What Are the Loops in Shell Scripting?

Shell scripting is a powerful tool that allows us to automate tasks by writing scripts in a shell programming language. One key feature of shell scripting is the ability to use loops to repeat a certain block of code multiple times. In this article, we will explore the different types of loops in shell scripting and how they can be used effectively.

1. For Loop:

The for loop is one of the most commonly used loops in shell scripting.

It allows us to iterate over a list of values and perform actions on each item in the list. The syntax for a for loop is as follows:

for variable in list
do
    # code to be executed
done

The variable represents each item in the list. During each iteration, the code inside the loop is executed. Let’s say we want to print numbers from 1 to 5 using a for loop:

for number in 1 2 3 4 5
do
    echo $number
done

This will output:

1
2
3
4
5

2. While Loop:

The while loop allows us to repeatedly execute a block of code as long as a certain condition is true. The syntax for a while loop is as follows:

while condition
do 
    # code to be executed 
done 

The condition is evaluated at the beginning of each iteration. If it evaluates to true, the code inside the loop is executed. Let’s say we want to print numbers from 1 to 5 using a while loop:

counter=1
while [ $counter -le 5 ]
do
    echo $counter
    counter=$((counter+1))
done

3. Until Loop:

The until loop is similar to the while loop, but it executes the code block until a certain condition becomes true. The syntax for an until loop is as follows:

until condition
do 
    # code to be executed 
done 

The condition is evaluated at the beginning of each iteration. If it evaluates to false, the code inside the loop is executed. Let’s say we want to print numbers from 1 to 5 using an until loop:

counter=1
until [ $counter -gt 5 ]
do
    echo $counter
    counter=$((counter+1))
done

4. Nested Loops:

In shell scripting, it is also possible to have loops within loops, known as nested loops.

This allows us to perform more complex tasks by combining multiple iterations. Let’s say we want to print a pattern of asterisks using nested loops:

for ((i=1; i<=5; i++))
do  
    for ((j=1; j<=i; j++))
    do
        echo -n "* "
    done
    echo
done

This will output:

*
* *
* * *
* * * *
* * * * *

Conclusion:

Loops are an essential part of shell scripting as they allow us to automate repetitive tasks. The for loop, while loop, and until loop provide different ways to iterate over a set of values or execute code until a condition is met. By understanding and utilizing these loop structures, you can significantly enhance the power and efficiency of your shell scripts.