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1. User variables
1.变量名和变量值之间赋值符号(=)☆☆左右两侧绝对不能有空格☆☆，否则报错。重点
2.和python一样，shell脚本语言是一种弱类型语言，在脚本当中使用变量不需要也无法指定变量的“类型”。
3.默认状态下，shell脚本的变量都是字符串，即一连串的单词列表。
4.变量的命名，和其他语言一样，只能包含字母、数字、下划线，且必须以字母、下划线开头，不能以数字开头。
5.shell中的解释器，在通过$引用变量时，若此变量没有定义，则不会报错，会输出为空。
a=10
b=111
al=abc
2. Variable reference
$How to use the variable name and ${variable name} of the defined variable can be used in both ways.
1. $Variable name. When referencing a variable in this way, the variable name cannot be followed by other content, but it can be separated by a space and the subsequent content.
2. ${variable name}, when referencing a variable in this way, the variable name can be followed by other content. Because the bash interpreter will automatically recognize ${variable name}.
Code demo:
a=1
echo this is num $a
this is num 1
echo this is num $a good
this is num 1 good
echo this is num $agood #解释器会把agood当前变量名，因为没有定义变量agood，所以输出为空
this is num
echo this is num ${
a}
this is num 1
echo this is num ${
a}good
this is num 1good
echo this is num ${
a} good
this is num 1 good