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Date: Thu, 14 Aug 2014 11:49:12 -0300 (ADT)
From: Andrew Hamilton-Wright <AHamiltonWright@MtA.ca>
To: Rick Miller <vmiller@hostileadmin.com>
Subject: Re: /bin/sh script not behaving as expected
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On Thu, 14 Aug 2014, Rick Miller wrote:

   [ ... condensed ... ]

> for d in ${disks}; do
>   if [ -z "${disk}" -o "${disk}" '>' "${d}" ]; then
>      : ${disk:=${d}};
>   fi
> done

   [ ... condensed ... ]

> # $disk; If $disk is unset/null, $disk is set to $d.  If $disk is set, it
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
I believe that this is your problem -- you want to update the value
of disk whenever your if statement is true, whereas the above will
do so only when the if statement is true _and_ your variable is unset.

Is there a reason you are not simply using:
   disk=${d}
as the assignment in your if statement?

Andrew.