Shell Note 8

2. Shell control Structure: conditional structure, branching structure, cyclic structure

4. If, conditional structure

   
   

6. If expression then command table [else command table]fi you can use semicolons to combine multiple commands in one line. Therefore, in the aesthetic and space-saving considerations, the above format is abbreviated as: if expression; then command table [else command table]fi

8. If statements can be nested indefinitely

10. If expression then command table [elif expression then command table] ... [Else Command table]fi

12. You can change the direct command table to an IF condition statement

14. Case structure

16. Case expression in-mode 11[| mode 12] ...) command table 1;; mode 21[| mode 22] ...) command table 2;;.. *) command table n;; Esac

18. Example

20. 1 #! /bin/bash 2 #filename: a.sh 3 Case $4 dir |  Path) echo "Current path is ' pwd '";; 5 date |  Time) echo "Today is ' date '";;  6 *) echo "Invalid argument!!"  7 echo "Enter argument:dir/path/date/time";; 8 ESAC

22. Case and if statements can be nested

24. Select structure

    The Select Loop structure allows you to generate menus in a simpler way. The generated menu is numbered, and after the user enters the selected number, the corresponding code is executed according to the user's choice.

26. Select variable in list do command table done

28. List, which is a sequence of strings.

30. Using the Select structure will generate a digitized menu on the screen, prompting the user to make a selection, and the default prompt is #. The user simply needs to enter the corresponding number at the prompt to complete the selection.

32. Typically, select is used with the case structure, allows the user to select from the menu, and executes the corresponding command based on the selection.

34. The select structure is a loop that requires you to exit the loop using the break command, or you can use the Exit command to end the script.

36. Example

38.   1 #! /bin/bash  2  #filename: a.sh   3 echo  "---------Select one week plan---------"   4 select  N in Mon Tue Wed Thu Fri Sat Sun  5 do   6   case  $N  in  7   mon)  echo  "Monday";   8   tue)  echo  "Tuesday";;   9   wed)  echo  "Wednesday";;  10   thu)  echo  "Thursday";;  11   FRI)  echo  "Friday";;  12   sat)  echo  "Saturday";;  13   sun)  echo  "Sunday";;  14   *)  echo  "wrong"  15       break;;  16   esac 17 done 

40. When you run the script with SH a.sh, the select:not found is displayed. Need to use bash a.sh to run scripts

42. # bash a.sh---------Select one week plan---------1) Mon2) Tue3) Wed4) Thu5) Fri6) Sat7) sun#? 1monday#?

44. While loop

46. While expression do command table done

48. While loops can be nested

50. Example

52. 1 #! /bin/bash 2 #filename: a.sh 3 i=1 4 While [$i-le] 5 does 6 echo "i= $i" 7 i=$ (($i + 1)) 8 done

54. Note that there is a space behind [],[] after the while and there is a space in front of it, otherwise an error occurs.

56. For loop

58. For variable [in List]do command table done

60. Example

62. 1 #! /bin/bash 2 #filename: a.sh 3 for I in 1 2 3 4 5 6 4 does 5 echo "i= $i" 6 i=$ (($i + 1)) 7 done

64. If the In list is omitted, for each positional parameter in the current execution script as a list, the for executes one of the list at a time. That is, the "for variable" implicitly means "for variable [email protected]".

66. Example (ARG, can be any name, not fixed)

68. 1 #! /bin/bash 2 #filename: a.sh 3 i=1 4 for ARG 5 does 6 echo "i= $i, Parameters: $arg" 7 i=$ (($i + 1)) 8 done

70. Until cycle

72. Until is the continuation of the loop when the condition is false, and stops execution when the condition is true.

74. Until command table 1test expression do command table 2done

76. Example

78. 1 #!  /bin/bash 2 #filename: a.sh 3 i=1 4 echo "Please enter the number N:" 5 read n 6 until 7 test $i-ge $N 8 do 9 echo "I= $i" i=$ (($i + 1)) done

80. Break statement

82. Break statement, which can end the execution of a structure such as while, for, until, or select, which jumps out of the structure. After exiting the loop, it goes to the done statement and resumes execution.

84. If you have a multi-layered structure nested, you can also add a number after the break keyword to indicate the number of layers of the structure you want to jump out of. For example, break 2

86. Example

88. 1 #!  /bin/bash 2 #filename: a.sh 3 i=1 4 echo "Please enter the number N:" 5 read n 6 until 7 test $i-ge $N 8 do 9 if    [$i-le 4];then echo "Hello" one-i=$ (($i + 1))-Else echo "i= $i" echo "World" 16 Fi + Done

90. Continue statements

92. The continue statement is the same as the break usage and is used to skip the remaining code in this loop.

Shell Note 8