Condition test of shell basic teaching

1. test command

• Test whether the expression is established, if it is established, return 0 (Boolean value), otherwise return other values
• Boolean value is one of true (True) or false (False), the Boolean value in shell programmingEstablished as 0, Does not hold and is expressed asNon-zero
• Format 1 test conditional expression
• Format 2 [Conditional expression] (There must be spaces on both sides of the expression)

2. File testing

Format
[Operator file or directory]

常用的测试操作符:
-d       #测试是否为目录(Directory)
-e       #测试目录或文件是否存在(Exist)
-f       #测试是否为文件/文件是否存在(File)
-r       #测试当前用户是否有权限读取(Read)
-w       #测试当前用户是否有权限写入(Write)
-x       #测试当前用户是否有权限执行(eXcute)

Example:

3. Integer value comparison

Format
[integer 1 operator integer 2]

常用的测试操作符：
-eq      #等于(Equal)
-ne      #不等于(Not Equal)
-gt      #大于(Greater Than)
-lt      #小于(Lesser Than)
-le      #小于或等于(Lesser or Equal)
-ge      #大于或等于(Greater or Equal)

Expansion:

4. String comparison

Format 1
[String 1 = String 2] or [String 1 == String 2]
[String 1 != String 2]
Format 2
[-z String] Check whether the string is empty (Zero), Variables that are not defined or assigned a null value will be treated as empty strings
[-n string] Check whether there is a string

常用的测试操作符：
=           #字符串内容相同
!=          #字符串的内容不同，!号表示相反的意思
-z          #检查字符串内容是否为空
-n          #检查字符串是否存在

5. Logic Test

Format 1
[Expression 1] Operator [Expression 2]
Format 2
[Command 1 Operator Command 2]

-a或&&        #逻辑与（而且的意思）
-o或||        #逻辑或（或者的意思）
！            #逻辑否

Use -a or -o in the same bracket, use && or || between different brackets

Note: If &&, || operators appear in [], an error will be reported, but &&, || can normally exist in the conditional judgment structure of [[]]
Example: