- 论坛徽章:
- 0
|
如何用磁带可备份和恢复数据呀
# man tar
-------------------------------------------------------------------------------
Commands Reference, Volume 5
-------------------------------------------------------------------------------
tar Command
Purpose
Manipulates archives.
Syntax
tar { -c | -r | -t | -u | -x } [ -b Blocks ] [ -B ] [ -d ] [ -F ] [ -h ] [ -i ]
[ -L InputList ] [ -l ] [ -m ] [ -N Blocks ][ -o ] [ -p ] [ -s ] [ -v ] [ -w ] [
-Number ] [ -f Archive ] [ -S Blocksb | -S Feet | -S Feet@Density ] [ File |
Directory | -C Directory ] ...
Description
Attention: Because of limitations on header block space in the tar command,
user numbers (UIDs), and group identification numbers (GIDs) larger than
65,535 will be corrupted when restored to certain systems. The size
constraint affects only the ownership and permissions causing no damage to
the data. Corruption of the ownership occurs on the following systems:
o Those that do not use uname and gname fields to check ownership.
o Those that do not have the same user and group IDs as the archiving system.
Note:
1. The tar command is not enabled for files greater than 2 Gig in size due
to limitations imposed by XPG/4 and POSIX.2 standards.
2. tar does not preserve the sparse nature of any file that is sparsely
allocated. Any file that was originally sparse before the restoration
will have all space allocated within the filesystem for the size of the
file.
The tar command manipulates archives by writing files to, or retrieving files
from an archive storage medium. The files used by the tar command are
represented by the File parameter. If the File parameter refers to a directory,
then that directory and recursively all files and directories within it are
referenced as well.
The tar command looks for archives on the default device (usually tape), unless
you specify another device with the -f Archive flag. When specifying path names
that are greater than 100 characters for the United States Tape Archiver (USTAR)
format, remember that the path name is composed of a prefix buffer, a / (slash),
and a name buffer.
The prefix buffer can be a maximum of 155 bytes and the name buffer can hold a
maximum of 100 bytes. If the path name cannot be split into these two parts by a
slash, it cannot be archived. This limitation is due to the structure of the tar
archive headers, and must be maintained for compliance with standards and
backwards compatibility. In addition, the length of a destination for a hard or
symbolic link ( the 'link name') cannot exceed 100 bytes.
When writing to an archive, the tar command uses a temporary file (the /tmp/tar*
file) and maintains in memory a table of files with several links. You receive
an error message if the tar command cannot create the temporary file, or if
there is not enough memory available to hold the link tables.
Two groups of flags exist for the tar command: the required flags and the
optional flags. The required flags control the actions of the tar command and
include the -c, -r, -t, -u, and -x flags. At least one required flag must be
selected for the tar command to function. Having selected a required flag, you
can select an optional flag but none are necessary to control the tar command.
Notes:
1. When the storage device is an ordinary file or a block special file,
the -u and -r flags backspace. However, raw magnetic tape devices do
not support backspacing. So when the storage device is a raw magnetic
tape, the -u and -r flags rewind the tape, open it, and then read it
again.
2. Records are one block long on block magnetic tape, but they are
typically less than half as dense on raw magnetic tape. As a result,
although a blocked raw tape must be read twice, the total amount of
tape motion is less than when reading one-block records from a block
magnetic tape once.
3. The structure of a streaming tape device does not support the addition
of information at the end of a tape. Consequently when the storage
device is a streaming tape, the -u and -r flags are not valid options.
An attempt to use these flags results in the following error message:
tar: Update and Replace options not valid for a
streaming tape drive.
4. No recovery exists from tape errors.
5. The performance of the tar command to the IBM 9348 Magnetic Tape Unit
Model 12 can be improved by changing the default block size. To change
the block size, enter the following at the command line:
chdev -1 <device_name>; -a block_size=32k
For more information on using tape devices see the rmt special file.
Flags
Flags for the tar command are in two groups, the required and the optional. You
must supply at least one required flag to control the tar command.
Required Flags
-c Creates a new archive and writes the files specified by one or more File
parameters to the beginning of the archive.
-r Writes the files specified by one or more File parameters to the end of the
archive. This flag is not valid for any tape devices because such devices do not
support the addition of information at the end of a tape.
-t Lists the files in the order in which they appear in the archive. Files can
be listed more than once.
-u Adds the files specified by one or more File parameters to the end of the
archive only if the files are not in the archive already, or if they have been
modified since being written to the archive. The -u flag is not valid for any
tape devices because such devices do not support the addition of information at
the end of a tape.
-x Extracts the files specified by one or more File parameters from the archive.
If the File parameter refers to a directory, the tar command recursively
extracts that directory from the archive. If you do not specify the File
parameter, the tar command extracts all of the files from the archive. When an
archive contains multiple copies of the same file, the last copy extracted
overwrites all previously extracted copies. If the file being extracted does not
already exist on the system, the file is created. If you have the proper
permissions, the tar command restores all files and directories with the same
owner and group IDs as they have on the tape. If you do not have the proper
permissions, the files and directories are restored with your owner and group
IDs. It is not possible to ask for any occurrence of a file other than the last.
Optional Flags
-B Forces input and output blocking to 20 blocks per record. With this option,
the tar command can work across communications channels where blocking may not
be maintained.
-b Blocks Specifies the number of 512 bytes blocks per record. Both the default
and the maximum is 20, which is appropriate for tape records. Due to the size of
interrecord gaps, tapes written with large blocking factors can hold much more
data than tapes with only one block per record.
The block size is determined automatically when tapes are read (the -x or -t
function flags). When archives are updated with the -u and -r functions, the
existing record size is used. The tar command writes archives using the
specified value of the Blocks parameter only when creating new archives with the
-c flag.
For output to ordinary files with the -f flag, you can save disk space by using
a blocking factor that matches the size of disk blocks (for example, the -b4
flag for 2048-byte disk blocks).
-C Directory Causes the tar command to perform a chdir subroutine to the
directory specified by the Directory variable. Using the -C flag allows multiple
directories not related by a close common parent to be archived, using short
relative path names. For example, to archive files from the /usr/include and
/etc directories, you might use the following command:
tar c -C /usr/include File1 File2 -C /etc File3 File4
The -CDirectory flag must appear after all other flags and can appear in the
list of file names given.
-d Makes separate entries for block files, special character files, and
first-in-first-out (FIFO) piped processes. Normally, the tar command will not
archive these special files. When writing to an archive with the -d flag, the
tar command makes it possible to restore empty directories, special files, and
first-in-first-out (FIFO) piped processes with the -x flag.
Note: Although anyone can archive special files, only a user with root user
authority can extract them from an archive.
-F Checks the file type before archiving. Source Code Control Systems (SCCS),
Revision Control Systems (RCS), files named core, errs, a.out, and files ending
in .o (dot o) are not archived.
-f Archive Uses the Archive variable as the archive to be read or written. When
this flag is not specified, the tar command uses a system-dependent default file
name of the form /dev/rmt0. If the Archive variable specified is - (minus sign),
the tar command writes to standard output or reads from standard input. If you
write to standard output, the -c flag must be used.
-h Forces the tar command to follow symbolic links as if they were normal files
or directories. Normally, the tar command does not follow symbolic links.
-i Ignores header checksum errors. The tar command writes a file header
containing a checksum for each file in the archive. When this flag is not
specified, the system verifies the contents of the header blocks by recomputing
the checksum and stops with a directory checksum error when a mismatch occurs.
When this flag is specified, the tar command logs the error and then scans
forward until it finds a valid header block. This permits restoring files from
later volumes of a multi-volume archive without reading earlier volumes.
-L InputList The Inputlist argument to the -L option should be always a name of
file listing the files and directories that need to be archived.
Alternatively the InputList argument can be a variable holding the name of the
file that lists the files and directories that is to be archived.
This option helps in writing the files and directories listed in the InputList
argument to the archive. Directories from the InputList argument are not treated
recursively. For directories contained in the InputList argument, the tar
command writes only the directory to the archive, not the files and
subdirectories rooted in the directory.
If additional files and directories follow the InputList argument on the command
line, the contents of the InputList argument are archived after these files and
directories. These additional files or directories are archived with their
default behavior, which is to treat them recursively.
-l Writes an error message to standard output for each file with a link count
greater than 1 whose corresponding links were not also archived. For example, if
file1 and file2 are hard-linked together and only file1 is placed on the
archive, then the -l flag will issue an error message. Error messages are not
displayed if the -l flag is not specified.
-m Uses the time of extraction as the modification time. The default is to
preserve the modification time of the files.
-N Blocks Allows the tar command to use very large clusters of blocks when it
deals with streaming tape archives. Note however, that on input, the tar command
cannot automatically determine the block size of tapes with very long block
sizes created with this flag. In the absence of a -N Blocks flag, the largest
block size that the tar command can automatically determine is 20 blocks.
-o Provides backwards compatibility with older versions (non-AIX) of the tar
command. When this flag is used for reading, it causes the extracted file to
take on the User and Group ID (UID and GID) of the user running the program,
rather than those on the archive. This is the default behavior for the ordinary
user.
-p Restores fields to their original modes, ignoring the present umask. The
setuid, setgid, and tacky bit permissions are also restored to the user with
root user authority. This flag restores files to their original mode but does
not restore directories to their original mode.
-s Tries to create a symbolic link If the tar command is unsuccessful in its
attempt to link (regular link) two files with the -s flag.
-SBlocksb, -S Feet, -S Feet@Density Specifies the number of 512KB blocks per
volume (first format), independent of the tape blocking factor. You can also
specify the size of the tape in feet by using the second form, in which case the
tar command assumes a default Density variable. The third form allows you to
specify both tape length and density. Feet are assumed to be 11 inches long to
be conservative. This flag lets you deal more easily with multivolume tape
archives, where the tar command must be able to determine how many blocks fit on
each volume.
Note:
1. Tape drives vary in density capabilities. The Density variable
calculates the amount of data a system can fit on a tape.
2. When using 1/4-inch tape devices, be sure to take into account the
number of tracks on the tape device when specifying the value for the
Feet variable. For example, a 4-track,1/4-inch tape drive with a
600-foot tape and a density of 8000 bpi can be specified using the -S
Feet@Density flag as follows:
-S 2400@8000
where 600 feet multiplied by 4 tracks equals 2400 feet.
-v Lists the name of each file as it is processed. With the -t flag, -v gives
more information about the tape entries, including file sizes, times of last
modification, User Number (UID), Group Number (GID), and permissions.
-w Displays the action to be taken, followed by the file name, and then waits
for user confirmation. If the response is affirmative, the action is performed.
If the response is not affirmative, the file is ignored.
-Number Uses the /dev/rmtNumber file instead of the default. For example, the -2
flag is the same as the -f/dev/rmt2 file.
Exit Status
This command returns the following exit values:
0 Successful completion.
>;0 An error occurred.
Examples
1. To write the file1 and file2 files to a new archive on the default tape
drive, enter:
tar -c file1 file2
2. To extract all files in the /tmp directory from the archive file on the
/dev/rmt2 tape device and use the time of extraction as the modification
time, enter:
tar -xm -f/dev/rmt2 /tmp
3. To create a new archive file that contains the file1 file and pass the
archive file to the dd command to be written to the /dev/rmt1 device,
enter:
tar -cvf - file1 | dd of=/dev/rmt1 conv=sync
4. To display the names of the files in the out.tar disk archive file on the
current directory, enter:
tar -vtf out.tar
5. To expand the compressed tar archive file, fil.tar.z, pass the file to the
tar command, and extract all files from the expanded tar archive file,
enter:
zcat fil.tar.Z | tar -xvf -
6. To archive the contents of /usr/include and /usr/bin files using short
relative path names, enter:
cd /usr
tar -cvf/dev/rmt0 -C./include . -C ../bin .
Note: When specifying multiple instances of the -C flag with relative
path names, the user must take the previous -C flag request into
account.
7. To archive to an 8-mm device when using the -S flag, enter:
tar -cvf /dev/rmt0 -S 4800000b /usr
Note: When archiving to an 8-mm device, the -S Feet and -S Feet@Density
flags are not recommended, since the 8-mm device does not use the
concept of density when writing to a tape.
8. To archive a list of all C files that is listed in the file through the
InputList argument of the -L option, enter:
tar -cvf fl.tar -L fl_list
Where fl_list is a file consisting a list of all .c files in it. This can
be obtained as follows:
ls *.c >; fl_list
9. To archive a list of all C files by setting a variable using the -L option,
enter:
ls *.c >; fl_list
fl=fl_list
tar -cvf var.tar -L $fl
Files
/dev/rmt0 Specifies the default tape device.
/bin/tar Specifies the symbolic link to the tar command.
/usr/bin/tar Contains the tar command.
/tmp/tar* Specifies a temporary file.
Note:In AIX 3.2, the entire /bin directory is a symbolic link to /usr/bin.
Related Information
The cat command, dd command, pax command.
The rmt special file.
File Systems Overview for System Management in AIX 5L Version 5.1 System
Management Concepts: Operating System and Devices explains file system types,
management, structure, and maintenance.
Directory Overview in AIX 5L Version 5.1 System User's Guide: Operating System
and Devices explains working with directories and path names.
Files Overview in AIX 5L Version 5.1 System User's Guide: Operating System and
Devices provides information on working with files.
-------------------------------------------------------------------------------
# |
|
免费注册
发表于 2003-06-18 12:42
