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Re: Taking over a previously diverted file



On 10/18/06, Steve Langasek <vorlon@debian.org> wrote:
On Wed, Oct 18, 2006 at 09:47:43AM +0700, Theppitak Karoonboonyanan wrote:

> Is there some specific reason for using "mv -i ... < /dev/null" ?
> It looks strange to me, but many times strange stuffs have
> their own reasons. Can it be "mv -f ..." ?

If $TEXDIR/babel.sty exists and $TEXDIR/babel.sty.undivert exists, the admin
has changed something by hand and we can't be sure any more which of these
is the file belonging to thailatex.  Calling "mv -f" would ignore that
uncertainty; using "mv -i" treats that uncertainty as a reason not to
continue.  (The < /dev/null avoids any prompts being sent by the maintainer
script: we just want to fail out, not interact with the user.)

This could also be written as

  if [ -e $TEXDIR/babel.sty ] && ! [ -e $TEXDIR/babel.sty.undivert ]; then
    mv $TEXDIR/babel.sty $TEXDIR/babel.sty.undivert
  fi

which may indeed be a clearer way to write it.

Ah.. This is the key to understanding the whole logic.
Thank you for enlightening me.

> What happens if the "if [ -e $TEXDIR/babel.sty ]" clause fails?
> Does it mean the babel.sty.undivert won't be created and the
> second mv will fail?

The only reason that $TEXDIR/babel.sty should not exist here is if the
postinst has already moved it aside in preparation for the undivert. If
this clause fails for any other reason, that's a big (admin-induced)
problem, and the second mv will fail *by design* to avoid declaring the
package "installed" when files have been randomly removed...

OK. I got it.

> >- The script is idempotent; we know that $TEXDIR/babel.sty will exist the
> >- The diversion removal will always succeed unless the user has manually
> >  adjusted files.
> >- The final mv will always succeed unless the user has manually adjusted
> >  files, and as a result give us a $TEXDIR/babel.sty file that we're
> >  assured
> >  of being the copy from thailatex.

> I'm not so sure about this. Should the final mv be guarded with
> "if [ -e $TEXDIR/babel.sty.undivert]" ?

If $TEXDIR/babel.sty.undivert is missing, should the script succeed?  I
believe that it should not, but there's no reason why your maintainer script
*must* fail in a situation like this.

All right. Thank you for your explanation. It's now clear to me.

Regards,
--
Theppitak Karoonboonyanan
http://linux.thai.net/~thep/



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