For Loops

For loops are one of the most powerful and commonly used control structures in Bash. They allow you to iterate over lists, arrays, file sets, command output, and numeric ranges. Mastering for loops is essential for processing multiple items, automating repetitive tasks, and building efficient shell scripts!

Basic For-In Loop

The most common type of for loop iterates over a list of items. Each item is assigned to the loop variable and processed.

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# Basic syntax
for variable in item1 item2 item3; do
    # code to execute for each item
done

# Example
for fruit in apple banana cherry; do
    echo "Fruit: $fruit"
done

# Output:
# Fruit: apple
# Fruit: banana
# Fruit: cherry

Iterating Over Arrays

For loops are perfect for processing arrays. Use "${array[@]}" to iterate over all elements.

# Array iteration
fruits=("apple" "banana" "cherry")
for fruit in "${fruits[@]}"; do
    echo "$fruit"
done

# Iterating with index
for i in "${!fruits[@]}"; do
    echo "Index $i: ${fruits[$i]}"
done

C-Style For Loop

Bash supports C-style for loops for numeric iteration. This syntax is useful for counting and numeric ranges.

# C-style syntax
for ((initialization; condition; increment)); do
    # code
done

# Example: Count from 1 to 5
for ((i=1; i<=5; i++)); do
    echo "Count: $i"
done

# Example: Count by 2
for ((i=0; i<=10; i+=2)); do
    echo "$i"
done

# Example: Countdown
for ((i=5; i>=1; i--)); do
    echo "Countdown: $i"
done

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Using Brace Expansion for Ranges

Brace expansion provides an easy way to generate number sequences without using C-style loops.

# Number ranges
for num in {1..5}; do
    echo "Number: $num"
done

# With step (Bash 4+)
for num in {0..10..2}; do
    echo "$num"
done

# Character ranges
for letter in {a..e}; do
    echo "$letter"
done

# Reverse ranges
for num in {5..1}; do
    echo "Countdown: $num"
done

Iterating Over Command Output

For loops can iterate over the output of commands. Each line becomes an iteration.

# Iterate over file listing
for file in $(ls /bin); do
    echo "File: $file"
done

# Iterate over lines in a file
for line in $(cat file.txt); do
    echo "Line: $line"
done

# Better: using while read for files (preserves spaces)
while IFS= read -r line; do
    echo "$line"
done < file.txt

Iterating Over Script Arguments

For loops can process command-line arguments passed to your script.

# Iterate over all arguments
for arg in "$@"; do
    echo "Argument: $arg"
done

# Shorthand: "$@" is default for for loop
for arg; do
    echo "Argument: $arg"
done

# Iterate with index
i=1
for arg in "$@"; do
    echo "Argument $i: $arg"
    ((i++))
done

Nested For Loops

You can nest for loops inside other for loops to create multi-dimensional iterations.

# Nested loops
for i in {1..3}; do
    for j in {a..c}; do
        echo "$i$j"
    done
done

# Output:
# 1a, 1b, 1c, 2a, 2b, 2c, 3a, 3b, 3c

Common Mistakes

# Wrong - elements with spaces get split
files=("file one.txt" "file two.txt")
for file in ${files[@]}; do
    echo "$file"  # Prints: file, one.txt, file, two.txt
done

# Correct - quote the expansion
for file in "${files[@]}"; do
    echo "$file"  # Prints: file one.txt, file two.txt
done

# Wrong - $ not needed in (( ))
for (($i=1; $i<=5; $i++)); do  # Syntax error!
    echo "$i"
done

# Correct - no $ in arithmetic context
for ((i=1; i<=5; i++)); do
    echo "$i"
done

# Wrong - splits on spaces, not lines
for line in $(cat file.txt); do
    echo "$line"
done

# Correct - use while read
while IFS= read -r line; do
    echo "$line"
done < file.txt

Exercise: Process Multiple Files

#!/bin/bash
# For loop exercises

# 1. For-in loop
echo "Fruits:"
for fruit in apple banana cherry; do
    echo "  - $fruit"
done

# 2. C-style loop
echo -e "\nCounting 1-10:"
for ((i=1; i<=10; i++)); do
    echo "  $i"
done

# 3. Array iteration
echo -e "\nColors:"
colors=("red" "green" "blue")
for color in "${colors[@]}"; do
    echo "  $color"
done

# 4. Brace expansion
echo -e "\nNumbers 1-5:"
for num in {1..5}; do
    echo "  $num"
done

# 5. Nested loop (multiplication table)
echo -e "\nMultiplication table (2x):"
for i in {1..5}; do
    for j in {1..3}; do
        result=$((i * j))
        echo -n "  $i×$j=$result"
    done
    echo ""
done

Summary

What's Next?

Perfect! You've mastered for loops for iteration. Next, we'll learn about While & Until Loops - control structures that continue looping based on conditions rather than iterating over lists. These are essential for reading files, waiting for conditions, and creating interactive scripts!