Shell script good practices changes.

It's considered good practice to:

1. Wrap the variable name in {}. As in ${variable} instead of $variable.
2. Put variables in ".

Also some minor error message tuning.

Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Rob Norris <robn@despairlabs.com>
Signed-off-by: Turbo Fredriksson <turbo@bayour.com>
Closes #18000