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Re: Indirect expansion and arrays


From: Chet Ramey
Subject: Re: Indirect expansion and arrays
Date: Tue, 02 Oct 2012 09:26:29 -0400
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.7; rv:15.0) Gecko/20120907 Thunderbird/15.0.1

On 10/2/12 7:38 AM, Techlive Zheng wrote:
> 
> 
>> On 7/29/10 4:55 PM, Bernd Eggink wrote:
>>> It seems that indirect expansion doesn't work with arrays:
>>>
>>> $ a=(x y z)
>>> $ b=a
>>> $ echo "${!b[0]} ${!b[1]} ${!b[2]}"
>>> x
>>>
>>> Is that intended? The documentation isn't explicit about it.
> 
>> It does, but it doesn't work in the way you are trying.  The `!' binds to
>> an entire variable reference, in this case 'b[0]'.  The idea behind that
>> was to permit the use of an array of variable names, for instance, that
>> could be easily referenced using indirect expansion.
> 
>> The following code will display
>> "x variable y variable z variable"
> 
>> a=(x y z)
> 
>> x='x variable'
>> y='y variable'
>> z='z variable'
> 
>> echo "${!a[0]} ${!a[1]} ${!a[2]}"
> 
>> Chet
>> -- 
>> ``The lyf so short, the craft so long to lerne.'' - Chaucer
>>               ``Ars longa, vita brevis'' - Hippocrates
>> Chet Ramey, ITS, CWRU    address@hidden   
>> http://cnswww.cns.cwru.edu/~chet/
> 
> I think that indirect reference for fash variable should be used literally.
> 
>     $ a=(x y z)
>     $ address@hidden
>     $ echo "${!b}" # this would work
> 
> Combine with Chet Ramey's reply, a strucure like below would work.
> 
>     $ c=(a[0] a[1] a[2])
>     $ echo "${!c[0]} ${!c[1]} ${!c[2]}" #this would work too

They both do work, in the sense that they both display `x y z'.

The next version of bash will have ksh93-style nameref variables, so
indirect references to a nameref variable will work as they do in ksh.
That's slightly different than how they work now, but backwards
compatible, since bash-4.2 doesn't have namerefs.

Chet
-- 
``The lyf so short, the craft so long to lerne.'' - Chaucer
                 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, ITS, CWRU    address@hidden    http://cnswww.cns.cwru.edu/~chet/



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