Subj : Re: about to blow a gaske
To   : Deuce
From : Angus Mcleod
Date : Sat May 21 2005 09:15 pm

  Re: Re: about to blow a gaske
  By: Deuce to Angus Mcleod on Sat May 21 2005 10:09:00

 >  >      > first.ext second.ext third.ext fourth.ext etcetera.ext
 >  >
 >  > then the shell should try to truncate 'first.ext', then execute
 >  > 'second.ext' with 'third.ext fourth.ext etcetera.ext' as arguments and
 >  > redirect STDOUT to 'first.ext'.
 >  >
 >  > I'm not sure why '*' evaluated to a list of files, doesn't bring about th
 >  > same result.
 > 
 > You would need a ; between first.ext and second.ext for the results you
 > described.  The reason you get an ambiguous redirect is that you're trying t
 > redirect to multiple files... the shell has no clue what you mean by that.

No, I can't agree with that logic.  If I place a ; in there, I am actually 
running two separate processes, as in a "list".  

If I run

	> output < input sort

what happens?  'sort' reads 'input' as a STDIN, sorts it, and writes the 
results to 'output' on STDOUT.  If I do

	> output ls

then the output from the 'ls' command should go to 'output' on STDOUT.

If I have a file called 'second.ext' which is a symlink to '/usr/bin/cat' 
and I run 

	second.ext third.ext fourth.ext etcetera.ext

then I should get the contents of the files 'third.ext', 'fourth.ext'. 
and 'etcetera.ext' concatenated on STDOUT.  Checking...

	$ ln -s /usr/bin/cat second.ext
	$ echo THIRD > third.ext
	$ echo FOURTH > fourth.ext
	$ echo ETCETERA > etcetera.ext
	$ second.ext third.ext fourth.ext etcetera.ext
	THIRD
	FOURTH
	ETCETERA
	$

Yep!

Therefore, if I ran the original example:

	$ > first.ext second.ext third.ext fourth.ext etcetera.ext
	$ cat first.ext
	THIRD
	FOURTH
	ETCETERA
	$

Yep!  Works exactly as I thought it should.

But *why* does it *not* work, if the entire list of files is generated 
with a pattern?

	$ echo *.ext
	etcetera.ext first.ext fourth.ext second.ext third.ext
	$ rm first.ext second.ext
	$ ln -s /usr/bin/cat first.ext
	$ > *.ext
	-bash: *.ext: ambiguous redirect
	$ > etcetera.ext first.ext fourth.ext second.ext third.ext
	$ cat etcetera.ext
	FOURTH
	SECOND
	THIRD
	$
	
See?  It works when you type the entire list, but if you use a shell 
substitution, it fails. 

The answer has to be something to do with the way the command line 
expansion occurs.  Perhaps some precedence issue?  Dunno.  Interesting.



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