[m-rev.] for review: shell script fixes

[Zoltan Somogyi]

I just learned about a linter for shell scripts called shellcheck,
so I tried it out. It pointed out the issues fixed by this diff,
as well as other things that aren't problems.

For review by anyone. The main issue is whether the unused
variables that this diff deletes may nevertheless be needed.

Another issue this brought up for me is: should we prefer
modern bash-isms such as $(...) over the old Bourne sh
equivalents such as `...`? Any opinions?

Zoltan.
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