[root@xuegod23 ~]# a=""
[root@xuegod23 ~]# [ -z "$a" ]&& echo ok
ok
[root@xuegod23 ~]# [ -n "$a" ]&& echo ok
[root@xuegod23 ~]# a="   "
[root@xuegod23 ~]# [ -n "$a" ]&& echo ok
ok