08-03-2011
Another tip from the 1980's ... adjust your file selection list fed to the tar command to include some "First" and "Last" file. You can then write a script to verify the integrity of your tapes (before sending off-site?) by reading back the "First" and "Last" files. If you accidentally overflowed your tape, or the tape is "bad", you'll be missing the "Last" file when your tar -x completes, prompting further investigation.
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The scenario is as follow:
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Hi all!
I'm new in this forum. I need to ask a few question.
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Hi Guy`s I`m a newbie to Unix and I`m starting to love it
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TAR(5) File Formats Manual TAR(5)
NAME
tar - tape archive file format
DESCRIPTION
Tar, (the tape archive command) dumps several files into one, in a medium suitable for transportation.
A ``tar tape'' or file is a series of blocks. Each block is of size TBLOCK. A file on the tape is represented by a header block which
describes the file, followed by zero or more blocks which give the contents of the file. At the end of the tape are two blocks filled with
binary zeros, as an end-of-file indicator.
The blocks are grouped for physical I/O operations. Each group of n blocks (where n is set by the b keyletter on the tar(1) command line
-- default is 20 blocks) is written with a single system call; on nine-track tapes, the result of this write is a single tape record. The
last group is always written at the full size, so blocks after the two zero blocks contain random data. On reading, the specified or
default group size is used for the first read, but if that read returns less than a full tape block, the reduced block size is used for
further reads.
The header block looks like:
#define TBLOCK 512
#define NAMSIZ 100
union hblock {
char dummy[TBLOCK];
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];
} dbuf;
};
Name is a null-terminated string. The other fields are zero-filled octal numbers in ASCII. Each field (of width w) contains w-2 digits, a
space, and a null, except size and mtime, which do not contain the trailing null and chksum which has a null followed by a space. Name is
the name of the file, as specified on the tar command line. Files dumped because they were in a directory which was named in the command
line have the directory name as prefix and /filename as suffix. Mode is the file mode, with the top bit masked off. Uid and gid are the
user and group numbers which own the file. Size is the size of the file in bytes. Links and symbolic links are dumped with this field
specified as zero. Mtime is the modification time of the file at the time it was dumped. Chksum is an octal ASCII value which represents
the sum of all the bytes in the header block. When calculating the checksum, the chksum field is treated as if it were all blanks. Link-
flag is NULL if the file is ``normal'' or a special file, ASCII `1' if it is an hard link, and ASCII `2' if it is a symbolic link. The
name linked-to, if any, is in linkname, with a trailing null. Unused fields of the header are binary zeros (and are included in the check-
sum).
The first time a given i-node number is dumped, it is dumped as a regular file. The second and subsequent times, it is dumped as a link
instead. Upon retrieval, if a link entry is retrieved, but not the file it was linked to, an error message is printed and the tape must be
manually re-scanned to retrieve the linked-to file.
The encoding of the header is designed to be portable across machines.
SEE ALSO
tar(1)
BUGS
Names or linknames longer than NAMSIZ produce error reports and cannot be dumped.
4.2 Berkeley Distribution November 7, 1985 TAR(5)