Bash Loops: A Practical Guide

Loops in Bash allow you to automate repetitive tasks. This document covers:
1️⃣ Reading from files vs. direct input
2️⃣ for vs while loops (key differences)
3️⃣ Nested loops (loop inside a loop)

---

1. Reading Lists: File vs. Direct Input

Method 1: Direct Input (Hardcoded List)

for item in "apple" "banana" "cherry"; do
    echo "Fruit: $item"
done

Output:

Fruit: apple  
Fruit: banana  
Fruit: cherry  

Method 2: Read from a File

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

Assumes fruits.txt contains:

apple  
banana  
cherry  

Key Notes:

• IFS= prevents whitespace issues.
• -r avoids interpreting backslashes.

---

2. for vs while Loops

Feature	for Loop	while Loop
Use Case	Iterate over a known list	Run until a condition is false
Syntax	for var in list; do ...	while [ condition ]; do ...
Termination	Ends after list is processed	Ends when condition is false
Example	for i in {1..3}; do ...	while [ $i -le 3 ]; do ...

Example: for Loop (Counting)

for i in {1..3}; do
    echo "Number: $i"
done

Example: while Loop (Condition-Based)

counter=1
while [ $counter -le 3 ]; do
    echo "Count: $counter"
    ((counter++))
done

---

3. Nested Loops (Loop Inside a Loop)

Example: Multiplication Table (Nested for)

for i in {1..3}; do
    for j in {1..3}; do
        echo "$i x $j = $((i * j))"
    done
done

Output:

1 x 1 = 1  
1 x 2 = 2  
1 x 3 = 3  
2 x 1 = 2  
...  
3 x 3 = 9  

Example: while Inside for

for user in "alice" "bob"; do
    echo "User: $user"
    attempts=1
    while [ $attempts -le 2 ]; do
        echo "  Login attempt #$attempts"
        ((attempts++))
    done
done

Output:

User: alice  
  Login attempt #1  
  Login attempt #2  
User: bob  
  Login attempt #1  
  Login attempt #2  

---

Cheat Sheet: When to Use Which Loop

Scenario	Recommended Loop
Known list (files, numbers, etc.)	for
Condition-based (e.g., "retry until success")	while
Complex iterations (e.g., grids)	Nested loops (for/while)

---

Final Tips

✔ Avoid infinite while loops → Always update the condition (e.g., ((counter++))).
✔ Use break/continue → Exit early or skip iterations.
✔ Quote variables → Prevents word splitting (e.g., "$item").