Shell Scripting Basics

#!/bin/bash
# This is a comment
echo "Hello, World!"

# Create the script
nano hello.sh

# Make it executable
chmod +x hello.sh

# Run the script
./hello.sh

#!/bin/bash

# String variables
name="John"
echo "Hello, $name!"

# Numeric variables
age=25
echo "Age: $age"

# Command substitution
current_date=$(date)
echo "Today is: $current_date"

#!/bin/bash

echo "Script name: $0"
echo "First argument: $1"
echo "Second argument: $2"
echo "All arguments: $@"
echo "Number of arguments: $#"
echo "Process ID: $$"
echo "Exit status of last command: $?"

#!/bin/bash

# Check if arguments are provided
if [ $# -eq 0 ]; then
    echo "Usage: $0 <name> [age]"
    exit 1
fi

name=$1
age=${2:-"unknown"}  # Default value if not provided

echo "Name: $name"
echo "Age: $age"

#!/bin/bash

read -p "Enter a number: " num

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

#!/bin/bash

filename="test.txt"

if [ -f "$filename" ]; then
    echo "File exists"
elif [ -d "$filename" ]; then
    echo "Directory exists"
else
    echo "Neither file nor directory exists"
fi

#!/bin/bash

read -p "Enter your name: " name

if [ "$name" = "admin" ]; then
    echo "Welcome, administrator!"
elif [ -z "$name" ]; then
    echo "Name cannot be empty"
else
    echo "Hello, $name!"
fi

#!/bin/bash

# Loop through arguments
for arg in "$@"; do
    echo "Argument: $arg"
done

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

# C-style for loop
for ((i=1; i<=5; i++)); do
    echo "Number: $i"
done

#!/bin/bash

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

# Reading a file line by line
while IFS= read -r line; do
    echo "Line: $line"
done < input.txt

#!/bin/bash

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

#!/bin/bash

# Function definition
greet() {
    echo "Hello, $1!"
}

# Function call
greet "World"
greet "Alice"

#!/bin/bash

# Function that returns a value
is_even() {
    if [ $(($1 % 2)) -eq 0 ]; then
        return 0  # Success (true)
    else
        return 1  # Failure (false)
    fi
}

# Usage
read -p "Enter a number: " num

if is_even $num; then
    echo "$num is even"
else
    echo "$num is odd"
fi

#!/bin/bash

calculate_area() {
    local length=$1
    local width=$2
    local area=$((length * width))
    echo $area
}

result=$(calculate_area 10 5)
echo "Area: $result"

#!/bin/bash

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

# Silent input (for passwords)
read -s -p "Enter password: " password
echo  # New line after silent input

# Read multiple values
read -p "Enter first and last name: " first last
echo "First: $first, Last: $last"

#!/bin/bash

# Writing to a file
echo "This is line 1" > output.txt
echo "This is line 2" >> output.txt

# Reading from a file
while IFS= read -r line; do
    echo "Read: $line"
done < output.txt

# Here document
cat << EOF > config.txt
server=localhost
port=8080
timeout=30
EOF

#!/bin/bash

# Function that might fail
backup_file() {
    if cp "$1" "$1.bak"; then
        echo "Backup created successfully"
        return 0
    else
        echo "Backup failed" >&2
        return 1
    fi
}

# Usage
if backup_file important.txt; then
    echo "Proceeding with next steps..."
else
    echo "Cannot proceed without backup"
    exit 1
fi

#!/bin/bash

# Cleanup function
cleanup() {
    echo "Cleaning up temporary files..."
    rm -f /tmp/temp_*.txt
    exit
}

# Set trap for common signals
trap cleanup INT TERM EXIT

echo "Script running... Press Ctrl+C to test cleanup"
sleep 10

#!/bin/bash

echo "=== System Information ==="
echo "Hostname: $(hostname)"
echo "OS: $(uname -s) $(uname -r)"
echo "Uptime: $(uptime -p)"
echo "Memory: $(free -h | grep '^Mem:' | awk '{print $3 "/" $2}')"
echo "Disk Usage: $(df -h / | tail -1 | awk '{print $5}')"

#!/bin/bash

# Backup script with error handling
SOURCE_DIR="/home/user/documents"
BACKUP_DIR="/home/user/backup"
TIMESTAMP=$(date +%Y%m%d_%H%M%S)

if [ ! -d "$SOURCE_DIR" ]; then
    echo "Source directory does not exist: $SOURCE_DIR" >&2
    exit 1
fi

mkdir -p "$BACKUP_DIR"

tar -czf "$BACKUP_DIR/backup_$TIMESTAMP.tar.gz" -C "$SOURCE_DIR" . 2>/dev/null

if [ $? -eq 0 ]; then
    echo "Backup completed: $BACKUP_DIR/backup_$TIMESTAMP.tar.gz"
else
    echo "Backup failed" >&2
    exit 1
fi

#!/bin/bash

calculate() {
    case $2 in
        +) echo $(($1 + $3)) ;;
        -) echo $(($1 - $3)) ;;
        \*) echo $(($1 * $3)) ;;
        /) 
            if [ $3 -eq 0 ]; then
                echo "Division by zero!" >&2
                return 1
            fi
            echo $(($1 / $3)) 
            ;;
        *) echo "Invalid operator" >&2; return 1 ;;
    esac
}

if [ $# -ne 3 ]; then
    echo "Usage: $0 <num1> <operator> <num2>"
    echo "Operators: +, -, *, /"
    exit 1
fi

result=$(calculate $1 "$2" $3)
if [ $? -eq 0 ]; then
    echo "$1 $2 $3 = $result"
fi

# Bad - word splitting can occur
echo $filename

# Good - preserves spaces and special characters
echo "$filename"

# Bad - script appears successful even on failure
some_command_that_might_fail

# Good - propagate error status
some_command_that_might_fail || exit 1

# Bad - continues even if mkdir fails
mkdir /important/directory
echo "success" > /important/directory/file.txt

# Good - check result
if mkdir /important/directory; then
    echo "success" > /important/directory/file.txt
else
    echo "Failed to create directory" >&2
    exit 1
fi

#!/bin/bash -x  # Enable debug mode

# Or enable debugging for specific sections
set -x
some_command
set +x

#!/bin/bash

# Add debug information
echo "Starting script execution"
echo "Arguments: $@"
echo "Working directory: $(pwd)"

# Your script logic here