Add help information upon misuse.

countdown.sh

@@ -22,16 +22,19 @@
 # 3. This notice may not be removed or altered from any source
 #    distribution.
 
-if [ -z $1 ]
+set -euo pipefail
+
+if [ $? -gt 1 ]
 then
-	COUNT=5
-else
-	COUNT=$1
+	echo "Usage: $0 [COUNT]"
+	exit 1
 fi
 
-for i in $(seq $COUNT -1 1)
+COUNT=${1:-5}
+
+for i in $(seq "$COUNT" -1 1)
 do
-	echo $i
+	echo "$i"
 	sleep 1
 done