Add more verbose output and apply fixes suggested by shellcheck.

tests/Modifiers_-_--interval.sh

@@ -2,11 +2,20 @@
 #
 # Check that the update interval can be set.
 
+# Dummy assignments for "shellcheck".
+testSubject="${testSubject:-false}"; workFile1="${workFile1:-.tmp1}"
+
 sleep 1 | "${testSubject}" -f -i 0.1 >/dev/null 2>"${workFile1}"
 
 # There should be more than 6 lines of output.
 #
-NUM=$(tr '\r' '\n' < "${workFile1}" | wc -l | tr -d ' ')
-test $NUM -gt 6
+lineCount=$(tr '\r' '\n' < "${workFile1}" | wc -l | tr -dc '0-9')
+if ! test "${lineCount}" -gt 6; then
+	echo "fewer than 7 lines of output"
+	tr '\r' '\n' < "${workFile1}"
+	exit 1
+fi
+
+exit 0
 
 # EOF