How to Use Bash “for” Loop with Variable [12 Examples]

In the world of bash scripting, loops are like building blocks, helping to do things automatically and making tasks easier. One special tool in these loops is the “for” loop, and it’s great for efficient iteration through a set of values. You need to know how to use variables in a loop for precise control over the script. This article will dive into the various examples of utilizing the Bash “for” loop with variables.

12 Examples of Bash “for” Loop with Variable

Loop variables in Bash scripting play a fundamental role in executing repetitive tasks by dynamically adapting to values during each iteration. Whether iterating through arrays of numbers, strings, or processing files, the loop variable provides a flexible mechanism for accessing and handling different elements. In this article, I will go through some of the common examples of using variables in the for loop.

Example 1: Print All Array Elements

To print all array elements, use a “for” loop to go through all elements of the array. Run the following bash script to do so:

#!/bin/bash

# Define an array of numbers
numbers=(1 2 3 4 5)

for num in "${numbers[@]}"; do
echo "Number: $num"
done

Here, all array elements are echoed on separate lines.

Example 2: Calculate the Sum of Numbers

Calculating the sum of some predefined numbers is a very common task in Bash scripting. The script uses a “for” loop to iterate through the numbers of a list and adds them to a running total. After the loop is complete, the total sum is echoed. Execute this script to calculate the sum of numbers:

#!/bin/bash

sum=0

# Iterate over a list of numbers
for number in 1 2 3 4 5
do
# Add the current number to the sum
sum=$((sum + number))
done

# Print the sum after the loop
echo "Sum: $sum"

The sum of all the numbers on the list is echoed.

Example 3: Print Array of Strings

Working with arrays in Bash scripting allows for efficiently handling multiple similar values. This example introduces a bash script that iterates through an array of strings, specifically fruit names, and then prints each fruit along with a label as shown below:

#!/bin/bash

# Define an array of strings
fruits=("Apple" "Banana" "Orange" "Grapes")

for fruit in "${fruits[@]}"; do
echo "Fruit: $fruit"
done

Example 4: Print Words in a Sentence

Efficiently breaking down a sentence into individual words is a common need in Bash scripting. To print all words of a sentence separately in individual lines use a “for” loop to iterate over the sentence as demonstrated in the following script:

#!/bin/bash

sentence="Bash scripting is great for automation"

for word in $sentence; do
echo "Word: $word"
done

Here all words of the sentence are printed on separate lines.

Example 5: Check Odd/Even Numbers

Analyzing and classifying numbers as odd or even is a fundamental aspect of programming logic. This example presents a bash script that prompts the user to enter a number, validates the input, and then uses “if” conditional statements and a “for” loop to determine if each number from 1 to the user’s input is odd or even. To do so execute the script below:

#!/bin/bash

echo "Enter a number: "
read user_input

# Check if the input is a valid number
if ! [[ $user_input =~ ^[0-9]+$ ]]; then
echo "Invalid input. Please enter a valid number."
exit 1
fi

# Check if the number is greater than 0
if (( user_input <= 0 )); then
echo "Invalid input. Please enter a positive integer greater than 0."
exit 1
fi

# Use a for loop to iterate through a range
for (( i=1; i<=$user_input; i++ )); do
# Check if the current number is odd or even
if (( i % 2 == 0 )); then
echo "$i is even"
else
echo "$i is odd"
fi
done

Here, all integers from zero to the given input number 5 are checked and echoed as even or odd.

Example 6: Check Variable in For Loop

To check if some variables are present in the bash script use a conditional statement within the “for” loop. This example showcases a bash script that iterates through a numerical range, creating variable names based on the iteration index. Inside the loop, it checks for the existence of these variables using the [[ -v $variable ]] command. Run the following script to do so:

#!/bin/bash

number_3=10

for i in {1..5}; do
variable="number_$i"
# Checking if the variable exists
if [[ -v $variable ]]; then
echo "$variable exists."
else
echo "$variable does not exist."
fi
done

Here only the number_3 variable exists in the script and that is also shown in the output.

Example 7: Iterate with a Counter

Counter is a variable used to keep track of the number of times a particular event or operation occurs. It is often employed in loops to control the iteration process. The following script initializes a counter variable (count) to 1 and continues the C-styled “for” loop while the counter is less than or equal to 5 showcasing the efficient use of a counter variable in a loop structure:

#!/bin/bash

for ((count=1; count<=5; count++)); do
echo "Iteration $count"
done

Here using a C-styled for loop 1 to 5 is counted.

Example 8: Read Variables from a File

Reading and processing data from a text file is important for task automation in Bash scripting. This example showcases a bash script that reads the user’s name and age from a text file using a for loop variable and prints them in a formatted way.

Content of the cat names_and_ages.txt text file:

Alice 25
Bob 30
Charlie 22
Diana 28

Run the following script to read data from a text file in Bash shell:

#!/bin/bash

# Read each line of the text file into an array
lines=($(cat names_and_ages.txt))

# Iterate over the array
for ((i = 0; i < ${#lines[@]}; i += 2)); do
name=${lines[i]}
age=${lines[i + 1]}
echo "Name: $name, Age: $age"
done

Here all data was read from the text file and then the formatted data was echoed.

Example 9: Count Lines from Files

The “for” loop can be used to count the number of lines in each file with the same extension within the current directory. The following script uses a “for” loop to iterate through the matching files, echoing a message for each file being processed. Inside the loop, the wc -l command, coupled with redirection <, counts and displays the number of lines in the current file. Run the following bash script to count the number of lines in each file with the “.sh” file extension within the current directory:

#!/bin/bash

for file in *.sh
do
echo "Processing file: $file"
line_count=$(wc -l < "$file")  # Count lines in each file
echo "This file has $line_count lines."
done

This script goes through all files with the “.sh” extension from the current directory and counts the number of lines of that file.

Example 10: Process All Files in a Directory

Iterating through files within a directory is a fundamental operation often used in shell scripting. The following script utilizes a “for” loop in conjunction with parameter expansion to iterate through the list of files in the current directory. Here $file is an example of parameter expansion. During the execution of the script, the variable “file” undergoes expansion to its current value in each iteration within the “for” loop. Execute the following command to list all files in the current directory:

#!/bin/bash

echo "Files in the current directory:"

for file in *; do
echo "$file"
done

This script shows all files including the hidden ones from the current directory into the terminal.

Example 11: List All Running Processes

Knowing the active processes proves essential for system monitoring and management. This example showcases a technique that leverages the ps, awk, and tail commands within a for loop to generate a comprehensive list of running processes on a Unix-like system. Execute the following script to list all processes running on the system:

#!/bin/bash

echo "Processes running on the system:"

for process in $(ps aux | awk '{print $11}' | tail -n +2); do
echo "$process"
done

The names of all processes running on my system are shown on the terminal.

Example 12: Multiplication Table (5×10)

A multiplication table is a mathematical table used to define a multiplication operation for an algebraic system. This script showcases a Bash implementation to generate a formatted multiplication table with a size of 5×10 within the shell interface, using nested “for” loops and formatting techniques for visual clarity. Run the following script to create a 5×10 multiplication table:

#!/bin/bash

echo "Multiplication Table:"

# Print header
echo -e "  | \c"
for ((i=1; i<=10; i++)); do
printf "%-4s" "$i"
done
echo

# Print separator
echo "-------------------------------------------"

# Print table
for ((i=1; i<=5; i++)); do
echo -n "$i | "
for ((j=1; j<=10; j++)); do
product=$((i * j))
printf "%-4s" "$product"
done
echo
done

The output shows a 5 by 10 multiplication table with proper alignment.

Conclusion

In conclusion, getting the hang of the variable in “for loop” in Bash opens up a world of possibilities. It makes your scripting adventures more efficient and enjoyable. I hope this article helps you to get some solid idea about how to use variable in bash for loop.

People Also Ask

What is a loop in Bash?

In Bash scripting, a “loop” is a powerful construct that enables the repetitive execution of a series of commands, making it a fundamental element for automation and task iteration. Loops play a crucial role in simplifying and optimizing code, allowing developers to efficiently perform tasks without the need for redundant or excessive manual input.

How do I run a for loop in Linux?

To run a for loop in Linux, use the following simple syntax:

for item in list; do
# Commands to be executed for each item
done

Here, replace “list” with the items you want to iterate over. This loop structure is invaluable for automating repetitive tasks, making it an essential tool in Linux scripting.

Related Articles

• 10 Common Bash “for” Loop Examples [Basic to Intermediate]
• How to Iterate Through List Using “for” Loop in Bash
• Bash Increment and Decrement Variable in “for” Loop
• How to Use Bash Parallel “for” Loop [7 Examples]
• How to Loop Through Array Using “for” Loop in Bash
• How to Use Bash “for” Loop with Range [5 Methods]
• How to Use Bash “for” Loop with “seq” Command [10 Examples]
• How to Use “for” Loop in Bash Files [9 Practical Examples]
• Usage of “for” Loop in Bash Directory [5 Examples]
• How to Use “for” Loop in One Line in Bash [7 Examples]
• How to Create Infinite Loop in Bash [7 Cases]
• How to Use Bash Continue with “for” Loop [9 Examples]

---

<< Go Back to For Loop in Bash | Loops in Bash | Bash Scripting Tutorial