When shell variables are unexpectedly empty, 'test' gets the wrong
number of arguments and becomes unhappy. Logical AND should not be
done with 'test EXPR1 -a EXPR2' in such cases, because 'test' logic
does not short-circuit. Replace with 'test EXPR1 && test EXPR2'.
Shell logic does short-circuit, and if the first test invocation
fails, the second will never occur, and will never encounter missing
arguments.
