Unix - Shell Loop Control

In this chapter, we will discuss shell loop control in Unix. So far you have looked at creating loops and working with loops to accomplish different tasks. Sometimes you need to stop a loop or skip iterations of the loop.
In this chapter, we will learn following two statements that are used to control shell loops−

• The break statement
• The continue statement
  

The infinite Loop

All the loops have a limited life and they come out once the condition is false or true depending on the loop.
A loop may continue forever if the required condition is not met. A loop that executes forever without terminating executes for an infinite number of times. For this reason, such loops are called infinite loops.

Example

Here is a simple example that uses the while loop to display the numbers zero to nine −

#!/bin/sh

a=10

until [ $a -lt 10 ]
do
   echo $a
   a=expr $a + 1`
done

This loop continues forever because a is always greater than or equal to 10 and it is never less than 10.

The break Statement

The break statement is used to terminate the execution of the entire loop, after completing the execution of all of the lines of code up to the break statement. It then steps down to the code following the end of the loop.

Syntax

The following break statement is used to come out of a loop −

break

The break command can also be used to exit from a nested loop using this format −

break n

Here n specifies the n^th enclosing loop to the exit from.

Example

Here is a simple example which shows that loop terminates as soon as a becomes 5 −

#!/bin/sh

a=0

while [ $a -lt 10 ]
do
   echo $a
   if [ $a -eq 5 ]
   then
      break
   fi
   a=`expr $a + 1`
done

Upon execution, you will receive the following result −

0
1
2
3
4
5

Here is a simple example of nested for loop. This script breaks out of both loops if var1 equals 2 and var2 equals 0 −

#!/bin/sh

for var1 in 1 2 3
do
   for var2 in 0 5
   do
      if [ $var1 -eq 2 -a $var2 -eq 0 ]
      then
         break 2
      else
         echo "$var1 $var2"
      fi
   done
done

Upon execution, you will receive the following result. In the inner loop, you have a break command with the argument 2. This indicates that if a condition is met you should break out of outer loop and ultimately from the inner loop as well.

1 0
1 5

The continue statement

The continue statement is similar to the break command, except that it causes the current iteration of the loop to exit, rather than the entire loop.
This statement is useful when an error has occurred but you want to try to execute the next iteration of the loop.

Syntax

continue

Like with the break statement, an integer argument can be given to the continue command to skip commands from nested loops.

continue n

Here n specifies the n^th enclosing loop to continue from.

Example

The following loop makes use of the continue statement which returns from the continue statement and starts processing the next statement −

#!/bin/sh

NUMS="1 2 3 4 5 6 7"

for NUM in $NUMS
do
   Q=`expr $NUM % 2`
   if [ $Q -eq 0 ]
   then
      echo "Number is an even number!!"
      continue
   fi
   echo "Found odd number"
done

Upon execution, you will receive the following result −

Found odd number
Number is an even number!!
Found odd number
Number is an even number!!
Found odd number
Number is an even number!!
Found odd number