The while loop in Shell

The while loop gives you the ability to execute a set of repetitive commands until certain conditions occur. It is often used when you need to manipulate iterative variable values.

Syntax in Unix / Linux

 while  command  do  cac lenh de thuc thi neu command la  true done 

Here, the command is evaluated. If the result value is true, the commands are executed. If the command is false, then the statement (s) will not be executed and the program will jump to the command line after the done command.

For example in Unix / Linux

Here is a simple example that uses a while loop to display numbers from 0 to 9.

 #!/bin/sh  a = 0 while [  $a  - lt  10 ] do  echo $a a = `expr $a + 1` done 

It will produce the following result:

 0 1 2 3 4 5 6 7 8 9 

Every time this loop executes, variable a is checked to see if it has a value of less than 10. If the value of a is less than 10, this condition has an exit status of 0. In In this case, the current value of variable a is displayed and then an increment of 1 is added.

According to Tutorialspoint

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