[Linux] Structured command IF

The syntax structure is as follows:

1. If-then statements

IF-thenstatement if  command   #根据conmmand的退出状态码, select Execute statement then    commandsfie.g.#! usr/bin/Bashtestuser=Richif grep $testuser/etc/passwdthen        for  user $testuser is:        -a/home/$testuser/fi

2. If-then-else statements

1 #!usr/bin/bash2if  command     #根据conmmand的退出状态码, select Execute statement 3  Then 4     Commands5Else6    commands7 fi

3. Nesting if (elif)

1#!usr/bin/Bash2 ifCommand3  Then4 commands5 elif Command6  Then7 commands8 elif Command9  ThenTen commands One Else A commands -Fi

4. Test command (single testing condition)

1 # There are 3 main types of test conditions2*Numerical comparison3*string comparison4*file Comparison5 6#usr/bin/Bash7 iftest Condition8  Then9 commandsTen fi One  A#!usr/bin/Bash - if[condition] # condition before and after space -  Then the commands -Fi

5. Compound condition test

1 # and test: Two conditions must be met (true)2 if[Condition1] &&[Condition2]3  Then4 commands5 fi6 7 8 #OR测试: Satisfies any one of the conditions (either true)9 if[Conditon1] | |[Condition2]Ten  Then One commands AFi

6. If-then Advanced Features

1 # Double parenthesis: for mathematical expressions2 ((expression))3 4 if((expression))5  Then6 commands7 ((expression))8 fi9 Ten # Two-way brackets: for string expressions (e.g. regular expressions) One [[expression]] A  - if[[expression]] -  Then the commands -Fi

7. Case command

1 # Compare the specified variable to a different pattern 2 3 4  Case inch 5 Pattern1 | pattern2) 6     commands1;; 7 pattern3) 8     commands2;; 9 *)    commands3;;  Esac

[Linux] Structured command IF