Introduction to shell programming in Linux (3)

Iv. Loop Structure statements

Common shell loop statements include for loop, while loop, and until loop.

ForLoop

Syntax: for variable in list

Do

Operation

Done

Note: The variable is used inside the loop to refer to the object in the List currently referred.

The list is the object to be operated inside the for loop. It can be a string or a file. If it is a file, it is the file name.

For example, delete all .gz files in the garbage box.

# Delete all file with extension of "GZ" in the dustbin

For I in $ home/dustbin/*. GZ

Do

Rm-F $ I

Echo "$ I has been deleted !"

Done

The execution result is as follows:

[Beichen @ localhost bin] $. f_rmgz

/Home/Beichen/dustbin/Nessus- 4.0.0 .2.tar.gz has been deleted!

/Home/Beichen/dustbin/gftp- 2.2.1 .Tar.gz has been deleted!

WhileLoop

Syntax: While expression

Do

Operation

Done

As long as the while expression is true, the operation between do and done will continue.

UntilLoop

Syntax: Until expression

Do

Operation

Done

Repeat the operations between do and done until the expression is true.

Example:

# Test

# Add from 1 to 100

Total = 0

Num = 0

Until test num-EQ 100

Do

Total = 'expr $ total + $ num' // note that the quotation marks here are backticks, the same below

Num = 'expr $ num + 1'

Done

Echo "the result is $ total"

The execution result is as follows:

[Beichen @ localhost bin] $

The result is 5050!

5. Condition statements

The condition statements in Shell programs mainly include the if statement and the case statement;

IfStatement

Syntax: If expression 1 then

Operation

Elif expression 2 then

Operation

Elif expression 3 then

Operation

.....

Else

Operation

Fi

In Linux, the IF end mark is to write the if statement in turn as fi, while Elif is short for else if.

In theory, there can be an infinite number of Elif instances.

CaseStatement

Syntax: Case string in

Value 1 | value 2)

Operation ::

Value 3 | value 4)

Operation ::

Value 5 | value 6)

Operation ::

*}

Operation ::

Esac

The role of case is to execute the operation after the value when the string is the same as a value. If the same operation has multiple values, use "|" to separate the values. At the end of each case operation, there are two ":" And a semicolon is required.

Example:

Case $ user in

Beichen)

Echo "you are Beichen !";;

Liangnian)

Echo "you are liangnian"; // note that there is only one semicolon

Echo "Welcome !";; // Here are two semicolons

Root)

Echo "you are root! : Echo welcome !";; // Write the two commands in one line and use a semicolon as the Separator

*)

Echo "who are you? $ User ?";;

Esac

Execution result:

[Liangnian @ localhost bin] $ Test

You are liangnian

Welcome!

There are so many basic things about shell programming. If you want to learn more about shell programming, read related books.