Shell script -- program conditional statements (conditional tests, if statements, case branch statements, echo usage, for loops, while loops)

shell Script —— Programming conditional statement （ Conditions for testing 、if sentence 、case Branch statement ）

One 、 Conditions for testing

1、test test

test expression Is it true , Return if established 0, Otherwise, other values are returned
Be careful Space Space Space

Format 1：test Conditional expression

2、 File test

Format

[  The operator   File or directory  ]

3、 Integer test

Integer comparison

[  Integers 1  The operator   Integers 2 ]

-eq	be equal to equal
-ne	It's not equal to not equal
-gt	Greater than greater than
-lt	Less than lesser than
-le	Less than or equal to lesser or equal
-ge	Greater than or equal to greater or equal

4、 String test

String comparison

 Format 1：[  character string 1 =  character string 2 ]
 Format 2：[  character string 1 !=  character string 2 ]
 Format 3：
[ -z  character string  ] # Check if the string is empty zero, Variables that are undefined or given null values are treated as empty strings 
[ -n  character string  ] # Check if there is a string 

（ 5、 ... and ） Logic test

Two 、if sentence

1、if Single branch statements

2、if Two branch statements

Example

3、if Multi branch statement

Example

3、 ... and 、case Branch statement （ Like a menu , There are options to use CASE)

Example

Four 、echo command - Output character or extract shell The value of the variable

1、 Format

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2、 Common parameters

5、 ... and 、for loop

Read different values of variables , Used to execute the same set of commands one by one

Example

Example ： Output 20 Even numbers within

notes ：i++ : i=1+1 Assign first and then calculate i=1 after Again +1
++i : 1+1=i Operation before assignment 1+1 after Again =i

6、 ... and 、while loop

As long as the conditions are met , And then it goes back and forth , Not immediately

1、 Grammatical structure

while expression
do
command
done

2、 Example

Print 1-5 The number of

notes ： Dead cycle , Output all the time 1

notes ： Not true, so output

7、until loop

Test a condition repeatedly , As long as the condition is not true, it will be repeated

Example

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7、until loop

Test a condition repeatedly , As long as the condition is not true, it will be repeated

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Example

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