Re: need bash indirect variable help

From	Jerry Peters <jerry@example.invalid>
Newsgroups	comp.os.linux.misc
Subject	Re: need bash indirect variable help
Date	2014-06-09 20:44 +0000
Organization	A noiseless patient Spider
Message-ID	<ln56ah$3u2$1@dont-email.me> (permalink)
References	<lmvmr3$oqu$1@dont-email.me> <ln2h7e$s1t$1@dont-email.me> <ln2m0f$1og$1@dont-email.me> <87mwdnm2wl.fld@barrow.com>

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Floyd L. Davidson <floyd@apaflo.com> wrote:
> Todd <Todd@invalid.invalid> wrote:
>>On 06/08/2014 01:31 PM, Jerry Peters wrote:
>>>
>>> In bash:
>>> ~$ x=z
>>> ~$ a=x
>>> ~$ echo ${!a}
>>> z
>>>
>>
>>Hi Jerry,
>>
>>Thank you!
>>
>>What exactly does the "!" do?
> 
> See the bash man page, under the heading of "Parameter
> Expansion", where the third paragraph down describes it.
> Indirection, or they suggest "indirect expansion", uses
> the contents of a variable as a the variable acted on.
> 
> It is the equivalent of the also suggested eval command,
> which might be prefered just because it will be more
> obvious what the eval command is doing while the bang
> operator will be easily missed or confused.

The ${!x} notation has the advantage that you don't have to be as
careful as you do with eval, ie it's harder to sneak in an arbitrary
command if you're in some way processing user input.

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Thread

need bash indirect variable help Todd <Todd@invalid.invalid> - 2014-06-07 11:49 -0700
  Re: need bash indirect variable help William Unruh <unruh@invalid.ca> - 2014-06-07 23:42 +0000
    Re: need bash indirect variable help Eric Pozharski <whynot@pozharski.name> - 2014-06-08 12:19 +0300
      Re: need bash indirect variable help Eric Pozharski <whynot@pozharski.name> - 2014-06-09 15:07 +0300
  Re: need bash indirect variable help Richard Kettlewell <rjk@greenend.org.uk> - 2014-06-08 09:03 +0100
    Re: need bash indirect variable help Todd <Todd@invalid.invalid> - 2014-06-08 02:35 -0700
  Re: need bash indirect variable help Huibert Bol <huibert.bol@quicknet.nl> - 2014-06-08 10:29 +0200
    Re: need bash indirect variable help Todd <Todd@invalid.invalid> - 2014-06-08 02:36 -0700
  Re: need bash indirect variable help Jerry Peters <jerry@example.invalid> - 2014-06-08 20:31 +0000
    Re: need bash indirect variable help Todd <Todd@invalid.invalid> - 2014-06-08 14:53 -0700
      Re: need bash indirect variable help floyd@apaflo.com (Floyd L. Davidson) - 2014-06-08 14:50 -0800
        Re: need bash indirect variable help Jerry Peters <jerry@example.invalid> - 2014-06-09 20:44 +0000

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