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ΓòÉΓòÉΓòÉ 1. -Preface- ΓòÉΓòÉΓòÉ
This file documents the tape archive of the GNU system.
Copyright (C) 1988 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.
ΓòÉΓòÉΓòÉ 2. Preface ΓòÉΓòÉΓòÉ
This manual describes the GNU tape archiver, tar, and how you can use it to
store copies of a file or a group of files in an archive. This archive may be
written directly to a magnetic tape or other storage medium, stored as a file,
or sent through a pipe to another program. tar can also be used to add files
to an already existing archive, list the files in an archive, or extract the
files in the archive.
GNU tar was written by John Gilmore, and modified by many people. The GNU
enhancements were written by Jay Fenlason.
Using the Tape Archiver
You can use the GNU tape archiver, tar, to store copies of a file or a group
of files in an archive. This archive may be written directly to a magnetic
tape or other storage medium, stored as a file, or sent through a pipe to
another program. tar can also be used to add files to an already existing
archive, list the files in an archive, or extract the files in the archive.
Why What tar archives are good for.
Commands How to tell tar what to do.
Options Options that change the way tar behaves.
FullDumps Using tar to perform full dumps.
IncDumps Using tar to perform incremental dumps.
Problems Common problems using tar.
Rem Tape The remote tape server.
Format The format of a tar archive.
ΓòÉΓòÉΓòÉ 3. The Uses of Tape Archives ΓòÉΓòÉΓòÉ
The tape archiver tar allows you to store many files in an archive file or tar
file which describes the names and contents of the constituent files. Later
you can extract some or all of these files from the archive.
Tar files are not restricted to magnetic tapes. The tar program can equally
well use an ordinary file, or a pipe, or any device, as the archive. But they
were originally designed for use with magnetic tapes, and that is how the name
``tar'' came about.
Archive files can be used for transporting 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, 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.
A magnetic tape can store several files in sequence, but has no names for
them, just relative position on the tape. A tar file or something like it is
the only way to store several files on one tape and retain their names. Even
when the basic transfer mechanism can keep track of names, as FTP can, the
nuisance of handling multiple files, directories, and multiple links, may make
a tar file a much easier method.
Archive files are also used for long-term storage, which you can think of as
transportation from one time to another.
Piping one tar to another is an easy way to copy a directory's contents from
one disk to another, while preserving the dates, modes, owners and
link-structure of all the files therein.
The GNU version of tar has special features that allow it to be used to make
incremental and full dumps of all the files in a filesystem.
ΓòÉΓòÉΓòÉ 4. The Different Operations tar Can Perform ΓòÉΓòÉΓòÉ
One program, tar, is used to create an archive, to extract files from an
archive, to modify an archive, or to list the contents. Each time you run
tar, you must give a command to specify which one of these things you want to
do.
The command must always be in the first argument to tar. This argument can
also contain options (see Options). For compatibility with Unix tar, the
first argument is always treated as containing command and option letters even
if it doesn't start with '-'. Thus, 'tar c' is equivalent to 'tar -c': both
of them specify the '-c' command to create an archive.
In addition, a set of long-named options are provided which can be used
instead of or inter-mixed with the single-letter flags. The long-named
options are meant to be easy to remember and logical, while the single letter
flags may not always be. Long-named options are preceded by a '+'. In the
list of single-letter commands below, each corresponding long-named option is
listed next to it, in parentheses.
The remaining arguments to tar are either options, if they start with '-' or
'+', or files to operate on.
The file names that you give as arguments are the files that tar will act on
-- for example, they are the files to put in the archive, or the files to
extract from it. If you don't give any file name arguments, the default
depends on which command you used. Some commands use all relevant files; some
commands have no default and will report an error if you don't specify files.
If a file name argument actually names a directory, then that directory and
all files and subdirectories in it are used.
Here is a list of all the tar commands:
'-c'
'+create' This command tells tar to create a new archive that contains the
file(s) specified on the command line. If you don't specify files,
all the files in the current directory are used.
If the archive file already exists, it is overwritten; the old
contents are lost.
'-d'
'+compare'
'+diff' This command causes tar to compare the archive with the files in the
file system. It will report differences in file size, mode, owner,
and contents. If a file exists in the archive, but not in the file
system, tar will report this.
If you specify file names, those files are compared with the tape
and they must all exist in the archive. If you don't specify files,
all the files in the archive are compared.
'-r'
'+append' This command causes tar to add the specified file(s) to the end of
the archive. This assumes that the archive file already exists and
is in the proper format (which probably means it was created
previously with the tar program). If the archive is not in a format
that tar understands, the results will be unpredictable.
You must specify the files to be used; there is no default.
'-t'
'+list' This command causes tar to display a list of the files in the
archive. If you specify file names, only the files that you specify
will be mentioned (but each of them is mentioned only if it appears
in the archive).
'-u'
'+update' This command causes tar to add the specified files to the end of the
archive, like '-r', but only when a file doesn't already exist in
the archive or is newer than the version in the archive (the
last-modification time is compared). Adding files to the end of an
archive can be very slow.
You must specify the files to be used; there is no default.
'-x'
'+extract'
'+get' This command causes tar to extract the specified files from the
archive. If no file names are given, all the files in the archive
will be extracted.
'-A'
'+catenate'
'+concatenate'This command is used for concatenating several archive files
into one big archive file. The files to operate on should all be
archive files. They are all appended to the end of the archive file
which tar works on. (The other files are not changed).
You might be tempted to use cat for this, but it won't ordinarily
work. A tar archive contains data which indicates the end of the
archive, so more material added to the end with cat would be
ignored. This command works because it removes the end-of-archive
markers from the middle of the result.
'-D'
'+delete' This command causes tar to delete the specified files from the
archive. This command is extremely slow. Warning: Use of this
command on archives stored on magnetic tape may result in a
scrambled archive. There is no safe way (except for completely
re-writing the archive) to delete files from a magnetic tape.
ΓòÉΓòÉΓòÉ 5. Options That Change How tar Works ΓòÉΓòÉΓòÉ
Options may be specified as individual arguments starting with '-'. In this
case, if the option wants an argument (as does, for example, '-f') then the
argument should come after the option, separated from it by a space. All
options are optional. Some options make sense with any command, while others
are meaningful only with particular commands.
General Options Options that are always meaningful.
Creation Options Options for creating or updating an archive.
Extraction Options Options for listing or extracting files.
Option Syntax Old syntax for options
ΓòÉΓòÉΓòÉ 5.1. Options That Are Always Meaningful ΓòÉΓòÉΓòÉ
'-b number'
'+block-size number'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 blocks of number*512 bytes.
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 blocks 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.
With GNU tar the blocking factor is limited only by the maximum
block size of the device containing the archive, or by the amount of
available virtual memory.
'-f filename'
'+file filename'This option is used to specify the file name of the archive
tar works on.
If this option is not given, but the environment variable TAPE is
set, its value is used; otherwise, a default archive name (which was
picked when tar was compiled) is used. The default is normally set
up to be the ``first'' tape drive or other transportable I/O medium
on the system.
If the filename is '-', tar reads the archive from standard input
(when listing or extracting), or writes it to standard output (when
creating). If the '-' filename 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 filename contains ':/dev/', it is interpreted as
'hostname:filename'. If the hostname contains an ``at'' sign ('@'),
it is treated as 'user@hostname:filename'. In either case, tar will
invoke the command rsh (or remsh) to start up an /etc/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 a copy of
'/etc/rmt'. /etc/rmt is free software from the University of
California, and a copy of the source code can be found with the
sources for tar. /etc/rmt will have to be modified to run on
non-BSD4.3 systems.
'-C dir'
'+directory dir'This option causes tar to change into the directory dir before
continuing. This option is usually interspersed with the files tar
is to work on. For example,
tar -c iggy ziggy -C baz melvin
will place the files 'iggy' and 'ziggy' from the current directory on the
tape, followed by the file 'melvin' from the directory 'baz'. This option is
especially useful when you have several widely separated files that you want
to store in the same directory in the archive.
Note that the file 'melvin' is recorded in the archive under the precise name
'melvin', not 'baz/melvin'. 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 -x', all three files will be created in the current directory.
Contrast this with the command
tar -c iggy ziggy bar/melvin
which records the third file in the archive under the name 'bar/melvin' so
that, if plain 'tar -x' is used, the third file will be created in a
subdirectory named 'bar'.
'-M'
'+multi-volume'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.
When this option is used, tar will not abort when it cannot read or
write any more data. Instead, it will ask you to prepare a new
volume. If the archive is on a magnetic tape, you should change
tapes now; if the archive is on a floppy disk, you should change
disks, etc.
Each volume of a multi-volume archive is an independent tar archive,
complete in itself. For example, you can list or extract any volume
alone (just don't specify '-M'). However, if one file in the
archive is split across volumes, the only way to extract it
successfully is with a multi-volume extract command ('-xM') starting
on or before the volume where the file begins.
'-N date'
'+after-date date'This option causes tar to only work on files whose
modification or inode-changed times are newer than the date given.
The main use is for creating an archive; then only new files are
written. If extracting, only newer files are extracted.
Remember that the entire date argument should be quoted if it
contains any spaces.
The date is parsed using getdate.
'-R'
'+record-number'If '-R' is used, tar prints, along with every message it would
normally produce, the record number within the archive where the
message occurred. This option is especially useful when reading
damaged archives, since it helps pinpoint the damaged sections.
This can also be useful when making a log of a file-system backup
tape, since the results allow you to find the file you want to
retrieve on several backup tapes and choose the tape where the file
appears earliest (closest to the front of the tape).
'-T filename'
'+files-from filename'Instead of taking the list of files to work on from the
command line, the list of files to work on is read from the file
filename. If filename is given as '-', the list is read from
standard input. Note that using both '-T -' and '-f -' will not
work unless you are using the '-c' command.
'-v'
'+verbose'This option causes tar to be verbose about the actions it is taking.
Normally, the '-t' command to list an archive prints just the file
names (one per line) and the other commands are silent.
'-tv' prints a full line of information about each file, like the
output of 'ls -l'. '-v' with any other command (aside from '-t')
prints just the name of each file operated on.
The output from '-v' appears on the standard output except when
creating or updating an archive to the standard output, in which
case the output from '-v' is sent to the standard error.
'+version'
This option causes tar to print out its version number to the
standard error. It has no equivalent short option name.
'-w'
'+interactive'This option causes tar to print a message for each action it
intends to take, and ask for confirmation on the terminal. To
confirm, you must type a line of input. If your input line begins
with 'y', the action is performed, otherwise it is skipped.
The actions which require confirmation include adding a file to the
archive, extracting a file from the archive, deleting a file from
the archive, and deleting a file from disk.
If tar is reading the archive from the standard input, tar will open
the file '/dev/tty' to ask for confirmation on.
'-X file'
'+exclude file'This option causes tar to read a list of filename regular
expressions, one per line, from the file file; tar will ignore files
with those names. Thus if tar is called as 'tar -c -X foo .' and
the file 'foo' contains '*.o' none of the files whose names end in
'.o' in the current directory (or any subdirectory of the current
directory) will be added to the archive. Multiple -X options may be
given.
'-z'
'-Z'
'+compress'
'+uncompress'The archive should be compressed as it is written, or
decompressed as it is read, using the compress program. This option
works 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) block size. The default compression
parameters are used; if you need to override them, avoid the '-z'
option and run compress explicitly.
If the '-z' option is given twice, or the '+compress-block' option
is used, tar will pad the archive out to the next block boundry (see
General Options). This may be useful with some devices that require
that all write operations be a multiple of a certain size.
Note that the '-z' option will not work with the '-M' option, or
with the '-u', '-r', '-A', or '-D' commands.
ΓòÉΓòÉΓòÉ 5.2. Options for Creating Or Updating an Archive ΓòÉΓòÉΓòÉ
These options are used to control which files tar puts in an archive, or to
control the format the archive is written in (see Format).
Except as noted below, these options are useful with the '-c', '-r', '-u',
'-A', and '-D' commands. Also note that the '-B' option, (see Extraction
Options), is also useful with the '-r', '-u', '-A', and '-D' commands.
'-G'
'+incremental'This option should only be used when creating an incremental
backup of a filesystem. When the '-G' option is used, tar writes,
at the beginning of the archive, an entry for each of the
directories that will be operated on. The entry for a directory
includes a list of all the files in the directory at the time the
dump was done, and a flag for each file indicating whether the file
is going to be put in the archive. This information is used when
doing a complete incremental restore.
Note that this option causes tar to create a non-standard archive
that may not be readable by non-GNU versions of the tar program.
'-h'
'+dereference'If '-h' is used, when tar encounters a symbolic link, it will
archive the linked-to file, instead of simply recording the presence
of a symbolic link. If the linked-to file is archived again, an
entire second copy of it will be archived, instead of a link. This
could be considered a bug.
'-l'
'+one-file-system'This option causes tar to not cross filesystem boundaries
when archiving parts of a directory tree. This option only affects
files that are archived because they are in a directory that is
archived; files named on the command line are archived regardless,
and they can be from various file systems.
This option is useful for making full or incremental archival
backups of a file system, as with the Unix dump command.
Files which are skipped due to this option are mentioned on the
standard error.
'-o'
'+old-archive'
'+old'
'+portability'This option causes tar to write an old format archive, which
does not include information about directories, pipes, fifos,
contiguous files, or device files, and specifies file ownership by
numeric user- and group-ids rather than by user and group names. 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.
When updating an archive, do not use '-o' unless the archive was
created with the '-o' option.
'-S'
'+sparse' This option causes all files to be put in the archive to be tested
for sparseness, and handled specially if they are. The '-S' option
is useful when many dbm files, for example, are being backed up, and
running out of space on the tape is an issue. Using this option
dramatically decreases the amount of space needed to store such a
file.
In later versions, this option may be removed, and the testing and
treatment of sparse files may be done automatically with any special
GNU options. For now, it is an option needing to be specified on
the command line with the creation or updating of an archive.
'-V name'
'+volume name'This option causes tar to write out a volume header at the
beginning of the archive. If '-M' is used, each volume of the
archive will have a volume header of 'name Volume N', where N is 1
for the first volume, 2 for the next, and so on.
'-W'
'+verify' 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.
ΓòÉΓòÉΓòÉ 5.3. Options for Listing Or Extracting Files ΓòÉΓòÉΓòÉ
The options in this section are meaningful with the '-x' command. Unless
otherwise stated, they are also meaningful with the '-t' command.
'-B'
'+read-full-blocks'If '-B' is used, tar will not panic if an attempt to read a
block from the archive does not return a full block. Instead, tar
will keep reading until it has obtained a full block.
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
block from the pipe.
This option is also useful with the commands for updating an
archive.
'-G'
'+incremental'The '-G' option means the archive is an incremental backup. Its
meaning depends on the command that it modifies.
If the '-G' option is used with '-t', tar will list, for each
directory in the archive, the list of files in that directory at the
time the archive was created. This information is put out in a
format that is not easy for humans to read, but which is unambiguous
for a program: each filename is preceded by either a 'Y' if the file
is present in the archive, an 'N' if the file is not included in the
archive, or a 'D' if the file is a directory (and is included in the
archive). Each filename is terminated by a null character. The
last file is followed by an additional null and a newline to
indicate the end of the data.
If the '-G' option is used with '-x', then when the entry for a
directory is found, all files that currently exist in that directory
but are not listed in the archive are deleted from the directory.
This behavior is convenient when you are restoring a damaged file
system from a succession of incremental backups: it restores the
entire state of the file system to that which obtained when the
backup was made. If you don't use '-G', the file system will
probably fill up with files that shouldn't exist any more.
'-i'
'+ignore-zeros'The '-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 cat-ing 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.
'-k'
'+keep-old-files'The '-k' option prevents tar from over-writing existing files
with files with the same name from the archive.
The '-k' option is meaningless with '-t'.
'-K filename'
'+starting-file filename'The '-K' option causes tar to begin extracting or
listing the archive with the file filename, and to consider only the
files starting at that point in the archive. This is useful if a
previous attempt to extract files failed when it reached filename
due to lack of free space. (This assumes, of course, that there is
now free space, or that you are now extracting into a different file
system.)
'-m'
'+modification-time'When this option is used, tar leaves the modification
times 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 '-t'.
'-O'
'+to-stdout'When this option is used, instead of creating the files specified,
tar writes the contents of the files extracted to its standard
output. This may be useful if you are only extracting the files in
order to send them through a pipe.
This option is meaningless with '-t'.
'-p'
'+same-permissions'
'+preserve-permissions'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 meaningless with '-t'.
'-P'
'+absolute-paths'This option should be used when the absolute pathname of a
file should be preserved in the archive. tar normally strips the
leading `/' from the name of the file, thus making /usr/foo/bar/baz
into usr/foo/bar/baz. Using the '-P' option keeps the pathname
intact, and is useful in that it is not necessary to change to the
root directory when extracting files.
'-s'
'+same-order'
'+preserve-order'This option tells tar that the list of filenames 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.
'+preserve'The '+preserve' option has no equivalent short option name. It is
equivalent to '-p' plus '-s'.
ΓòÉΓòÉΓòÉ 5.4. Old Syntax for Options ΓòÉΓòÉΓòÉ
For compatibility with Unix tar, the first argument can contain option letters
in addition to the command letter; for example, 'tar cv' specifies the option
'-v' in addition to the command '-c'. The first argument to GNU tar is always
treated as command and option letters even if it doesn't start with '-'.
Some options need their own arguments; for example, '-f' is followed by the
name of the archive file. When the option is given separately, its argument
follows it, as is usual for Unix programs. For example:
tar -c -v -b 20 -f /dev/rmt0
When options that need arguments are given together with the command, all the
associated arguments follow, in the same order as the options. Thus, the
example above could also be written in the old style as follows:
tar cvbf 20 /dev/rmt0
Here '20' is the argument of '-b' and '/dev/rmt0' is the argument of '-f'.
The long-named options can be used instead of the single-letter flags. They
are meant to be obvious and easy to remember, possibly more so than their
corresponding single-letter options. The above example using long-named
options would look like this:
tar +create +verbose +block-size +file 20 /dev/rmt0
ΓòÉΓòÉΓòÉ 6. Using tar to Perform Full Dumps ΓòÉΓòÉΓòÉ
Full dumps should only be made when no other people or programs are modifying
files in the filesystem. If files are modified while tar is making the
backup, they may not be stored properly in the archive, in which case you
won't be able to restore them if you have to.
You will want to use the '-V' option to give the archive a volume label, so
you can tell what this archive is even if the label falls off the tape, or
anything like that.
Unless the filesystem you are dumping is guaranteed to fit on one volume, you
will need to use the '-M' option. Make sure you have enough tapes on hand to
complete the backup.
If you want to dump each filesystem separately you will need to use the '-l'
option to prevent tar from crossing filesystem boundaries when storing
(sub)directories.
The '-G' option is not needed, since this is a complete copy of everything in
the filesystem, and a full restore from this backup would only be done onto a
completely empty disk.
Unless you are in a hurry, and trust the tar program (and your tapes), it is a
good idea to use the -W (verify) option, to make sure your files really made
it onto the dump properly. This will also detect cases where the file was
modified while (or just after) it was being archived.
ΓòÉΓòÉΓòÉ 7. Using tar to Perform Incremental Dumps ΓòÉΓòÉΓòÉ
Performing incremental dumps is similar to performing full dumps, although a
few more options will usually be needed.
You will need to use the '-N date' option to tell tar to only store files that
have been modified since date. date should be the date and time of the last
full/incremental dump.
A standard scheme is to do a 'monthly' (full) dump once a month, a 'weekly'
dump once a week of everything since the last monthly and a 'daily' every day
of everything since the last (weekly or monthly) dump.
Here is a copy of the script used to dump the filesystems of the machines here
at the Free Software Foundation. This script is run (semi-)automatically late
at night when people are least likely to be using the machines. This script
dumps several filesystems from several machines at once (by using a
network-filesystem). The operator is responsible for ensuring that all the
machines will be up at the time the dump happens. If a machine is not
running, its files will not be dumped, and the next day's incremental dump
will not store files that would have gone onto that dump.
#!/bin/csh
# Dump thingie
set now = `date`
set then = `cat date.nfs.dump`
/u/hack/bin/tar -c -G -v\
-f /dev/rtu20\
-b 126\
-N "$then"\
-V "Dump from $then to $now"\
/alpha-bits/gp\
/gnu/hack\
/hobbes/u\
/spiff/u\
/sugar-bombs/u
echo $now > date.nfs.dump
mt -f /dev/rtu20 rew
Output from this script is stored in a file, for the operator to read later.
This script uses the file 'date.nfs.dump' to store the date/time of the last
dump.
Since this is a streaming tape drive, no attempt to verify the archive is
done. This is also why the high blocking factor (126) is used. The tape
drive must also be rewound by the mt command after the dump is made.
ΓòÉΓòÉΓòÉ 8. Common Problems Using tar ΓòÉΓòÉΓòÉ
Unless you use the -P option, GNU tar will not allow you to create an archive
that contains absolute pathnames. (An absolute pathname is one that begins
with a '/'.) 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 pathnames as
if the pathnames were not absolute. If the archive contained a file
'/usr/bin/computoy', GNU tar would extract the file to 'usr/bin/computoy' in
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 '-P' option, or use the command 'tar -C / ┬╖┬╖┬╖'.
Some versions of UNIX, (Ultrix 3.1 is know 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 blocksize than the default 20.
In order to update an archive, tar must be able to backspace the archive in
order to re-read or re-write a block 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
ioctl(┬╖┬╖┬╖,MTIOCTOP,┬╖┬╖┬╖)).
This means that the '-r', '-u', '-A', and '-D' 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 '-M', '-V', and '-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 '-G' option.
ΓòÉΓòÉΓòÉ 9. The Remote Tape Server ΓòÉΓòÉΓòÉ
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 '/etc/rmt' on any machine whose tape drive
you want to use. tar calls '/etc/rmt' by running an rsh or remsh to the remote
machine, optionally using a different login name if one is supplied.
A copy of the source for the remote tape server is provided. It is Copyright
(C) 1983 by the Regents of the University of California, but can be freely
distributed. Instructions for compiling and installing it are included in the
'Makefile'.
The remote tape server may need to be modified in order to run on a non-4.3BSD
system.
ΓòÉΓòÉΓòÉ 10. The Format of a tar Archive ΓòÉΓòÉΓòÉ
This chapter is based heavily on John Gilmore's tar(5) manual page for the
public domain tar that GNU tar is based on.
ΓòÉΓòÉΓòÉ 10.1. The Standard Format ΓòÉΓòÉΓòÉ
A tar tape or file contains a series of records. Each record contains
RECORDSIZE 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 record which describes the file,
followed by zero or more records which give the contents of the file. At the
end of the archive file there may be a record filled with binary zeros as an
end-of-file marker. A reasonable system should write a record of zeros at the
end, but must not assume that such a record exists when reading an archive.
The records may be blocked for physical I/O operations. Each block of N
records (where N is set by the '-b' option to tar) is written with a single
write() operation. On magnetic tapes, the result of such a write is a single
tape record. When writing an archive, the last block of records should be
written at the full size, with records after the zero record containing all
zeroes. When reading an archive, a reasonable system should properly handle
an archive whose last block is shorter than the rest, or which contains
garbage records after a zero record.
The header record is defined in C as follows:
/*
* Standard Archive Format - Standard TAR - USTAR
*/
#define RECORDSIZE 512
#define NAMSIZ 100
#define TUNMLEN 32
#define TGNMLEN 32
#define SPARSE_EXT_HDR 21
#define SPARSE_IN_HDR 4
struct sparse {
char offset[12];
char numbytes[12];
};
union record {
char charptr[RECORDSIZE];
struct header {
char name[NAMSIZ];
char mode[8];
char uid[8];
char gid[8];
char size[12];
char mtime[12];
char chksum[8];
char linkflag;
char linkname[NAMSIZ];
char magic[8];
char uname[TUNMLEN];
char gname[TGNMLEN];
char devmajor[8];
char devminor[8];
/* these following fields were added by JF for gnu */
/* and are NOT standard */
char atime[12];
char ctime[12];
char offset[12];
char longnames[4];
/* the next three fields were added by JK to deal with
shrinking down sparse files */
struct sparse sp[SPARSE_IN_HDR];
char isextended;
char ending_blanks[12]; /* number of nulls at the
end of the file, if any */
} header;
struct extended_header {
struct sparse sp[21];
char isextended;
} ext_hdr;
};
/* The checksum field is filled with this while the checksum is computed. */
#define CHKBLANKS " " /* 8 blanks, no null */
/* The magic field is filled with this if uname and gname are valid. */
#define TMAGIC "ustar " /* 7 chars and a null */
/* The magic field is filled with this if this is a GNU format dump entry */
#define GNUMAGIC "GNUtar " /* 7 chars and a null */
/* The linkflag defines the type of file */
#define LF_OLDNORMAL '\0' /* Normal disk file, Unix compatible */
#define LF_NORMAL '0' /* Normal disk file */
#define LF_LINK '1' /* Link to previously dumped file */
#define LF_SYMLINK '2' /* Symbolic link */
#define LF_CHR '3' /* Character special file */
#define LF_BLK '4' /* Block special file */
#define LF_DIR '5' /* Directory */
#define LF_FIFO '6' /* FIFO special file */
#define LF_CONTIG '7' /* Contiguous file */
/* Further link types which were defined later. */
#define LF_DUMPDIR 'D' /* This is a dir entry that contains
the names of files that were in
the dir at the time the dump
was made */
#define LF_MULTIVOL 'M' /* This is the continuation
of a file that began on another
volume */
#define LF_SPARSE 'S' /* This is for sparse files */
#define LF_VOLHDR 'V' /* This file is a tape/volume header */
/* Ignore it on extraction */
/* 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 /* Save text (sticky bit) */
/* 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 */
All characters in header records 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 record 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 fileds are zero-filled octal numbers in ASCII. Each
numeric field of width w contains w minus 2 digits, a space, and a null,
except size, and mtime, which do not contain the trailing null.
The name field is the pathname 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 (``stick'') modes. Values for
these bits are defined above. When special permissions are required to create
a file with a given mode, and the user restoring files from the archive does
not hold such permissions, the mode bit(s) specifying those special
permissions are ignored. Modes which are not supported by the operating
system restoring files from the archive will be ignored. Unsupported modes
should be faked up when creating or updating an archive; e.g. the group
permission could be copied from the other permission.
The uid and gid fields are the numeric user and group ID of the file owners,
respectively. If the operating system does not support numeric user or group
IDs, these fields should be ignored.
The size field is the size of the file in bytes; linked files are archived
with this field specified as zero. See Extraction Options; in particular the
'-G' option.
The mtime field is the modification time of the file at the time it was
archived. It is the ASCII representation of the octal value of the last time
the file was modified, represented as an integer number of seconds since
January 1, 1970, 00:00 Coordinated Universal Time.
The chksum field is the ASCII representation of the octal value of the simple
sum of all bytes in the header record. Each 8-bit byte in the header is added
to an unsigned integer, initialized to zero, the precision of which shall be
no less than seventeen bits. When calculating the checksum, the chksum field
is treated as if it were all blanks.
The typeflag field specifies the type of file archived. If a particular
implementation does not recognize or permit the specified type, the file will
be extracted as if it were a regular file. As this action occurs, tar issues
a warning to the standard error.
The atime and ctime fields are used in making incremental backups; they store,
respectively, the particular file's access time and last inode-change time.
The offset is used by the -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 longnames field of the header belongs with something that is not yet
implemented in tar, and is therefore empty.
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 alloted 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 record 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_headers to describe a sparse file.
LF_NORMAL
LF_OLDNORMALThese flags represent a regular file. In order to be compatible
with older versions of tar, a typeflag value of LF_OLDNORMAL should
be silently recognized as a regular file. New archives should be
created using LF_NORMAL. Also, for backward compatibility, tar
treats a regular file whose name ends with a slash as a directory.
LF_LINK 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.
LF_SYMLINKThis represents a symbolic link to another file. The linked-to name
is specified in the linkname field with a trailing null.
LF_CHR
LF_BLK 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.
LF_DIR 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.
LF_FIFO This specifies a FIFO special file. Note that the archiving of a
FIFO file archives the existence of this file and not its contents.
LF_CONTIG 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 ID represented by these names will
be used rather than the values within the uid and gid fields.
ΓòÉΓòÉΓòÉ 10.2. GNU Extensions to the Archive Format ΓòÉΓòÉΓòÉ
The GNU format uses additional file types to describe new types of files in an
archive. These are listed below.
LF_DUMPDIR
'D' This represents a directory and a list of files created by the '-G'
option. The size field gives the total size of the associated list
of files. Each filename 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 filename is terminated by a null.
There is an additional null after the last filename.
LF_MULTIVOL
'M' This represents a file continued from another volume of a
multi-volume archive created with the '-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.
LF_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.
LF_VOLHDR
'V' This file type is used to mark the volume header that was given with
the '-V' option when the archive was created. The name field
contains the name given after the '-V' option. The size field is
zero. Only the first file in each volume of an archive should have
this type.
You may have trouble reading a GNU format archive on a non-GNU system if the
options '-G', '-M', '-S', or '-V' 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.
ΓòÉΓòÉΓòÉ 11. Concept Index ΓòÉΓòÉΓòÉ
(.txt)
(.txt)