disk_io now checks if iostat is available. "done" is now always printed before exiting

lib/main.sh

@@ -178,7 +178,8 @@ main_terminate () {
   if [ -d $TEMP_DIR ]; then
     rmdir $TEMP_DIR
   fi
-  verbose "done"
+
+  echo "done"
 }
 
 main_print_docs () {

metrics/disk_io.sh

@@ -11,6 +11,11 @@ defaults () {
 }
 
 start () {
+  if ! command_exists iostat; then
+    echo "Warning: disk_io requires the command 'iostat' to be available"
+    return 1
+  fi
+
   readonly fifo=$TEMP_DIR/$(unique_id)_disk_io
   mkfifo $fifo