Loops in Scripts

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From time to time anyone who writes scripts will find an occasion to repeat a set of commands for a certain number of times. This is referred to as a loop. The type of loop used most often in Linux shell scripts is the for loop. The general concept of a for loop is for as long as some condition is true, keep looping. The general concept of a while loop is similar. It is while some condition is true, keep looping.

There are a few rules that apply to all loops in shell scripts.

  • The variable used in the loop condition must be initialized before execution of the loop begins.

  • A test to see if the loop’s conditions are met is made at the beginning of each iteration.

  • The body of a loop ends with a statement that modifies the value of the test variable.

The basic syntax of a for loop is this:

  for { variable name } in { list }   do   execute one for each item in the list until the list is   not finished (and repeat all statement between do and done)   done

Let’s take a look at a script that illustrates the for loop. Type the following lines into the text editor of your choice and save it as scriptten.sh:

  #!/bin/bash   echo Count down   for i in 1 2 3 4 5   do     echo $i   done   echo blast off!   exit 0

If you run this from the shell, you should see something like what is displayed in Figure 20.15.

click to expand
Figure 20.15: The for loop.

You can see that a for loop is simple. Take a variable named i and count through the numbers 1 through 5, assigning that variable to i in each loop. Each time, echo that value to the screen. In other words, take the variable i, start it off with a value of 1, and keep incrementing through a value of 5.



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Moving From Windows to Linux
Moving From Windows To Linux (Charles River Media Networking/Security)
ISBN: 1584502800
EAN: 2147483647
Year: 2004
Pages: 247
Authors: Chuck Easttom

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