Different Loops with If Statements in Bash [5 Examples]

Using loops with if statements is a powerful mechanism in Bash for executing conditional logic over a series of data or as long as a condition evaluates to true. Bash provides different loop structures that can be combined with if statements to automate iterative tasks while making decisions.

In this article, I’ll cover the basics as well as different examples of using loops with if statements in Bash.

Understanding Bash Loops

Loops are fundamental constructs in Bash that can be used to iterate over a collection of data or execute a block of code repeatedly until a particular condition is met. By leveraging loops, a user can automate repetitive tasks, handle repetitive data processing, and control the flow of Bash scripts.

If Statement with Different Loops in Bash

In Bash scripting, using different loop structures with if statements can help execute different conditional logic effectively. Generally, Bash provides 3 types of loop constructs such as for loop, while loop, and until loop which can be combined with if statements.

Following are the basic structures of these loops with if statements in Bash:

1. “for” Loop with If Statement

for variable in [sequence]; do
#Code to execute for each iteration
if [ condition ]; then
#Code to execute if the condition is true
fi
done

2. “while” Loop with If Statement

while [ condition ]; do
#Code to execute while the condition is true
if [ condition ]; then
#Code to execute if the condition is true
fi
done

3. “until” Loop with If Statement

until [ condition ]; do
#Code to execute until the condition is true
if [ condition ]; then
#Code to execute if the condition is true
fi
done

Loop Control Statement

Loop control statements allow users to control the execution and flow of loops within scripts. There are mainly two loop control statements in Bash i.e. continue statement and break statement. Following is a detailed explanation of these two statements:

1. Continue Statement

The continue statement is used to jump to the following iteration, leaving the remaining code with the current iteration behind. This statement is particularly useful when you need to bypass certain conditions within a loop without ending the entire loop.

Following is a Bash script with the ‘continue’ statement:

#!/bin/bash

#'continue' statement in a loop
for n in {1..4}; do
if [ $n -eq 2 ]; then
echo "Iteration skipped for n = $n"
continue #Skipping rest of the code in this iteration
fi
echo "Iteration $n"
done

The above image states that the continue statement has skipped one iteration when n=2 and jumps to the next iteration.

2. Break Statement

The break statement is used to terminate the innermost loop, immediately exiting the execution of the loop.

Here is a Bash script with the ‘break’ statement:

#!/bin/bash

#'break' statement in a loop
for n in {1..6}; do
echo $n
if [ $n -eq 2 ]; then
break
fi
done

The above image states that the break statement has terminated the loop after the second iteration.

5 Examples of Loops with If Statements in Bash

Following are 5 examples demonstrating the versatility of combining loops with if statements in Bash:

1. Processing Files in a Directory

Processing files in a directory requires iteration over each file to perform particular actions. Combining loops with if statements ensures that each file is processed selectively according to the conditions or attributes of the file.

Check out the following script to iterate through files and automate the task of file processing in a directory:

#!/bin/bash

#Checking the regular files in a directory and printing their sizes
for file in /home/nadiba/new_dir/*; do
if [ -f "$file" ]; then
echo "File location: $file"
echo "File size: $(du -h "$file" | cut -f1)"
fi
done

From the image, you can see all the files existing in the home directory in my system with their file size.

2. Reading Lines from a File in Bash

To read lines from a file, use a loop with an if statement in Bash. But before reading lines from a file you must ensure that the file exists in the system. Otherwise, it will cause error.

Here’s a Bash script showing how to read lines from a file using a while loop in Bash:

#!/bin/bash

file_name="file1.txt"

#Checking if the file does not exist
if [ ! -f "$file_name" ]; then
echo "$file_name does not exist."
exit 1
fi

#Reading lines from the file
while IFS= read -r line; do
#Checking for empty lines
if [ -z "$line" ]; then
continue #Skipping rest of the code for this iteration
fi
#Printing non-empty lines
echo "$line"
done < "$file_name"

In the above image, you can see all the lines in the file file1.txt in my system.

3. Skipping Even Numbers with “for” Loop

To skip even numbers and print only the odd numbers, first, it is necessary to figure out which numbers are odd and which are even. This can be accomplished by using a modulus operator % that calculates the remainder of a division between two integers.

Here’s how to skip even numbers using a for loop in Bash:

#!/bin/bash

#A for loop to iterate from 1 to 15
for ((n=1; n<=15; n++)); do
if ((n % 2 == 0)); then
continue
fi
echo "$n"
done

In the image, all the odd numbers from 1 to 15 are highlighted, while the even numbers are missing.

4. Validating User Input Using “until” Loop

User input validation is such an aspect that ensures that a program gets the real data. In Bash, using the until loop is a great way to ask users for input repeatedly until the the correct information is provided.

Navigate through the following Bash script to validate user input using an until loop with an if statement:

#!/bin/bash

#Defining regular expression pattern
pattern="^[0-9]+$"

#Declaring variables
value=""
trial=0

#An until loop to repeatedly prompt the user until a valid input is entered
until [[ "$value" =~ $pattern ]]; do
read -p "Enter any number: " value
((trial++)) #Incrementing the value of 'trial'

#Checking if the entered value does not match the pattern
if ! [[ "$value" =~ $pattern ]]; then
echo "Invalid! Please enter a valid number"
fi

#Checking if maximum trial exceeds
if [ "$trial" -ge 3 ]; then
echo "Maximum trial exceeds! No more chance"
exit 1
fi
done

As you can see from the image after executing the script once, when I entered 1, it displayed as a valid number. Then, again executing the script when I entered a or $, it displayed as invalid and when I entered 5, it showed maximum trial exceeded (3 attempts).

5. Skipping Specific Options Using “select” Loop

The select loop is a specialized loop that provides an interactive way to choose an option from a list. It doesn’t directly support if statements within the loop structure. However, this loop structure can be used with an if statement to handle the behavior of the specific options.

To skip specific options using the select loop, review the Bash script below:

#!/bin/bash

linux_distro=("Arch linux" "Ubuntu" "Kali Linux" "Quit")
select distro in "${linux_distro[@]}"; do

#Checking if the selected distro is 'Ubuntu'
if [[ "$distro" == "Ubuntu" ]]; then
echo "Not available! Select another distro."
continue #Skipping the rest of the code for 'Ubuntu'
fi

#Checking if the selected option is 'Quit'
if [[ "$distro" == "Quit" ]]; then
echo "Exit!"
break #Exiting the loop
fi

#Printing the selected option for other distros
echo "Selected distro: $distro"
done

From the image, you can see that when I selected option 2 (Ubuntu), the script skipped the option from displaying; instead printed that it was not available.

Conclusion

Throughout the article, you have learned the fundamentals of using loops with if statements in Bash. Moreover, you have explored how to iterate over files, read lines from files, manipulate data, select or skip specific options etc. By mastering the use of loops with if statements, you can create sophisticated Bash scripts capable of automating complicated tasks and handling different scenarios effectively.

People Also Ask

What are loop control statements in Bash?

Loop control statements in Bash are those that are used to alter and control the flow of execution of loops. These statements are generally useful for controlling or skipping certain iterations. There are two types of loop control statements in Bash. They are the continue statement and the break statement.

Is it possible to use an if statement with nested loops in Bash?

Yes, it is possible to use an if statement with nested loops in Bash. For example:

#!/bin/bash

#Nested loop iterations
for ((p=1; p<=5; p++)); do #Outer loop to iterate over rows
echo "Row $p:"
for ((q=1; q<=5; q++)); do #Inner loop to iterate over columns
#Checking if the row number is even
if ((p % 2 == 0)); then
#If the row number is even, print the square of the column number
echo -n "$((q*q)) "
else
#If the row number is odd, print the column number
echo -n "$q "
fi
done
echo ""
done

Can I combine multiple if statements with loops in Bash?

Yes, you can combine multiple if statements with loops in Bash to create complex conditional logic and control the flow of the Bash scripts. For example:

#!/bin/bash

#Combining multiple if statements with loops
for ((n = 1; n <= 6; n++)); do
echo "Iteration $n"

if [ $n -eq 3 ]; then
echo "First condition met in iteration $n"
fi

if [ $n -lt 9 ]; then
echo "Second condition met in iteration $n"
fi

if [ $n -gt 4 ]; then
echo "Third condition met in iteration $n"
fi
done

When should I use loops within if statements in Bash scripts?

You should use loops within if statements when you need to perform conditional execution based on specific conditions that involve iterative or repetitive tasks. It enables dynamic script behavior, allowing you to deal with different scenarios effectively.

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