思路：date -d " - 12hours"  +%H判断时间 
#!/bin/bash
d=`date  +%H`
if [ $d -ge 0 -a $d -lt 7 ];then
        tag=1
elif [ $d -ge 7 -a $d -lt 12 ];then
        tag=2
elif [ $d -ge 12 -a $d -lt 18 ];then
        tag=3
else
        tag=4
fi
case $tag in
        1)
        echo "early in the morning !"
        ;;
        2)
        echo "Good morning !"
        ;;
        3)
        echo "Good afternoon !"
        ;;
        4)
        echo "Good evening !"
        ;;
        *)
        echo "Script error !"
        ;;
esac