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Conditional statements of shell programming

2022-07-04 01:33:00 【Official certification】

One 、 Conditions for testing

1、test command

Test whether the expression holds , If not, return 0, Otherwise, other values are returned

      
      
       
        Format 1：test  Conditional expression 
       
       
 Format 2：[  Conditional expression  ]
      
      
      
      
       
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2、 File test

      
      
       
       [  The operator   File or directory  ]
      
      
      
      
       
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Common test operators

shell Conditional statements for programming _ The operator

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3、 Integer comparison

      
      
       
       [  Integers 1  Operands   Integers 2 ]
      
      
      
      
       
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Common test operators

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①awk usage

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shell Conditional statements for programming _ character string _07

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②bc Tool usage

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4、 String comparison

      
      
       
        Format 1 [  character string 1 =  character string 2 ]
       
       
     [  character string 1 !=  character string 2 ] 
       
       

       
       
 Format 2 [ -z  character string  ]
      
      
      
      
       
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Common test operators

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5、 Logic test

      
      
       
        Format 1：[  expression 1 ]  The operator  [  expression 2 ]
       
       
 Format 2： command 1  The operator   command 2
      
      
      
      
       
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Common test operators

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a=5
[ $a -ne 1 ] && [ $a != 2 ] Equate to [ $a -ne 1 -a $a != 2 ]

      
      
       
       [ -d 11 ] && [ -f 2.txt ] && echo "yes"
       
       
[ -d 11 -a -f 2.txt ] && echo "yes"
       
       
[[ -d 11 && -f 2.txt ]] && echo "yes"
       
       

       
       
a=6
       
       
[ $a -ne 5 ] && [ $a -gt 5 ] && echo "yes"
       
       
[ $a -ne 5 -a $a -gt 5] && echo "yes"
       
       
[[ $a -ne 5 && $a -gt 5 ]] $$ echo "yes"
       
       

       
       
[[ $a -ne 5  || $a -gt 5 ]] && echo "yes"
       
       
[$a -ne 5 ] || [ $a -gt 5 ] && echo "yes"
      
      
      
      
       
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&&、|| Operators can exist normally [[ ]] Conditional judgment structure , But if it appears in a [ ] In structure , You're going to report a mistake

example ：

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Two 、if Structure of statement

1、 Single branch structure

      
      
       
       if      Conditional test operation                        if  Used disk >80%
       
       
  then  Command sequence                           then  Call the police    
       
       
fi                    fi
      
      
      
      
       
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shell Conditional statements for programming _ The operator _14

Determine the mount point directory , If it doesn't exist, it will be created automatically

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2、 Two branch structure

Determine whether the target host is alive or not , Display the inspection results

      
      
       
       if  Conditional test operation                    if 80 Whether the port is listening 
       
       
   then                            then  
       
       
    Command sequence  1                      The website service is already running   
       
       
   else                            else
       
       
    Command sequence  2                        start-up httpd service 
       
       
fi                              fi
      
      
      
      
       
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3、 Multi branch structure

      
      
       
       if  Conditional test operation                 if  fraction 85-100 Between 
       
       
   then                         then 
       
       
    Command sequence 1                       Judged as excellent 
       
       
elif                         elif
       
       
 Conditional test operation 2                 Fraction in 70-84 Between 
       
       
   then                          then
       
       
    Command sequence 2                        It is judged as qualified 
       
       
else                          else
       
       
 Command sequence 3                       Judged as unqualified 
       
       
fi                            fi
      
      
      
      
       
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Example :

Prompt the user for input 100 The number of seconds in the meter race , The number of seconds required is greater than the judgment 0 And less than or equal to 10 Seconds into the trials , Greater than 10 Seconds are eliminated , If you enter other characters, you will be prompted to re-enter , And judge the gender of men and women , Boys enter the boys' group , Girls enter the girls' group , If the input is wrong, you will be prompted with an error .

shell Conditional statements for programming _ The operator _20

3、 ... and 、case Structure of statement

case Multibranched structure

      
      
       
       case  A variable's value  in 
       
       
 Model a )
       
       
    Command sequence 
       
       
;;
       
       
 Model 2 )
       
       
    Command sequence  
       
       
;;
       
       
……
       
       
*)
       
       
    Default command sequence 
       
       
esac
      
      
      
      
       
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case Statement notes

①case The line must begin with a word “in”, The first mock exam is a single bracket. ")" end

② Double a semicolon ";;" Indicates the end of the command sequence

③ In pattern string , You can use square brackets to indicate a continuous range , Such as "[0-9]"; You can also use vertical bars | Represents or , such as a|b

final *) Represents the default mode , Among them * It's equivalent to a wildcard

Example ：

Or the above question , Just use case Statement , Look at the diagram

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