Re: Backup/Copy tree with alias file

From: Alan E. Feldman (spamsink2001_at_yahoo.com)
Date: 09/15/03 

Date: 15 Sep 2003 12:39:40 -0700

Kilgallen@SpamCop.net (Larry Kilgallen) wrote in message news:<UzoV7QQVVvYJ@eisner.encompasserve.org>...
> In article <b096a4ee.0309071319.3cf17ad4@posting.google.com>, spamsink2001@yahoo.com (Alan E. Feldman) writes:
>
> > So why not have the initial cluster code use [SYSn.foo] and then
> > [VMS$COMMON.foo] ? (Substitute VMS$COMMON with whatever it was called
> > back then.)
>
> That would prevent having two different versions of the operating
> system on the same disk. (That is not presently supported, but
> this piece of VMS does not prevent it, and to the best of my knowledge
> the only piece of VMS that prevents it is upgrade and installation.
>
> > The question remains: Why is [VMS$COMMON] aliased as [SYSn.SYSCOMMON]?
>
> I thought I explained that in my earlier post.

In your earlier post you wrote:

[begin quote]
There is at least what you see from

 $ SEARCH SYS$STARTUP:VMS$INITIAL-050_VMS.COM SYSCOMMON

and I thought I remembered something from an executable image used in
startup, but I don't have time to look it up.
[end quote]

OK. So what does this SEARCH command give us? Let's take a look (with
the addition of a /WINDOW qualifier):

$define/system/exec/nolog/translation=(terminal,concealed)
sys$specific 'root
        no_common_root =
f$search("SYS$SPECIFIC:[000000]SYSCOMMON.DIR") .eqs. ""
$if no_common_root then -
define/system/exec/nolog/translation=(terminal,concealed) sys$sysroot
'root
$if no_common_root then -
define/system/exec/nolog/translation=(terminal,concealed) sys$common
'root

OK. So -- and my apologies if I'm missing something obvious -- if
VMS$COMMON weren't aliased as SYSCOMMON, then couldn't the DCL command
procedure have been written to simply look for
SYS$SYSDEVICE:[000000]VMS$COMMON.DIR instead? In which case why bother
with the alias? I don't think one would write the f$search command
above and then add the alias to give it something to look for. Is
there some reason to allow for SYS$SPECIFIC trees without the common
tree?

Yes, it is more flexible to have VMS$COMMON.DIR aliased, but it comes
with a cost. You have to remember /EXCLUDE=[SYS%.SYSCOMMON...] to
avoid accessing the same files repeatedly for various commands. And,
of course, one shouldn't manually delete a system directory tree
without such a qualifier lest one end up deleting the VMS$COMMON tree!

So what's the big benefit of the syscommon alias that balances out the
costs?

Disclaimer: JMHO
Alan E. Feldman

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