Overview of Unix Shell Loops and Different Loop Types like:
- Unix Do While Loop
- Unix For Loop
- Unix Until Loop
In this tutorial, we will cover the control instructions that are used to iterate a set of commands over a series of data.
Unix offers three loop structures of which we can repeat a part of a program at a specified number of times.
Unix Video #17:
Loops in Unix
You may use different loops based on the situation.
#1) Unix For loop statement
Example: This program will add 1+2+3+4+5 and the result will be 15
for i in 1 2 3 4 5 do sum=`expr $sum + $i` done echo $sum
#2) Unix While loop statement
Example: This program will print the value of ‘a’ five times, from 1 to 5.
a=1 while [ $a -le 5 ] do echo “value of a=” $a a=`expr $a + 1` done
#3) Unix Until loop statement
This program will print the value of ‘a’ two times from 1 to 2.
a=1 until [ $a -ge 3 ] do echo “value of a=” $a a=`expr $a + 1` done
While running these loops, there may be a need to break out of the loop in some condition before completing all the iterations or to restart the loop before completing the remaining statements. This can be achieved with the ‘break’ and ‘continue’ statements.
The following program illustrates the ‘break’ operation:
num=1 while [ $num -le 5 ] do read var if [ $var -lt 0 ] then break fi num=`expr $num + 1` done echo “The loop breaks for negative numbers”
Our upcoming tutorial will brief you more about working with Functions in Unix.