[9-13] Shell Series 3--branching structure if and case statements

The purpose of this paper is to review 3 kinds of IF statements and case multi-branch structure statements that the shell implements for branching, and to supplement the execution efficiency of if and case.

Knowledge Reserve

• The language structure in the shell script is still the 3 kinds: sequential structure, selection structure, loop structure

• The branching structure is the same concept as the selection structure, and the execution path is chosen according to certain conditions, rather than the physical order in which the statements appear strictly

• The IF statement and Case statement are specific representations of the branching structure implemented in the shell, with the IF and switch statements in the C language

• The following statement I use structure flowchart, code implementation [code format], specific examples to illustrate

  
  

  
  

If single branch structure

Flow chart

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Code implementation

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if [Expression];then Statement1 Statement2 ...... Fi

Example: If the current year is 2015, show "You have to study hard!"

Note that there are spaces before and after the [expression] expression! Otherwise syntax error

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#!/bin/bash if  [ $ (date +%y)  -eq  2015  ];then         echo  "you should study hard!" Fi

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If dual branch structure

Flow chart

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Code implementation

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if [Expreesion];then Statment1 ...... Else Statement2 ...... Fi

Example: Read the user input two number, compare the output large number

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# !/bin/bash read  -p  "enter 2 number: "  a b if  [  $a  -ge  $b  ];then     echo   else     echo  Fi

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If multi-branch structure

Flow chart

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Code implementation

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if [Expression1];then Statement1 elif [Expression2];then Statement2 elif [Expression3];then Statement3 Else Statment4 Fi

Example: Input test score, 90 points [including] above is a, 80-90 is B, 60-80 is C, etc., 60 below is D, etc.

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#!/bin/bash Read- P"Enter Your score:"Score if[$score-ge -];then Echo"Grade A" elif[$score-ge the];then Echo"Grade B" elif[$score-ge -];then Echo"Grade C" Else Echo"Grade D" Fi

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If attention point

Judging conditions can use [expression] and [[expression]] Two, the effect is usually consistent but special case, [[]] is a keyword, for the string is better judged, it is recommended to use [[]] This

[[]] The judgment condition can use logical operations to combine multiple expressions, such as

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Span style= "color: #008000;" > #当且仅当满足exp1和exp2时候才会执行command1 if     command1 Fi  #Common logical operators      -a  logic with     -o  logical OR     -!  non-

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Form one if [Expression];then Command1 Fi Form two if [Expression] Then Command1 Fi

Case Multi-branch structure

Flow chart

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Code implementation

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Case Value in Mode 1) Command1 ...... ;; Mode 2) Command1 ...... ;; *) Command1 ...... ;; Esac

Example: The user enters a number 1-3, if present, returns the number, otherwise the error

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#!/bin/bash Read- P"Enter A number 1-3:"num Case $numinch 1) Echo"1" ;; 2) Echo"2" ;; 3) Echo"3" ;; *) Echo"sure the number from 1-3" ;; Esac

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Case note Point

Each case branch, that is, a number of instructions contained in pattern N, must finally be ";; "Identifies the pattern matching execution end

Start with a case and end with a ESAC. ---> Exact word write-down

Execution efficiency of IF and case

Usually there is no difference between the two, can be used instead, but the efficiency of execution is different when there are many branches.

When the computer processes the case branch, it generates a jump table, jumps to the appropriate branch based on the value of the variable, and executes over

in the processing if branch, the number of jumps is variable, according to the order structure of a single decision to jump, such as the input results show grade that example, if the input is 95 then do not jump 1 times, if it is 45 points need to jump

Summary:

When selecting branches more, choosing the CASE...ESAC structure will improve the efficiency of the program, but it is not enough to handle only the variables of character or number type.

If...else structure is more flexible, if...else structure can be used to determine whether the expression is established, such as if (A+B>C), if...else the application of a wider range.

This article is from the "Blue Warehouse" blog, be sure to keep this source http://bluebox.blog.51cto.com/8852456/1696838

[9-13] Shell Series 3--branching structure if and case statements