the filesystem. Various options cause tar to
extract more information than just file contents, such as the owner,
the permissions, the modification date, and so forth. This section
presents options to be used with `--extract' when certain special
considerations arise. You may review the information presented in
section How to Extract Members from an Archive for more basic information about the
`--extract' operation.
Normally, tar will request data in full record increments from
an archive storage device. If the device cannot return a full record,
tar will report an error. However, some devices do not always
return full records, or do not require the last record of an archive to
be padded out to the next record boundary. To keep reading until you
obtain a full record, or to accept an incomplete record if it contains
an end-of-archive marker, specify the --read-full-records (-B) option
in conjunction with the --extract (--get, -x) or --list (-t) operations.
See section Blocking.
The --read-full-records (-B) option is turned on by default when
tar reads an archive from standard input, or from a remote
machine. This is because on BSD Unix systems, attempting to read a
pipe returns however much happens to be in the pipe, even if it is
less than was requested. If this option were not enabled, tar
would fail as soon as it read an incomplete record from the pipe.
If you're not sure of the blocking factor of an archive, you can read the archive by specifying --read-full-records (-B) and --blocking-factor=512-size (-b 512-size), using a blocking factor larger than what the archive uses. This lets you avoid having to determine the blocking factor of an archive. See section The Blocking Factor of an Archive.
@FIXME{need sentence or so of intro here}
Normally, tar stops reading when it encounters a block of zeros
between file entries (which usually indicates the end of the archive).
--ignore-zeros (-i) allows tar to completely read an archive
which contains a block of zeros before the end (i.e. a damaged
archive, or one which was created by cat-ing several archives
together).
The --ignore-zeros (-i) option is turned off by default because many
versions of tar write garbage after the end-of-archive entry,
since that part of the media is never supposed to be read. GNU
tar does not write after the end of an archive, but seeks to
maintain compatablity among archiving utilities.
@FIXME{Is this in the right place? It doesn't exist anywhere else in the book (except the appendix), and has no further explanation. For that matter, what does it mean?!}
tar Writes Files@FIXME{need to mention the brand new option, --backup}
Normally, tar writes extracted files into the file system without
regard to the files already on the system; i.e., files with the same
names as archive members are overwritten when the archive is extracted.
If the name of a corresponding file name is a symbolic link, the file
pointed to by the symbolic link will be overwritten instead of the
symbolic link itself (if this is possible). Moreover, special devices,
empty directories and even symbolic links are automatically removed if
they are found to be on the way of the proper extraction.
To prevent tar from extracting an archive member from an archive
if doing so will overwrite a file in the file system, use
--keep-old-files (-k) in conjunction with `--extract'. When
this option is specified, tar will report an error stating the
name of the files in conflict instead of overwriting the file with the
corresponding extracted archive member.
@FIXME{these two P's have problems. i don't understand what they're trying to talk about well enough to fix them; i may have just made them worse (in particular the first of the two). waiting to talk with hag.}
The --unlink-first (-U) option removes existing files, symbolic links, empty directories, devices, etc., prior to extracting over them. In particular, using this option will prevent replacing an already existing symbolic link by the name of an extracted file, since the link itself is removed prior to the extraction, rather than the file it points to. On some systems, the backing store for the executable is the original program text. You could use the --unlink-first (-U) option to prevent segmentation violations or other woes when extracting arbitrary executables over currently running copies. Note that if something goes wrong with the extraction and you did use this option, you might end up with no file at all. Without this option, if something goes wrong with the extraction, the existing file is not overwritten and preserved.
@FIXME{huh?} If you specify the --recursive-unlink option,
tar removes anything that keeps you from extracting a file
as far as current permissions will allow it. This could include removal
of the contents of a full directory hierarchy. For example, someone
using this feature may be very surprised at the results when extracting
a directory entry from the archive. This option can be dangerous; be
very aware of what you are doing if you choose to use it.
tar from over-writing
existing files with files with the same name from the archive.
The --keep-old-files (-k) option is meaningless with --list (-t).
Prevents tar from overwriting files in the file system during
extraction.
Some people argue that GNU tar should not hesitate to overwrite
files with other files when extracting. When extracting a tar
archive, they expect to see a faithful copy of the state of the filesystem
when the archive was created. It is debatable that this would always
be a proper behaviour. For example, suppose one has an archive in
which `usr/local' is a link to `usr/local2'. Since then,
maybe the site removed the link and renamed the whole hierarchy from
`/usr/local2' to `/usr/local'. Such things happen all the time.
I guess it would not be welcome at all that GNU tar removes the
whole hierarchy just to make room for the link to be reinstated (unless it
also simultaneously restores the full `/usr/local2', of course!
GNU tar is indeed able to remove a whole hierarchy to reestablish a
symbolic link, for example, but only if --recursive-unlink
is specified to allow this behaviour. In any case, single files are
silently removed.
Normally, tar sets the modification times of extracted files to
the modification times recorded for the files in the archive, but
limits the permissions of extracted files by the current umask
setting.
To set the modification times of extracted files to the time when the files were extracted, use the --touch (-m) option in conjunction with --extract (--get, -x).
To set the modes (access permissions) of extracted files to those recorded for those files in the archive, use `--same-persmissions' in conjunction with the --extract (--get, -x) operation. @FIXME{Should be aliased to ignore-umask.}
@FIXME{Following paragraph needs to be rewritten: why doesnt' this cat files together, why is this useful. is it really useful with more than one file?}
To write the extracted files to the standard output, instead of creating the files on the file system, use --to-stdout (-O) in conjunction with --extract (--get, -x). This option is useful if you are extracting files to send them through a pipe, and do not need to preserve them in the file system. If you extract multiple members, they appear on standard output concatenated, in the order they are found in the archive.
tar writes
the contents of the files extracted to its standard output. This may
be useful if you are only extracting the files in order to send them
through a pipe. This option is meaningless with --list (-t).
@FIXME{Why would you want to do such a thing, how are files separated on the standard output? is this useful with more that one file? Are pipes the real reason?}
@FIXME{the various macros in the front of the manual think that this option goes in this section. i have no idea; i only know it's nowhere else in the book...}
If a previous attempt to extract files failed due to lack of disk
space, you can use --starting-file=name (-K name) to start extracting only
after member name of the archive. This assumes, of course, that
there is now free space, or that you are now extracting into a
different file system. (You could also choose to suspend tar,
remove unnecessary files from the file system, and then restart the
same tar operation. In this case, --starting-file=name (-K name) is
not necessary. See section Using tar to Perform Incremental Dumps, See section Asking for Confirmation During Operations,
and section Excluding Some Files.)
@FIXME{we don't need/want --preserve to exist any more (from melissa: ie, don't want that *version* of the option to exist, or don't want the option to exist in either version?}
@FIXME{i think this explanation is lacking.}
The --same-order (--preserve-order, -s) option tells tar that the list of file
names to be listed or extracted is sorted in the same order as the
files in the archive. This allows a large list of names to be used,
even on a small machine that would not otherwise be able to hold all
the names in memory at the same time. Such a sorted list can easily be
created by running `tar -t' on the archive and editing its output.
This option is probably never needed on modern computer systems.
GNU tar offers options for making backups of files before writing
new versions. These options control the details of these backups.
They may apply to the archive itself before it is created or rewritten,
as well as individual extracted members. Other GNU programs (cp,
install, ln, and mv, for example) offer similar
options.
Backup options may prove unexpectedly useful when extracting archives containing many members having identical name, or when extracting archives on systems having file name limitations, making different members appear has having similar names through the side-effect of name truncation. (This is true only if we have a good scheme for truncated backup names, which I'm not sure at all: I suspect work is needed in this area.) When any existing file is backed up before being overwritten by extraction, then clashing files are automatically be renamed to be unique, and the true name is kept for only the last file of a series of clashing files. By using verbose mode, users may track exactly what happens.
At the detail level, some decisions are still experimental, and may change in the future, we are waiting comments from our users. So, please do not learn to depend blindly on the details of the backup features. For example, currently, directories themselves are never renamed through using these options, so, extracting a file over a directory still has good chances to fail. Also, backup options apply to created archives, not only to extracted members. For created archives, backups will not be attempted when the archive is a block or character device, or when it refers to a remote file.
For the sake of simplicity and efficiency, backups are made by renaming old files prior to creation or extraction, and not by copying. The original name is restored if the file creation fails. If a failure occurs after a partial extraction of a file, both the backup and the partially extracted file are kept.
SIMPLE_BACKUP_SUFFIX
environment variable is used. And if SIMPLE_BACKUP_SUFFIX is not
set, the default is `~', just as in Emacs.
VERSION_CONTROL
environment variable is used. And if VERSION_CONTROL is not set,
the default backup type is `existing'.
This option corresponds to the Emacs variable `version-control';
the same values for method are accepted as in Emacs. This options
also more descriptive name. The valid methods (unique
abbreviations are accepted):
Some people express the desire to always use the op-backup option, by defining some kind of alias or script. This is not as easy as one may thing, due to the fact old style options should appear first and consume arguments a bit inpredictably for an alias or script. But, if you are ready to give up using old style options, you may resort to using something like (a Bourne shell function here):
tar () { /usr/local/bin/tar --backup $*; }
tar Usages@UNREVISED
@FIXME{Using Unix file linking capability to recreate directory
structures--linking files into one subdirectory and then
tarring that directory.}
@FIXME{Nice hairy example using absolute-names, newer, etc.}
You can easily use archive files to transport a group of files from
one system to another: put all relevant files into an archive on one
computer system, transfer the archive to another system, and extract
the contents there. The basic transfer medium might be magnetic tape,
Internet FTP, or even electronic mail (though you must encode the
archive with uuencode in order to transport it properly by
mail). Both machines do not have to use the same operating system, as
long as they both support the tar program.
For example, here is how you might copy a directory's contents from one disk to another, while preserving the dates, modes, owners and link-structure of all the files therein. In this case, the transfer medium is a pipe, which is one a Unix redirection mechanism:
$ cd sourcedir; tar -cf - . | (cd targetdir; tar -xf -)
The command also works using short option forms:
@FIXME{The following using standard input/output correct??}
$ cd sourcedir; tar --create --file=- . | (cd targetdir; tar --extract --file=-)
This is one of the easiest methods to transfer a tar archive.
You have now seen how to use all eight of the operations available to
tar, and a number of the possible options. The next chapter
explains how to choose and change file and archive names, how to use
files to store names of other files which you can then call as
arguments to tar (this can help you save time if you expect to
archive the same list of files a number of times), and how to
@FIXME{in case it's not obvious, i'm making this up in some sense
based on my imited memory of what the next chapter *really* does. i
just wanted to flesh out this final section a little bit so i'd
remember to sitck it in here. :-)}
If there are too many files to conveniently list on the command line,
you can list the names in a file, and tar will read that file.
See section Reading Names from a File.
There are various ways of causing tar to skip over some files,
and not archive them. See section Choosing Files and Names for tar.
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