Bash Scripting: While Loops

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What is a While Loop?

A while loop is a control flow statement that allows code to be executed repeatedly based on a given condition. The code inside the loop is executed as long as the condition evaluates to true. Once the condition becomes false, the loop terminates.

The basic structure of a while loop in bash is as follows:

while [ condition ]
do
    command1
    command2
    ...
    commandN
done

In this structure, condition is a test or command that returns a status. If the status is zero (which means true in a shell scripting context), the commands between do and done are executed. The loop continues to execute until the condition returns a non-zero status (which means false).

Basic While Loop Example

Let's start with a simple example. Suppose we want to print the numbers from 1 to 10. Here's how we can do it using a while loop:

counter=1
while [ $counter -le 10 ]
do
    echo "Current number is: $counter"
    ((counter++))
done

In this script, counter is a variable that starts at 1. The condition in the while loop checks if counter is less than or equal to 10. The echo command prints the current number, and ((counter++)) increments the counter by 1 at each iteration. The loop ends after counter exceeds 10.

Practical Use of a While Loop: Reading a File Line by Line

Now, let's look at a more practical example. Suppose we have a text file, and we want to read it line by line. Here's how we can do it using a while loop:

while IFS= read -r line
do
    echo "Line: $line"
done < file.txt

In this script, line is a variable that represents each line in the file. The IFS= and -r options ensure that the read command reads each line exactly as it is, preserving whitespace and backslashes. The echo command prints each line. The loop ends after it has read all lines in the file.

Using While Loops for User Input

While loops can also be used to handle user input in interactive scripts. For example, you can use a while loop to keep asking for user input until a valid input is received:

while :
do
    read -p "Enter a number: " number
    if [[ $number =~ ^[0-9]+$ ]]
    then
        break
    else
        echo "Invalid input. Please enter a number."
    fi
done

In this script, the loop continues indefinitely because the condition is simply : (which always evaluates to true). The read -p command asks for user input, and the if statement checks if the input is a number. If the input is a number, the break command terminates the loop. If the input is not a number, an error message is printed, and the loop continues.

Conclusion

While loops are a versatile tool in bash scripting. They allow you to automate tasks that need to be performed repeatedly until a certain condition is met. Whether you're printing numbers, reading files, or handling user input, while loops can make your scripts more efficient and easier to manage.

• Bash Scripting
• Bash Scripting: Introduction
• Variables
• Basic Math
• Control Statements
• For Loops
• Functions
• Case Statements
• Arguments

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Version 1.0

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