...ok now it actually works

bin/mpd-monitor

@@ -1,7 +1,11 @@
 #!/bin/sh
 
 # quit if another instance is already running
-[ -z $(pidof mpd-monitor) ] && echo "mpd-monitor already running";exit
+if [ $(pidof mpd-monitor >/dev/null) ];then
+  echo "mpd-monitor already running"
+  exit
+fi
+
 ID=$(mpc current -f %id% )
 mpc idleloop |
   while read -r event; do 
@@ -18,7 +22,7 @@ mpc idleloop |
     TITLE=$(mpc current -f %title%)
     ARTIST=$(mpc current -f %artist%)
 
-    if [ -z $TITLE ]; then
+    if [ -z "$TITLE" ]; then
       notify-send "End of queue or queue empty" -i :
     else
       notify-send "$TITLE" "$ARTIST" -i :