Conditional judgment of Shell Foundation

One 、 The role of conditional judgment

  1. Tell the program which code block should be executed 
  2. The judgment statement can be executed separately 
  3. Judge by $? The returned results ,0 Said right , Not 0 Indicates an error （！！！ Note that with conventional Boolean types 0、1 The difference between , The two are just the opposite .）

Two 、 The type of judgment

   1. Arithmetic judgment ： Use （（）） Can complete arithmetic judgment ; To really return to 0, Returns... If false 1

   2. String judgment 
       2.1 Determine whether a string is empty , Need to use -z、-n Parameters . among -n Indicates that the string is not empty , Return to true ;-z Indicates that true is returned when the string is empty .

       2.2 Determine whether two strings are the same ：[  character string 1 =  character string 2  ]     Note the string and []、=  Space between 

     2.3 Determine whether there are multiple identical characters in the string ：[[ “ character string ” ==  The character you want to match * ]]

     2.4 Determine whether there is only one character in the string ：[[  character string  ==  The character you want to match ？？ ]]

   3. logic ：&&（ All conditions need to be met ）、||（ Any condition is satisfied ）

   4. Internal keyword comparison 
       4.1. -d flie==> Determine whether it is a directory , Usage for  [ -d  file name ];echo $?
       4.2. -f file ==> Determine if it is a file 
       4.3 . -e file ==> Judge whether the file exists 
       4.4 . -r file: Determine whether the file is readable 
       4.5. -w file： Judge whether the document is writable 
       4.6  -x file ： Determine if the file is executable 
   
   5. Command return code :
       
        because shell After executing the order , There will be a return code , All can pass the return code $? Judge .

3、 ... and 、[] ,(()) And [[]] The difference between ： It is recommended to use [[]]

      1. All three can be used for conditional judgment 
      2.[[]] Support for wildcards , There is no need to escape 
      3.[] Escape must be used 
      4.（（）） It can only be used for numerical comparison 






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