Chapter 10: Shell Scripting

Learning Objectives

By the end of this chapter, you will be able to:

• Write and execute bash scripts
• Use variables and user input
• Implement conditionals and comparisons
• Use loops for repetitive tasks
• Create functions for reusable code
• Automate real-world system administration tasks

Prerequisites

• Completed Chapter 9: Processes & Services
• Comfortable with basic commands and pipes
• Understanding of file permissions

What is Shell Scripting?

A shell script is a text file containing a sequence of commands that the shell can execute. Think of it as:

• Automation - Run multiple commands with one command
• Programming - Use variables, loops, and conditionals
• Productivity - Save time on repetitive tasks

#!/bin/bash
# This is a simple script
echo "Hello, World!"
date
whoami

Why Write Scripts?

Creating Your First Script

Script Structure

Every bash script starts with a shebang:

#!/bin/bash
# ^ This tells Linux to run this file with bash

# Comments start with #
echo "This is my first script!"

Step 1: Create the Script

# Create the file
nano myscript.sh

# Or use echo
cat > myscript.sh << 'EOF'
#!/bin/bash
echo "Hello, World!"
echo "Today is $(date)"
echo "Current user: $USER"
EOF

Step 2: Make It Executable

chmod +x myscript.sh

# Verify permissions
ls -l myscript.sh
-rwxr-xr-x 1 user user ... myscript.sh
# ^^^
# The 'x' means executable

Step 3: Run the Script

# Execute directly
./myscript.sh

# Or with bash explicitly
bash myscript.sh

# Or with sh
sh myscript.sh

Variables

Defining Variables

#!/bin/bash

# No spaces around '='
name="Alice"
age=25
greeting="Hello, $name!"

echo $name           # Alice
echo $greeting       # Hello, Alice!
echo "I am $age"     # I am 25

Variable Rules

# Correct
NAME="value"
my_var="value"
_var2="value"

# WRONG - spaces around =
name = "Alice"    # Error

# WRONG - using $ on left side
$name="Bob"       # Error

Using Variables

#!/bin/bash

file="report.txt"
count=5

echo "File: $file"           # File: report.txt
echo "Count: $count"         # Count: 5

# Braces for clarity
echo "${file}_backup"        # report.txt_backup

Environment Variables

These are predefined variables available in every script:

#!/bin/bash

echo "Current user: $USER"
echo "Home directory: $HOME"
echo "Current directory: $PWD"
echo "Shell: $SHELL"
echo "Hostname: $HOSTNAME"
echo "Path: $PATH"

Command Substitution

Use the output of a command as a variable value.

Syntax: $(command) or `command`

#!/bin/bash

# Get today's date
today=$(date +%Y-%m-%d)
echo "Today is: $today"

# Count files
file_count=$(ls | wc -l)
echo "Files in current directory: $file_count"

# Get disk usage
disk_usage=$(df -h / | tail -1 | awk '{print $5}')
echo "Disk usage: $disk_usage"

Old Syntax (Backticks)

# Still works, but prefer $(...)
today=`date +%Y-%m-%d`

User Input

Reading Input with read

#!/bin/bash

echo "What is your name?"
read name
echo "Hello, $name!"

Prompt and Input on One Line

read -p "Enter your name: " name
echo "Hello, $name!"

Silent Input (for passwords)

read -s -p "Enter password: " password
echo  # newline
echo "Password received (not shown)"

Reading Multiple Values

read -p "Enter name age: " name age
echo "Name: $name, Age: $age"

Conditionals

if Statements

#!/bin/bash

count=10

if [ $count -eq 10 ]; then
    echo "Count is 10"
fi

if-else

#!/bin/bash

age=18

if [ $age -ge 18 ]; then
    echo "You are an adult"
else
    echo "You are a minor"
fi

if-elif-else

#!/bin/bash

score=75

if [ $score -ge 90 ]; then
    echo "Grade: A"
elif [ $score -ge 80 ]; then
    echo "Grade: B"
elif [ $score -ge 70 ]; then
    echo "Grade: C"
else
    echo "Grade: F"
fi

Comparison Operators

Examples

#!/bin/bash

# File check
if [ -f "/etc/passwd" ]; then
    echo "Password file exists"
fi

# String comparison
name="Alice"
if [ "$name" = "Alice" ]; then
    echo "Welcome, Alice!"
fi

# Number comparison
age=20
if [ $age -ge 18 ]; then
    echo "You can vote"
fi

# Combining conditions
if [ -f "data.txt" ] && [ -r "data.txt" ]; then
    echo "File exists and is readable"
fi

Logical Operators

-a    # AND (within single test)
-o    # OR (within single test)
!     # NOT
&&    # AND (between commands)
||    # OR (between commands)

# Example
if [ $age -ge 18 ] && [ $age -le 65 ]; then
    echo "Working age"
fi

Loops

for Loop

#!/bin/bash

# Loop through numbers
for i in 1 2 3 4 5; do
    echo "Number: $i"
done

# Loop through range
for i in {1..10}; do
    echo "Count: $i"
done

# Loop through files
for file in *.txt; do
    echo "Processing: $file"
done

while Loop

#!/bin/bash

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

until Loop

#!/bin/bash

count=1
until [ $count -gt 5 ]; do
    echo "Count: $count"
    count=$((count + 1))
done

Breaking Out of Loops

#!/bin/bash

# break - exit loop entirely
for i in {1..10}; do
    if [ $i -eq 5 ]; then
        break
    fi
    echo $i
done
# Output: 1 2 3 4

# continue - skip to next iteration
for i in {1..5}; do
    if [ $i -eq 3 ]; then
        continue
    fi
    echo $i
done
# Output: 1 2 4 5

Functions

Defining Functions

#!/bin/bash

# Define a function
greet() {
    echo "Hello, $1!"
}

# Call the function
greet "Alice"
greet "Bob"

Functions with Multiple Arguments

#!/bin/bash

greet() {
    name=$1
    age=$2
    echo "Hello, $name! You are $age years old."
}

greet "Alice" 25
greet "Bob" 30

Returning Values

#!/bin/bash

add() {
    result=$(($1 + $2))
    echo $result
}

sum=$(add 5 3)
echo "Sum is: $sum"

Functions with Return Code

#!/bin/bash

check_file() {
    if [ -f "$1" ]; then
        return 0    # Success
    else
        return 1    # Failure
    fi
}

if check_file "data.txt"; then
    echo "File exists"
else
    echo "File not found"
fi

Arithmetic

Arithmetic Expansion

#!/bin/bash

a=10
b=5

# Addition
echo $(($a + $b))      # 15

# Subtraction
echo $(($a - $b))      # 5

# Multiplication
echo $(($a * $b))      # 50

# Division
echo $(($a / $b))      # 2

# Modulo
echo $(($a % $b))      # 0

# Exponentiation
echo $(($a ** 2))      # 100

Shorthand Arithmetic

#!/bin/bash

count=5

count=$((count + 1))   # Increment
echo $count            # 6

((count++))            # Post-increment
echo $count            # 7

((count+=5))           # Add 5
echo $count            # 12

Special Variables

Positional Parameters

#!/bin/bash

# $0    - Script name
# $1    - First argument
# $2    - Second argument
# $@    - All arguments
# $#    - Number of arguments

echo "Script name: $0"
echo "First arg: $1"
echo "Second arg: $2"
echo "All args: $@"
echo "Number of args: $#"

Exit Status

#!/bin/bash

# $? - Exit status of last command

ls /nonexistent
if [ $? -ne 0 ]; then
    echo "Command failed"
fi

Practical Scripts

Example 1: Backup Script

#!/bin/bash
# backup.sh - Backup important files

SOURCE="$HOME/Documents"
DESTINATION="$HOME/backup"
DATE=$(date +%Y%m%d_%H%M%S)
BACKUP_DIR="$DESTINATION/backup_$DATE"

# Create backup directory
mkdir -p "$BACKUP_DIR"

# Copy files
echo "Backing up $SOURCE to $BACKUP_DIR"
cp -r "$SOURCE"/* "$BACKUP_DIR/"

# Compress
tar -czf "$BACKUP_DIR.tar.gz" "$BACKUP_DIR"
rm -rf "$BACKUP_DIR"

echo "Backup complete: $BACKUP_DIR.tar.gz"

Example 2: System Health Check

#!/bin/bash
# health.sh - Check system health

echo "=== System Health Check ==="
echo "Date: $(date)"
echo ""

# Disk usage
echo "--- Disk Usage ---"
df -h | grep -E "Filesystem|/$"

# Memory
echo ""
echo "--- Memory Usage ---"
free -h

# CPU
echo ""
echo "--- CPU Usage ---"
top -bn1 | grep "Cpu(s)"

# Top processes
echo ""
echo "--- Top 5 Processes by CPU ---"
ps aux | sort -rk 3 | head -n 6

echo ""
echo "=== Health Check Complete ==="

Example 3: Log Cleaner

#!/bin/bash
# clean_logs.sh - Remove old log files

LOG_DIR="/var/log"
DAYS=30

echo "Finding logs older than $DAYS days..."

# Find and remove old logs (dry run)
find "$LOG_DIR" -name "*.log" -type f -mtime +$DAYS

read -p "Delete these files? (y/n) " confirm

if [ "$confirm" = "y" ]; then
    find "$LOG_DIR" -name "*.log" -type f -mtime +$DAYS -delete
    echo "Old logs deleted."
else
    echo "Operation cancelled."
fi

Example 4: Batch File Renamer

#!/bin/bash
# rename.sh - Rename files in batch

# Add prefix to all files
PREFIX="backup_"

for file in *.txt; do
    if [ -f "$file" ]; then
        mv "$file" "${PREFIX}${file}"
        echo "Renamed: $file -> ${PREFIX}${file}"
    fi
done

Example 5: User Creation Script

#!/bin/bash
# create_user.sh - Create a new user

if [ $(id -u) -ne 0 ]; then
    echo "This script must be run as root"
    exit 1
fi

read -p "Enter username: " username

# Check if user exists
if id "$username" &>/dev/null; then
    echo "User $username already exists"
    exit 1
fi

# Create user
useradd -m "$username"

# Set password
passwd "$username"

echo "User $username created successfully"

Best Practices

1. Use Shebang

#!/bin/bash
# Always specify the interpreter

2. Use Comments

#!/bin/bash
# This script backs up important files
# Author: Your Name
# Date: 2025-02-07

# Create backup directory
mkdir -p backup

3. Quote Variables

# Good
echo "$name"
rm "$file"

# Bad (breaks with spaces)
echo $name
rm $file

4. Check for Errors

#!/bin/bash
set -e    # Exit on error

# Or check specific commands
if ! cp source.txt dest.txt; then
    echo "Copy failed"
    exit 1
fi

5. Use Meaningful Names

# Good
backup_source="$HOME/Documents"
backup_destination="$HOME/backup"

# Bad
a="$HOME/Documents"
b="$HOME/backup"

Debugging Scripts

Enable Debugging

#!/bin/bash
set -x    # Print each command before executing
# or run: bash -x script.sh

set -v    # Print script lines as they are read

Syntax Check

bash -n script.sh    # Check syntax without running

Common Errors

Summary

In this chapter, you learned:

• Script Basics: Shebang, creating, and executing scripts
• Variables: Defining and using variables, environment variables
• Input/Output: echo, read, command substitution
• Conditionals: if, else, elif, comparison operators
• Loops: for, while, until, break, continue
• Functions: Creating reusable code blocks
• Arithmetic: Math operations and calculations
• Best Practices: Comments, error checking, quoting

Chapter Quiz

Test your understanding of shell scripting:

Exercises

Exercise 1: Hello Script

1. Create a script that asks for your name
2. Greets you with the current date
3. Tells you how many files are in the current directory

Exercise 2: Number Guessing Game

1. Generate a random number: random=$((1 + RANDOM % 100))
2. Ask the user to guess
3. Tell them if it's too high or too low
4. Continue until they guess correctly

Exercise 3: File Backup

1. Create a script that backs up all .txt files
2. Create a backup/ directory if it doesn't exist
3. Copy all .txt files with a timestamp prefix
4. Print a summary of files copied

Exercise 4: System Monitor

1. Create a script that checks disk usage
2. If disk usage is above 80%, print a warning
3. Check if a specific service is running
4. Print a summary report

Exercise 5: Calculator

1. Create a script that takes two numbers
2. Ask for the operation (+, -, *, /)
3. Perform the calculation
4. Display the result

Expected Output

Exercise 1 Solution

#!/bin/bash
# hello.sh

echo "What is your name?"
read name

echo ""
echo "Hello, $name!"
echo "Today is $(date)"
echo "Files in this directory: $(ls | wc -l)"

$ chmod +x hello.sh
$ ./hello.sh
What is your name?
Alice

Hello, Alice!
Today is Wed Feb  7 12:34:56 CET 2025
Files in this directory: 15

Exercise 2 Solution

#!/bin/bash
# guess.sh

random=$((1 + RANDOM % 100))
guess=0
attempts=0

echo "Guess the number (1-100)"

while [ $guess -ne $random ]; do
    read -p "Enter your guess: " guess
    ((attempts++))

    if [ $guess -lt $random ]; then
        echo "Too low!"
    elif [ $guess -gt $random ]; then
        echo "Too high!"
    fi
done

echo "Correct! You guessed it in $attempts attempts."

Exercise 3 Solution

#!/bin/bash
# backup.sh

BACKUP_DIR="backup"
TIMESTAMP=$(date +%Y%m%d_%H%M%S)
COUNT=0

# Create backup directory
mkdir -p "$BACKUP_DIR"

# Copy all .txt files
for file in *.txt; do
    if [ -f "$file" ]; then
        cp "$file" "$BACKUP_DIR/${TIMESTAMP}_$file"
        ((COUNT++))
        echo "Backed up: $file"
    fi
done

echo ""
echo "Backup complete! Copied $COUNT files to $BACKUP_DIR/"

Exercise 4 Solution

#!/bin/bash
# monitor.sh

# Check disk usage
DISK_USAGE=$(df / | tail -1 | awk '{print $5}' | sed 's/%//')
echo "Disk usage: $DISK_USAGE%"

if [ $DISK_USAGE -gt 80 ]; then
    echo "WARNING: Disk usage is above 80%!"
fi

# Check if SSH is running
echo ""
if systemctl is-active --quiet ssh; then
    echo "SSH service: Running"
else
    echo "SSH service: Not running"
fi

# Memory check
echo ""
echo "Memory usage:"
free -h | grep "Mem:"

Exercise 5 Solution

#!/bin/bash
# calculator.sh

read -p "Enter first number: " num1
read -p "Enter second number: " num2
read -p "Enter operation (+, -, *, /): " op

case $op in
    +)
        result=$(($num1 + $num2))
        ;;
    -)
        result=$(($num1 - $num2))
        ;;
    \*)
        result=$(($num1 * $num2))
        ;;
    /)
        if [ $num2 -eq 0 ]; then
            echo "Error: Division by zero"
            exit 1
        fi
        result=$(($num1 / $num2))
        ;;
    *)
        echo "Invalid operation"
        exit 1
        ;;
esac

echo ""
echo "$num1 $op $num2 = $result"

Next Chapter

In Chapter 11, you'll learn Networking Basics - understanding IP addresses, checking connectivity, using SSH for remote access, and troubleshooting network issues.