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From: ccdn@levels.sait.edu.au (DAVID NEWALL)
Newsgroups: comp.unix.xenix,comp.unix.questions
Subject: Re: adding to the end of tarred Archive 60 meg tapes
Message-ID: <226@levels.sait.edu.au>
Date: 24 May 89 14:22:38 GMT
References: <479@mjbtn.MFEE.TN.US>
Organization: Sth Australian Inst of Technology
Lines: 36

In article <479@mjbtn.MFEE.TN.US>, root@mjbtn.MFEE.TN.US (Mark J. Bailey) writes:
> Could somebody please explain to me how I can make use of the 'r' option
> of tar to replace/add on at the end of a previous tar archive on an
> Archive 60 stream tape cartidge under SCO Xenix 2.3.0???  I have tried
> using the '/dev/nrct0' which is designed to leave the tape sitting where
> it ended instead of rewinding it as it does under /dev/rct0.

The simple answer to you question, is that you can't replace or add to a
tar file when that tar file is on a cartridge.  (I mean QIC; but this is
probably true for other cartridges, too).  Basically, in order to do what
you want, the drive must be capable of switching from read to write, and
back again.  QIC's can't.

However, you can store multiple "files" on the one cartridge.  After you
have written to tape (when you close the device, I think), an end of file
mark is appended.  The tape drive is capable of positioning the tape after
an end of file mark.  You can then switch from read to write mode (but not,
I think, back again) and write a new file.

What I am saying is that you can append a file to the end of a tape.  (But
not to the end of a tar image, stored on tape).

The good news, then, is that you can achieve the effect that you want.
When you want to add more files to your tape, you should seek to the end,
and then start writing.  How?  It's easy.  You have to know how many
"files" are already on the tape, though.  For each file that is already
there, execute something like "cat < /dev/nrct0 > /dev/null".  This will
advance over each file.  Then you do your normal "tar f /dev/rct0", which
adds one more file to the tape.  (I assume /dev/rct0 does not rewind the
tape on open?)


David Newall                     Phone:  +61 8 343 3160
Unix Systems Programmer          Fax:    +61 8 349 6939
Academic Computing Service       E-mail: ccdn@levels.sait.oz.au
SA Institute of Technology       Post:   The Levels, South Australia, 5095