🔄 Lesson 8.3: Loops and Functions

for Loops

A for loop repeats commands for each item in a list.

Looping Over a List

#!/bin/bash

# Loop over explicit values
for COLOR in red green blue; do
    echo "Color: $COLOR"
done

# Loop over files in a directory
for FILE in *.txt; do
    echo "Found text file: $FILE"
done

# Loop over command output
for USER in $(who | awk '{print $1}' | sort -u); do
    echo "Logged in: $USER"
done

C-Style for Loop

# Count from 1 to 10
for ((i = 1; i <= 10; i++)); do
    echo "Number: $i"
done

# Count by 2s
for ((i = 0; i <= 20; i += 2)); do
    echo "$i"
done

Using {start..end} Ranges

# Brace expansion — generates a sequence
for i in {1..5}; do
    echo "Step $i"
done

# With a step value (Bash 4+)
for i in {0..100..10}; do
    echo "$i%"
done

# Letters work too
for letter in {a..z}; do
    echo -n "$letter "
done
echo ""

# ❌ Fragile — breaks on filenames with spaces
for f in $(ls *.txt); do echo "$f"; done

# ✅ Correct — glob handles spaces properly
for f in *.txt; do echo "$f"; done

while Loops

A while loop repeats as long as a condition is true.

#!/bin/bash

# Count down from 5
COUNT=5
while [ "$COUNT" -gt 0 ]; do
    echo "$COUNT..."
    ((COUNT--))
done
echo "Liftoff!"

# Read a file line by line (the proper way)
while IFS= read -r LINE; do
    echo "Line: $LINE"
done < /etc/hostname

# Infinite loop with exit condition
while true; do
    read -p "Enter 'quit' to exit: " INPUT
    if [ "$INPUT" = "quit" ]; then
        break
    fi
    echo "You said: $INPUT"
done

Processing Command Output with a Pipe

# Process each line from a command
df -h | tail -n +2 | while read -r LINE; do
    USAGE=$(echo "$LINE" | awk '{print $5}' | tr -d '%')
    MOUNT=$(echo "$LINE" | awk '{print $6}')
    if [ "$USAGE" -gt 80 ] 2>/dev/null; then
        echo "⚠️  WARNING: $MOUNT is ${USAGE}% full!"
    fi
done

until Loops

until is the opposite of while — it loops until the condition becomes true.

#!/bin/bash

# Wait for a file to appear
echo "Waiting for /tmp/signal.txt to be created..."
until [ -f /tmp/signal.txt ]; do
    sleep 1
done
echo "File found!"

# Count up to 5
COUNT=1
until [ "$COUNT" -gt 5 ]; do
    echo "Count: $COUNT"
    ((COUNT++))
done

break and continue

#!/bin/bash

# break — exit the loop entirely
for i in {1..100}; do
    if [ "$i" -gt 5 ]; then
        break   # Stop at 5
    fi
    echo "$i"
done
# Output: 1 2 3 4 5

# continue — skip the rest of THIS iteration
for i in {1..10}; do
    if [ $((i % 2)) -eq 0 ]; then
        continue   # Skip even numbers
    fi
    echo "$i"
done
# Output: 1 3 5 7 9

Functions

Functions let you group commands into reusable blocks with a name. Define once, call anywhere.

#!/bin/bash

# Define a function (two equivalent syntaxes)
greet() {
    echo "Hello, World!"
}

function say_goodbye {
    echo "Goodbye!"
}

# Call the functions
greet            # Hello, World!
say_goodbye      # Goodbye!

# Functions must be defined BEFORE they're called
# (Bash reads top-to-bottom)

Function Arguments and Return Values

Functions receive arguments the same way scripts do — via $1, $2, $@, etc.

#!/bin/bash

greet() {
    local name="${1:-World}"    # First argument, default "World"
    echo "Hello, $name!"
}

greet "Ray"     # Hello, Ray!
greet           # Hello, World!

# A function with multiple arguments
add() {
    echo $(( $1 + $2 ))
}

RESULT=$(add 10 20)
echo "Sum: $RESULT"    # Sum: 30

Return Values

Bash functions have two ways to "return" data:

#!/bin/bash

# Method 1: return — sets an exit code (0-255 only)
is_even() {
    if [ $(( $1 % 2 )) -eq 0 ]; then
        return 0    # Success = true
    else
        return 1    # Failure = false
    fi
}

if is_even 42; then
    echo "42 is even"    # This runs
fi

# Method 2: echo — outputs text (capture with $())
get_extension() {
    local filename="$1"
    echo "${filename##*.}"    # Parameter expansion: remove everything before last dot
}

EXT=$(get_extension "report.pdf")
echo "Extension: $EXT"    # Extension: pdf

Local Variables

By default, variables in Bash are global — even inside functions. Use local to keep variables contained.

#!/bin/bash

GLOBAL_VAR="I'm global"

my_function() {
    local LOCAL_VAR="I'm local"
    GLOBAL_VAR="Modified by function"    # Changes the global!
    echo "Inside: LOCAL_VAR=$LOCAL_VAR"
    echo "Inside: GLOBAL_VAR=$GLOBAL_VAR"
}

my_function
echo "Outside: LOCAL_VAR=$LOCAL_VAR"       # Empty — local is gone
echo "Outside: GLOBAL_VAR=$GLOBAL_VAR"     # "Modified by function"

Inside: LOCAL_VAR=I'm local
Inside: GLOBAL_VAR=Modified by function
Outside: LOCAL_VAR=
Outside: GLOBAL_VAR=Modified by function

Capstone Script: Project Initializer

Here's a complete script that uses everything from this module — variables, input, conditionals, loops, and functions:

#!/bin/bash
# ============================================
# init-project.sh — Set up a new project directory
# Usage: ./init-project.sh [project-name]
# ============================================

# --- Configuration ---
PROJECTS_DIR="$HOME/projects"
TEMPLATES=("README.md" ".gitignore" "LICENSE")

# --- Functions ---

print_header() {
    echo ""
    echo "=============================="
    echo "  🚀 Project Initializer"
    echo "=============================="
    echo ""
}

create_readme() {
    local name="$1"
    local dir="$2"
    cat > "$dir/README.md" << EOF
# $name

## Description
A new project.

## Getting Started
\`\`\`bash
cd $name
\`\`\`

## Author
$(whoami)

## Created
$(date +%Y-%m-%d)
EOF
}

create_gitignore() {
    local dir="$1"
    cat > "$dir/.gitignore" << 'EOF'
# OS files
.DS_Store
Thumbs.db

# Editor files
*.swp
*.swo
.vscode/
.idea/

# Dependencies
node_modules/
__pycache__/
*.pyc

# Build output
dist/
build/
EOF
}

create_subdirs() {
    local dir="$1"
    shift   # Remove first argument, leaving the rest
    for subdir in "$@"; do
        mkdir -p "$dir/$subdir"
        echo "  📁 Created $subdir/"
    done
}

# --- Main Script ---

print_header

# Get project name from argument or prompt
PROJECT_NAME="${1:-}"
if [ -z "$PROJECT_NAME" ]; then
    read -p "Project name: " PROJECT_NAME
fi

# Validate name (letters, numbers, hyphens, underscores only)
if [[ ! "$PROJECT_NAME" =~ ^[a-zA-Z0-9_-]+$ ]]; then
    echo "❌ Invalid name. Use only letters, numbers, hyphens, underscores." >&2
    exit 1
fi

PROJECT_DIR="$PROJECTS_DIR/$PROJECT_NAME"

# Check if it already exists
if [ -d "$PROJECT_DIR" ]; then
    echo "❌ Directory already exists: $PROJECT_DIR" >&2
    exit 1
fi

# Ask what type of project
echo "What kind of project?"
echo "  1) Basic (just README and .gitignore)"
echo "  2) Web (adds src/, css/, js/ directories)"
echo "  3) Python (adds src/, tests/, requirements.txt)"
read -p "Choice [1]: " PROJECT_TYPE
PROJECT_TYPE="${PROJECT_TYPE:-1}"

# Create the project
echo ""
echo "Creating project: $PROJECT_NAME"
echo "Location: $PROJECT_DIR"
echo "---"

mkdir -p "$PROJECT_DIR"
echo "  📁 Created project directory"

# Create base files
create_readme "$PROJECT_NAME" "$PROJECT_DIR"
echo "  📄 Created README.md"

create_gitignore "$PROJECT_DIR"
echo "  📄 Created .gitignore"

# Create type-specific structure
case "$PROJECT_TYPE" in
    2)
        create_subdirs "$PROJECT_DIR" "src" "css" "js" "images"
        touch "$PROJECT_DIR/src/index.html"
        echo "  📄 Created src/index.html"
        ;;
    3)
        create_subdirs "$PROJECT_DIR" "src" "tests"
        touch "$PROJECT_DIR/requirements.txt"
        touch "$PROJECT_DIR/src/__init__.py"
        echo "  📄 Created requirements.txt"
        echo "  📄 Created src/__init__.py"
        ;;
    *)
        # Basic — no extra directories
        ;;
esac

# Initialize git if available
if command -v git &> /dev/null; then
    cd "$PROJECT_DIR" && git init --quiet
    echo "  🔀 Initialized git repository"
fi

# Done!
echo ""
echo "✅ Project '$PROJECT_NAME' is ready!"
echo "   cd $PROJECT_DIR"

Exercises

Knowledge Check

Summary

🚀 Where to Go from Here

• Practice daily — use the terminal for everyday tasks, even when a GUI exists
• Read man pages — man command is your always-available reference
• Explore further — cron jobs, advanced shell scripting, Docker, and Linux server administration
• Contribute — many open-source projects welcome Linux-savvy contributors