Bash Script Volume Control

#!/bin/bash

if [ "$1" ]
then
 let vol=$(amixer get Master|grep 'Mono:'| awk '{print $3}')
 echo "$vol"
 echo "$1"
 let vol+=$1
 echo "$vol"

 amixer cset iface=MIXER,name="Master Playback Volume" $vol >/dev/null

else
 echo "please give value"

 echo "example: $0 1"
 echo "example: $0 -1"
fi