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  From: Jake Hawkes <jake@infinitylimited.net>
  To  : Daryl Tester <dt@picknowl.com.au>
  Date: Fri, 28 Apr 2000 09:12:52 +0930

Re: Protecting files against power failure

Daryl Tester wrote:
> 
> Jake Hawkes wrote:
> 
> >>> exec 5>foo 6<foo
> 
> > ok, I grok this, but why does it work when executed one by one at the
> > prompt? In a shell script - sure, but at the prompt?
> 
> Same reason for both - the "exec 5>foo 6<foo" applies to the _current_
> shell.  When you execute "cat <&6", the shell is actually duplicating
> file descriptor #6 onto #0 (standard input), because that's what <&6
> means.  cat then reads to EOF on its standard input, then returns.
> Upon returns, file descriptor 6 is at the end of file mark.  You could
> even do something like ...
> 
> exec 5<foo 6>foo
> rm foo
> echo "hi" >&5
> cat <&6
> echo "there" >&5
> cat <&6
> # etc.
> 
> to your little heart's content.  I suspect that if you could pass
> this descriptor into your favourite programming/scripting language,
> you could even rewind it back to the beginning of the non-existant
> file and replay its contents (although, of course, this shouldn't
> affect the shell's file descriptor - care to hazard a guess why?).

this feels like a class :) Bueler... Bueler....


um, because the file descriptors are passed by value, not reference? 

--
Jake Hawkes, B.Eng (CSE)

"I'm only smart enough to know how stupid I am", Joe Strummer.

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