shell script flow control statement

Table of Contents

One, if---> Conditional judgment test statement

Second, for conditional loop statement

Three, while conditional loop statement

Summary:

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1. if---> Conditional judgment test statement

Automatically execute the corresponding commands according to the actual situation; if is divided into single-branch structure, double-branch structure and multi-branch structure.

1. if is divided into single-branch structure: if single-branch structure consists of if, then, fi (equivalent to spoken language: if...then...)

if conditional test action ------------------------>> if directory does not exist

then command number ----------------------->> then then create this directory

fi             ------------------------>> fi end

Requirements: Use the shell to automatically create a directory with a fixed name

2. The double-branch structure of if consists of if, then, else, fi

Function implementation method: perform a conditional match, if the condition matches, execute the corresponding preset command, otherwise, execute the unmatched preset command.Equivalent to the colloquial "if...then...or...then",

Requirements: Use a two-branch if conditional statement to verify whether a host is online

3. The if statement of if multi-branch structure: consists of "if, then, elif, else, fi"

Conditional judgment is performed multiple times, and the corresponding preset command will be executed once the match is successful

Requirements: Enter your grades to view grades

Second, for conditional loop statement

The for loop statement allows the script to read the information of multiple files at once, and then operate on the information one by one.But there is a range when dealing with data.The for statement couldn't be better.

Syntax composition: composed of for...in, do, done

Requirement: Create multiple users by calling the contents of the specified file using a shell script.

All users who created the user are placed in a file

Verification:

Delete multiple users at the same time

Three, while conditional loop statement

Whil and for are both conditional loop statements, the difference between the two is that the conditional loop of while does not know how many times it will be executed in the end

And for has scope and target.

Syntax composition: composed of while true, do, done

Demand: Price Guessing Game

Summary:

There are many used in shell scripts, if, for, while statements