Part 3: Control Structures in Bash (if, for, while, case, break, continue)

 Introduction

Control structures are what make your Bash scripts intelligent, dynamic, and able to make decisions.
With these tools, your script can:

• Check if a file exists
• Loop through lists or directories
• Perform repeated tasks
• Match patterns
• Break or skip loops when needed

This part is crucial because automation begins with conditions + loops.

🧠 SECTION 1 — IF, ELSE, ELIF (Conditional Statements)

🔹 Basic Syntax

if [ condition ]; then
    # commands
fi

🔹 Example: Check if a File Exists

file="data.txt"

if [ -f "$file" ]; then
    echo "File $file exists."
else
    echo "File $file not found."
fi

Common File Conditions

🔹 Using elif

num=20

if [ $num -gt 50 ]; then
    echo "Number is greater than 50"
elif [ $num -gt 10 ]; then
    echo "Number is greater than 10"
else
    echo "Number is 10 or less"
fi

🔹 Comparison Operators

🧠 SECTION 2 — FOR LOOPS

Used to repeat actions for a list.

🔹 Loop Through Numbers

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

🔹 Loop Through Files in Directory

for file in *.txt
do
    echo "Processing $file"
done

🔹 Loop Through an Array

fruits=("apple" "banana" "mango")

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

🧠 SECTION 3 — WHILE LOOPS

Repeat until condition becomes false.

🔹 Example: Print Numbers 1 to 5

count=1
while [ $count -le 5 ]
do
    echo "Count: $count"
    count=$((count+1))
done

🔹 Monitor a Log File Until a Keyword Appears

while true
do
    if grep -q "ERROR" app.log; then
        echo "Error found in log!"
        break
    fi
    sleep 3
done

This is used often for server monitoring scripts.

🧠 SECTION 4 — UNTIL LOOPS

Opposite of while: runs until condition is true.

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

🧠 SECTION 5 — CASE STATEMENT

Better than long if-else chains. Useful for menus.

🔹 Example: User Menu

echo "Choose an option:"
echo "1) Show date"
echo "2) List files"
echo "3) Show current user"

read choice

case $choice in
    1)
        date
        ;;
    2)
        ls -l
        ;;
    3)
        whoami
        ;;
    *)
        echo "Invalid option!"
        ;;
esac

🧠 SECTION 6 — LOOP CONTROL: BREAK & CONTINUE

🔹 BREAK — Stop the loop

for i in {1..10}
do
    if [ $i -eq 5 ]; then
        break
    fi
    echo "Number: $i"
done

🔹 CONTINUE — Skip current iteration

for i in {1..5}
do
    if [ $i -eq 3 ]; then
        continue
    fi
    echo "Value: $i"
done

🧰 Real-World Automation Example: Clean Old Log Files

#!/bin/bash

log_dir="/var/log/myapp"

for file in "$log_dir"/*.log
do
    if [ -f "$file" ]; then
        size=$(stat -c%s "$file")
        if [ $size -gt 500000 ]; then
            echo "Rotating $file ..."
            mv "$file" "$file.old"
        fi
    fi
done

✔️ Very useful in server maintenance scripts (DevOps).

🧰 Bonus: Server Health Check Script

#!/bin/bash

echo "Checking server health..."
echo "---------------------------"

# CPU usage
cpu=$(top -bn1 | grep "Cpu" | awk '{print $2}')
echo "CPU Usage: $cpu%"

# Memory
mem=$(free -m | awk 'NR==2{printf "%s/%sMB (%.2f%%)", $3,$2,$3*100/$2 }')
echo "Memory: $mem"

# Disk
disk=$(df -h / | awk 'NR==2{print $5}')
echo "Disk Usage: $disk"

# Check service
if systemctl is-active --quiet nginx; then
    echo "Nginx: Running"
else
    echo "Nginx: Not Running"
fi

 Summary

In Part 3, you learned:

✅ if, elif, else
✅ Numeric and file comparisons
✅ for, while, and until loops
✅ case pattern matching
✅ break & continue
✅ Real-world automation examples

These are the core fundamentals of every automation script in Bash.

🔜 Next in Series

👉 Part 4: Working with Files & Directories (grep, sed, awk, permissions, redirects, pipes)
This part includes powerful text-processing commands used in DevOps & Linux admin work.