conditional
To make shell The script program has certain “ intelligence ”, The first problem is how to distinguish between different situations to determine what to do . for example ： When the backup directory does not exist , Can automatically create ; When the source code compiles the program , If the configuration fails, the installation will not continue

1. File test –test
Use a special testing tool test command , You can test specific conditions , And judge whether the condition is true according to the return value ( The return value is 0 Indicates that the condition is true ).
Grammar format :
Format 1： test The operator Conditional expression
Format 2： [ The operator Conditional expression ] ps： Note that there should be at least one space between brackets and expressions
Common operators ：

The operator      describe
-d     Determine whether it is a directory
-f     Determine if it is a file
-e     Determine whether it is a directory or file
-r     Determine if you have read permission
-w     Determine if there is write permission
-x     Judge whether there is executive authority
-L     Determine whether it is a linked file
-b     Determine whether it is a device file
-c     Determine whether it is a character device
-s     Determine whether the file size is empty
Shell The environment returns the status value according to the command execution ($?) To judge whether the execution is successful , When the return value is 0 Time means success , otherwise ( Not 0 value ) Indicates failure or exception .

  stay shell Script , We often use [] Judge

Integer comparison
stay shell The numerical value size comparison in the script does not use ><= Equal operation comparator
Integer value comparison format ：[ Integers 1 The operator Integers 2 ]
Test operator ：

The operator      describe
-eq（equal）     be equal to
-nq （not equal）     It's not equal to
-lt (lesser than)     Less than
-le (lesser or equal)     Less than or equal to
-gt (greater than)     Greater than
-ge(greater or equal)     Greater than or equal to

String comparison

Format 1：[ character string 1 = character string 2 ]
Format 2：[ -z character string ]

 =： String content is the same
！=： String content is different ,！ Take the opposite
-z： The string content is empty  

Logic test

Format 1：[ expression 1 The operator expression 2 ]
Format 2： command 1 The operator command 2

 -a or &&： Logic and , And what it means
-o or ||： Logic or , Or it means
！： Logic is not , Meaning of no

IF sentence

Single branch statements

Format ：

if Conditional statements
 then command
fi 

Two branch statements

Format ：

if Conditions
 then command 1
 else command 2
fi 

  Multibranched structure

if Conditions 1
 then command 1
 elif Conditions 2
   then command 2
 else command 3
fi   

  measuring httpd Whether the service is installed and running ：

 #!/bin/bash
netstat -antup|grep ":80" &>/dev/null     # Check 80 Whether the port has service listening
if [ $? -eq 0 ]                           # Judge the monitoring status ,0 normal , Others are abnormal
 then echo "Apache Service is running !"            # by 0 Then the output service starts normally
 elif rpm -q httpd &>/dev/null           # Check for installation httpd service  
  then echo "Apache The service is already installed !"            # The test installation result is 0, Then the service is installed
       systemctl start httpd               # Start the service  
       echo "Apache The service is starting ......"
       echo "Apache Service started successfully , Running !"
 else                                      # If the above two are not satisfied , Then the service is not installed
     yum install -y httpd &>/dev/null       # install httpd service
     echo "Apache The service is being installed ......"
     echo "Apache Service installed successfully , Starting ......"
     systemctl start httpd                  # Opening service
     echo "Apache Service started successfully , Running !" 
fi

  Nested statement

Sometimes you need to make multiple judgments of different types of conditions

 CASE sentence

Command format

case Variable name in
Pattern 1）
     command 1
;;             // Indicates the end of the command
Pattern 2）
     command 2
;;
*）        // Default statement ,
        command 3    // Don't write
esac            // Reverse the ending

case in The pattern supports regular expressions , such as ：

Simulation results , Let the user enter a score
100 Divided into excellent ;90-99 For excellence ;70-89 For good
60-69 Pass for ;60 The following are the failing grades ; Enter other information to prompt the user for input error

 case in Statements are often used in production environments for service startup
such as ：httpd Start of service

summary

1. stay shell We often use [] To judge , Rather than using test
2. Numerical comparison in judgment requires eq、gt Wait for the operator , They can't be used ><
3. In actual production if A single sentence is rarely used , The common one is if There are many double branches and multiple branches
4.case Statements are often used to start services