Bash while loop

In our previous article, we explored the for loop construct in Bash scripting, which allows you to iterate over a sequence of values. However, Bash offers another powerful loop construct known as the while loop, designed to execute a set of commands repeatedly as long as a specified condition remains true. This article will cover the Bash while loop, exploring its syntax, usage, and of course, some practical examples.

bash while loop

The bash while loop is designed to execute a block of code repeatedly as long as a specified condition holds true.The basic syntax of the while loop is as follows:

while [ condition ]
do
    # Code to be executed
done

Let's break down this structure:

• The while keyword initiates the loop.
• [ condition ] is the condition that is evaluated before each iteration of the loop. If the condition is true, the commands within the loop body are executed. If the condition is false, the loop terminates.
• The do keyword signifies the beginning of the code block to be repeated.
• The actual code to be executed resides between do and done.
• The done keyword marks the end of the loop body.

Let's consider a simple example that demonstrates the usage of the while loop:

#!/bin/bash

counter=1

while [ $counter -le 5 ]
do
    echo "Counter: $counter"
    ((counter++))
done

In this example, the while loop iterates over a counter variable, starting from 1 and incrementing it by 1 after each iteration until it reaches 6. The condition [ $counter -le 5 ] checks if the counter is less than or equal to 5. If the condition is true, the loop body is executed, printing the current value of the counter and incrementing it. Once the counter exceeds 5, the loop terminates.

Infinity while loop

In some cases, you might want to create an infinite loop that runs indefinitely until a specific condition is met or until the loop is explicitly terminated. This can be achieved by using a condition that is always true, such as true or a non-zero value.

In the following example, the while loop runs indefinitely because the condition true is always evaluated as true. However, to prevent an endless loop, we introduce an if statement within the loop body that checks if the counter exceeds 10. If the condition is met, the break statement is executed, terminating the loop.

#!/bin/bash

counter=1

while true
do
    echo "Counter: $counter"
    ((counter++))

    if [ $counter -gt 10 ]
    then
        break
    fi
done

You may also see some bash scripts that use the shorthand syntax for infinity loops.

while : 
do
	 # Code to be executed 
done

break and continue statement

The break and continue statements are used to control the flow of execution within loops in Bash. They provide a way to alter the normal loop behavior based on specific conditions.

break statement

The break statement is used to exit or terminate a loop prematurely. When encountered within a loop, the break statement immediately stops the loop's execution and transfers control to the next statement following the loop.

Using the same code as above, the while loop runs indefinitely due to the true condition. However, when the counter exceeds 5, the break statement is executed, terminating the loop. The program then proceeds to execute the next statement, which is echo "Loop terminated.".

#!/bin/bash

counter=1

while true
do
    echo "Counter: $counter"
    ((counter++))

    if [ $counter -gt 5 ]
    then
        break
    fi
done

echo "Loop terminated."

continue statement

The continue statement is used to skip the remaining commands in the current iteration of a loop and move to the next iteration. It is typically used in conjunction with an if statement to selectively skip certain iterations based on a specific condition.

Here's an example that demonstrates the usage of the continue statement:

#!/bin/bash

counter=1

while [ $counter -le 10 ]
do
    ((counter++))

    if [ $((counter % 2)) -eq 0 ]
    then
        continue
    fi

    echo "Counter: $counter"
done

In this example, the while loop iterates from 1 to 10. Within the loop, the if statement checks if the counter is an even number using the modulus operator %. If the counter is even, the continue statement is executed, skipping the remaining commands in the current iteration and moving to the next iteration. However, if the counter is odd, the loop body executes, and the current value of the counter is printed.

Read a file using a while loop

One common use case for the while loop is reading data from a file line by line. This can be achieved by combining the while loop with the read command.

Here is an example:

#!/bin/bash

while IFS= read -r line
do
   # Process each line
   echo "Processing: $line"
done < "data.txt"

In this example, the while loop is used in conjunction with the read command to read each line from the specified file (data.txt). The read command reads a line of input and assigns it to the variable line. The -r option prevents backslash from acting as an escape character.

Before the read command, we set IFS= (IFS stands for Internal Field Separator). By default, the read command trims leading and trailing whitespace characters (spaces and tabs). Setting IFS= prevents this behavior, ensuring that leading/trailing whitespace is preserved.

Here is another method of reading files using pipes:

cat data.txt | while read line
do
	# Process each line
  echo "Processing: $line"
done

Conclusion

The Bash while loop is a construct that allows executing commands repeatedly based on a specified condition. This article covered the loop's syntax, control statements like break and continue, and practical examples like reading files line by line. This knowledge should help you automate boring tasks and as well improve your scripting skills.