变量的定义规则

一 . 变量名区分大小写
[root@MissHou ~]# A=hello
[root@MissHou ~]# a=world
[root@MissHou ~]# echo a
world

二. 变量名不能有特殊符号
[root@MissHou ~]# *A=hello
-bash: *A=hello: command not found
[root@MissHou ~]# ?A=hello
-bash: ?A=hello: command not found
[root@MissHou ~]# @A=hello
-bash: @A=hello: command not found

特别说明：对于有空格的字符串给变量赋值时，要用引号引起来
[root@MissHou ~]# A=hello world
-bash: world: command not found
[root@MissHou ~]# A="hello world"
[root@MissHou ~]# A='hello world'

三. 变量名不能以数字开头
[root@MissHou ~]# 1A=hello
-bash: 1A=hello: command not found
[root@MissHou ~]# A1=hello

注意：不能以数字开头并不代表变量名中不能包含数字呦。

四. 等号两边不能有任何空格
[root@MissHou ~]# A =123
-bash: A: command not found
[root@MissHou ~]# A= 123
-bash: 123: command not found
[root@MissHou ~]# A = 123
-bash: A: command not found
[root@MissHou ~]# A=123
[root@MissHou ~]# echo $A
123