Returns free disk space:
df | egrep '/$' | awk '{print $4}'
Returns free disk space in human readable form like K.M or G:
df -h | egrep '/$' | awk '{print $4}'
Returns the used Percentage as value :
df -h | egrep '/$' | awk '{print 100-$5}'
with Percentage character use :
df -h | egrep '/$' | awk '{print 100-$5."%"}'