[系统运维]Linux Shell中如何检查变量是否存在呢

#删除变量var
[root@localhost ~]# unset var
[root@localhost ~]# echo $var

[root@localhost ~]# var=" "
[root@localhost ~]# echo $var

[root@localhost ~]# 

当变量var不存在时,输出$var你将会得到一个空行.
当变量var=" ",你还是会得到一个空行.
大多数时候,我们将无法确定一个变量到底是不存在呢?还是等于空字符串?

接下来教你一招:
语句1:${varName? Error: The variable is not defined}

上述语句的含义是如果变量varName已定义且不为空,则此语句就相当于$varName;如果变量varName的值为空,则此语句的值也是空;但如果varName是未定义的,则此语句将返回一个错误,并显示问号?定义的错误信息Error: The variable is not defined(此信息可改成其它)

语句2:${varName:? Error: The variable is not defined}
语句2跟语句1的区别是:如果变量varName的值为空,此语句也返回一个错误.

// #定义一个变量var
[root@localhost ~]# var=123
// #检查变量var是否存在,存在则打印var的值
[root@localhost ~]# echo ${var? Error: The variable is not defined}
123
// #删除变量var
[root@localhost ~]# unset var
[root@localhost ~]# echo ${var? Error: The variable is not defined}
bash: var:  Error: The variable is not defined
[root@localhost ~]# var=123
[root@localhost ~]# echo ${var:? Error: The variable is not defined}
123
[root@localhost ~]# unset var
[root@localhost ~]# echo ${var:? Error: The variable is not defined}
bash: var:  Error: The variable is not defined
[root@localhost ~]# var=""
[root@localhost ~]# echo ${var:? Error: The variable is not defined}
bash: var:  Error: The variable is not defined
[root@localhost ~]# echo ${var? Error: The variable is not defined}

[root@localhost ~]#