Another BASH Scripting question

[Chris F.A. Johnson]

On Mon, 9 Feb 2004, D. Hugh Redelmeier wrote:

> | From: Chris F.A. Johnson <c.f.a.johnson-bJEeYj9oJeDQT0dZR+AlfA at public.gmane.org>
>
> | On Mon, 9 Feb 2004, D. Hugh Redelmeier wrote:
> |
> | > | From: Chris F.A. Johnson <c.f.a.johnson-bJEeYj9oJeDQT0dZR+AlfA at public.gmane.org>
> | > | On Mon, 9 Feb 2004, Madison Kelly wrote:
> | >
> | > |     Use eval:
> | > |
> | > | eval var=${server}_SNAT_IP
> | > | echo "$var | { .......
> | >
> | > If I understand what you are suggesting, then it does not work:
> | >
> | >     $ server=SRV2
> | >     $ SRV2_IB_TCP="22 80"
> | >     $ eval var=${server}_SNAT_IP
> | >     $ echo $var
> | >     SRV2_SNAT_IP
> | >
> | > I think that this is closer to what is needed:
> | >     $ eval z='$'${server}_IB_TCP
> | >     $ echo $z
> | >     22 80
> |
> |     Right. I meant:
> |
> | eval var=$\${server}_SNAT_IP
>
> I don't think so:
>     $ server=SRV2
>     $ SRV2_IB_TCP="22 80"
>     $ eval var=$\${server}_SNAT_IP
>     $ echo $var
>     23484{server}_SNAT_IP
>
> I think you meant:
>     $ eval var=\$${server}_IB_TCP
>     $ echo $var
>     22 80
>
> This just shows how easy it is to make a mistake with this corner of
> the language.  Test before publishing.
>
> |     That what happens when I post before my first cup of coffee.
>
> More coffee!

     I should have finished it; I was half way through my first cup.

> |     Exactly. I prefer the backslash.
>
> Single quotes seem a little more explicit to me.  In this case, either
> will do.
>
> |     It can get pretty hairy using backslashes with eval, but if I need
> |     more than 2 consecutive backslashes, I use a different syntax.
>
> Backslashes come in quatities that are a power of two :-)

     They can come in any number. :(

     I don't remember exactly what I was doing, but I recall a
     situation in which there was NO correct number of backslashes; X
     wasn't enough; X+1 was too many!

> | > If I'd designed the Bourne Shell, the following would work:
> | >
> | >     $ echo ${${server}_IB_TCP}
> | >     bash: ${${server}_IB_TCP}: bad substitution
> |
> |      In bash, you can use:
> |
> | v=${server}_SNAT_IP
> | var=${!v}
>
> Right.  I'd vaguely remembered this, but could not see it in the man
> page.  The reason is that ! is written as "exclamation point" in that
> section of the man page :-(
>
> I avoid bashisms as much as possible, but this one looks like the best
> approach.  It avoids the dreaded eval.

     I use bashisms when they're more efficient; this is one I never
     use, as there is a portable equivalent.

> It does seem awkward compared with my design.  It requires an extra
> variable and an extra statement.

-- 
	Chris F.A. Johnson
	=================================================================
	cfaj-uVmiyxGBW52XDw4h08c5KA at public.gmane.org                      http://cfaj.freeshell.org
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