Loops — Automating Repetitive Tasks in Bash Scripts¶

Loops allow Bash scripts to execute a block of code repeatedly until a specified condition is met or a collection of items has been processed. Instead of writing the same commands multiple times, loops enable automation with fewer lines of code, making scripts more efficient, scalable, and maintainable. Loops are extensively used in Linux administration, DevOps, cloud automation, infrastructure management, monitoring, and deployment pipelines.

Learning Path¶

Linux Mastery → Module 10: Bash Scripting → Lesson 3

Difficulty: Beginner → Intermediate

Reading Time: 90 Minutes

Course Progress

Course: Linux Mastery

Module: Bash Scripting

Lesson: 3 of 11

What You'll Learn¶

After completing this lesson, you'll be able to:

Understand loops
Use for loops
Use while loops
Use until loops
Iterate through files and directories
Read files line by line
Control loops using break and continue
Apply loops in production automation

Prerequisites¶

Complete:

Modules 1–9
Module 10 Lessons 1–2

Why Learn Loops?¶

Imagine backing up multiple directories.

Without loops:

tar -czf home.tar.gz /home

tar -czf etc.tar.gz /etc

tar -czf var.tar.gz /var

Using a loop:

for DIR in /home /etc /var
do
    tar -czf "$(basename $DIR).tar.gz" "$DIR"
done

The script becomes shorter, reusable, and easier to maintain.

What is a Loop?¶

A loop repeatedly executes a block of commands.

Example:

Start

↓

Condition

↓

Execute Commands

↓

Repeat

↓

Exit

Types of Loops¶

Bash supports:

for
while
until

Each is useful in different scenarios.

for Loop¶

The for loop is used when iterating over a known list of items.

Syntax:

for VARIABLE in LIST
do
    commands
done

Example:

for NAME in Alice Bob Charlie
do
    echo "$NAME"
done

Output:

Alice

Bob

Charlie

Numeric for Loop¶

Using brace expansion:

for i in {1..5}
do
    echo $i
done

Output:

C-Style for Loop¶

for (( i=1; i<=5; i++ ))
do
    echo $i
done

Loop Through Files¶

for FILE in *.txt
do
    echo "$FILE"
done

Loop Through Directories¶

for DIR in /home/*/
do
    echo "$DIR"
done

while Loop¶

A while loop runs as long as the condition is true.

Syntax:

while [ condition ]
do
    commands
done

Example:

COUNT=1

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

Reading a File Line by Line¶

while read LINE
do
    echo "$LINE"
done < users.txt

This technique is widely used for processing configuration files and logs.

until Loop¶

An until loop executes until a condition becomes true.

Syntax:

until [ condition ]
do
    commands
done

Example:

COUNT=1

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

break Statement¶

break exits a loop immediately.

Example:

for i in {1..10}
do
    if [ $i -eq 5 ]
    then
        break
    fi

    echo $i
done

Output:

continue Statement¶

continue skips the current iteration.

Example:

for i in {1..5}
do
    if [ $i -eq 3 ]
    then
        continue
    fi

    echo $i
done

Output:

Nested Loops¶

Loops can be placed inside other loops.

for i in {1..3}
do
    for j in {1..2}
    do
        echo "$i $j"
    done
done

Infinite Loop¶

while true
do
    echo "Running..."
    sleep 5
done

Stop using:

Ctrl + C

Common Commands¶

Simple loop.

for i in {1..5}

While loop.

while [ condition ]

Until loop.

until [ condition ]

Exit loop.

break

Skip iteration.

continue

Real Production Examples¶

Backup multiple directories.

for DIR in /home /etc /var
do
    tar -czf "$(basename $DIR).tar.gz" "$DIR"
done

Check multiple servers.

for HOST in server1 server2 server3
do
    ping -c1 "$HOST"
done

Process log file.

while read LINE
do
    echo "$LINE"
done < access.log

Production Perspective¶

Loops are widely used in:

Backup automation
Log processing
CI/CD pipelines
Infrastructure provisioning
Cloud automation
Kubernetes administration
Server monitoring
Configuration management

Most production Bash scripts rely on loops to automate repetitive tasks.

Hands-on Lab¶

Task 1¶

Print numbers.

for i in {1..10}
do
    echo $i
done

Task 2¶

Print file names.

for FILE in *
do
    echo "$FILE"
done

Task 3¶

Create five directories.

for i in {1..5}
do
    mkdir "project$i"
done

Task 4¶

Use a while loop.

COUNT=1

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

Task 5¶

Read a file.

while read LINE
do
    echo "$LINE"
done < /etc/passwd

Task 6¶

Use break.

for i in {1..10}
do
    [ $i -eq 6 ] && break
    echo $i
done

Task 7¶

Use continue.

for i in {1..5}
do
    [ $i -eq 3 ] && continue
    echo $i
done

Task 8¶

Create an infinite loop.

while true
do
    date
    sleep 2
done

Stop it with:

Ctrl + C

Command Deep Dive¶

Command	Purpose	Production Example
`for`	Iterate over items	File processing
`while`	Repeat while condition is true	Monitoring
`until`	Repeat until condition becomes true	Retry logic
`break`	Exit loop	Error handling
`continue`	Skip current iteration	Filtering
`read`	Read input line by line	Log processing

Common Loop Mistakes¶

Mistake	Solution
Missing `done`	Close every loop
Infinite loop	Update loop condition
Forgetting to increment variables	Modify loop counter
Using the wrong loop type	Choose the appropriate loop
Not quoting filenames	Quote variables containing paths

Production Troubleshooting Scenario¶

Scenario

A monitoring script should check multiple servers.

Without loops:

ping server1

ping server2

ping server3

Using a loop:

for HOST in server1 server2 server3
do
    if ping -c1 "$HOST" >/dev/null
    then
        echo "$HOST is reachable"
    else
        echo "$HOST is down"
    fi
done

The script is easier to maintain and can scale to hundreds of servers.

Best Practices¶

Choose the appropriate loop type.
Keep loop bodies simple.
Avoid unnecessary nested loops.
Quote filenames and variables.
Use break and continue to improve readability.
Test loops with small datasets before running them in production.

Common Mistakes¶

❌ Forgetting the closing done.

✅ Remember to the closing done.

❌ Creating unintended infinite loops.

✅ Avoid this mistake: creating unintended infinite loops.

❌ Not updating loop variables.

✅ Always updating loop variables.

❌ Using for when while is more appropriate.

✅ Avoid using for when while is more appropriate when a safer approach exists.

❌ Processing filenames without quoting variables.

✅ Avoid this mistake: processing filenames without quoting variables.

Interview Questions¶

Beginner¶

What is a loop?
What are the three loop types in Bash?
What is the purpose of a for loop?
What is the purpose of a while loop?

Intermediate¶

What is the difference between while and until?
What does break do?
What does continue do?
How do you read a file line by line in Bash?

Architect Level¶

How would you automate server health checks using loops?
How would you process thousands of log files efficiently?
How can poorly designed loops affect production systems?

Summary¶

In this lesson, you learned:

for loops
while loops
until loops
Nested loops
Reading files line by line
break
continue
Production automation techniques

Loops are one of the most powerful features of Bash scripting. They eliminate repetitive code, simplify automation, and enable scripts to process files, users, servers, and system resources efficiently. Mastering loops is essential for writing scalable and production-ready Bash scripts.

Key Takeaways¶

Loops automate repetitive tasks.
Use for loops when the number of iterations is known.
Use while loops when execution depends on a condition.
Use until loops to repeat until a condition becomes true.
Use break to exit loops and continue to skip iterations.
Keep loops simple, efficient, and easy to read.

What's Next?¶

Functions — Writing Reusable Code in Bash Scripts

You'll explore:

Creating functions
Function parameters
Returning values
Variable scope
Reusable code
Organizing large scripts
Production scripting best practices

By the end of the lesson, you'll be able to build modular, reusable Bash scripts using functions, making your automation easier to maintain and scale.