Linux Shell Scripting Basics: Automate Tasks with Bash

#!/bin/bash
# Script: backup.sh
# Purpose: Backup important files
# Author: Your Name
# Date: 2024-01-15

# Exit on error
set -e

# Exit on undefined variable
set -u

# Variables
BACKUP_DIR="/backup"
SOURCE_DIR="/home/user/documents"
DATE=$(date +%Y%m%d)

# Main logic
echo "Starting backup..."
tar -czf "$BACKUP_DIR/backup-$DATE.tar.gz" "$SOURCE_DIR"
echo "Backup complete: backup-$DATE.tar.gz"

# Exit successfully
exit 0

# Make it executable:
# chmod +x backup.sh
# Run it:
# ./backup.sh

#!/bin/bash

# Simple variables
NAME="John"
AGE=30
echo "User: $NAME, Age: $AGE"

# Command substitution
CURRENT_DATE=$(date +%Y-%m-%d)
USER_COUNT=$(who | wc -l)
DISK_USAGE=$(df -h / | awk 'NR==2 {print $5}')

echo "Date: $CURRENT_DATE"
echo "Users logged in: $USER_COUNT"
echo "Disk usage: $DISK_USAGE"

# Using braces for clarity
FILE="report"
echo "Creating ${FILE}_2024.txt"

# Quotes matter
LITERAL='$HOME'      # Stores literal "$HOME"
EXPANDED="$HOME"     # Expands to /home/user

# Read-only variable
readonly CONFIG_FILE="/etc/myapp.conf"

# Arrays
FILES=("file1.txt" "file2.txt" "file3.txt")
echo "First file: ${FILES[0]}"
echo "All files: ${FILES[@]}"

# Arithmetic
COUNT=5
COUNT=$((COUNT + 1))
echo "Count: $COUNT"

#!/bin/bash

# File tests
FILE="data.txt"
if [ -f "$FILE" ]; then
    echo "File exists"
else
    echo "File not found"
    exit 1
fi

# String comparison
USERNAME="admin"
if [ "$USERNAME" = "admin" ]; then
    echo "Admin user detected"
fi

# Check if variable is empty
if [ -z "$VAR" ]; then
    echo "Variable is empty"
fi

# Numeric comparison
AGE=25
if [ "$AGE" -ge 18 ]; then
    echo "Adult"
else
    echo "Minor"
fi

# Multiple conditions with [[ ]]
if [[ -f "$FILE" && -r "$FILE" ]]; then
    echo "File exists and is readable"
fi

# Pattern matching with [[ ]]
FILENAME="report.pdf"
if [[ "$FILENAME" == *.pdf ]]; then
    echo "PDF file detected"
fi

# Testing command success
if ping -c 1 google.com &> /dev/null; then
    echo "Internet connection available"
else
    echo "No internet connection"
fi

# elif for multiple conditions
SCORE=85
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

#!/bin/bash

# for loop with list
for fruit in apple banana orange; do
    echo "Fruit: $fruit"
done

# for loop with files
for file in *.txt; do
    echo "Processing: $file"
    wc -l "$file"
done

# for loop with command output
for user in $(cut -d: -f1 /etc/passwd); do
    echo "User: $user"
done

# for loop with range
for i in {1..5}; do
    echo "Number: $i"
done

# C-style for loop
for ((i=1; i<=10; i++)); do
    echo "Iteration $i"
done

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

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

# until loop (opposite of while)
COUNT=1
until [ $COUNT -gt 5 ]; do
    echo "Count: $COUNT"
    COUNT=$((COUNT + 1))
done

# Loop with break
for i in {1..10}; do
    if [ $i -eq 5 ]; then
        break  # Exit loop when i=5
    fi
    echo $i
done

# Loop with continue
for i in {1..5}; do
    if [ $i -eq 3 ]; then
        continue  # Skip iteration when i=3
    fi
    echo $i
done

#!/bin/bash

# Simple function
greet() {
    echo "Hello, $1!"
}
greet "World"  # Output: Hello, World!

# Function with local variables
calculate_sum() {
    local num1=$1
    local num2=$2
    local sum=$((num1 + num2))
    echo $sum
}
result=$(calculate_sum 5 10)
echo "Sum: $result"  # Output: Sum: 15

# Function with return status
check_file() {
    local file=$1
    if [ -f "$file" ]; then
        return 0  # Success
    else
        return 1  # Failure
    fi
}

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

# Function with multiple outputs
get_system_info() {
    echo "Hostname: $(hostname)"
    echo "Uptime: $(uptime -p)"
    echo "Users: $(who | wc -l)"
}
get_system_info

# Function that processes all arguments
print_args() {
    echo "Number of arguments: $#"
    for arg in "$@"; do
        echo "  Argument: $arg"
    done
}
print_args apple banana orange

# Practical example: backup function
backup_directory() {
    local source=$1
    local destination=$2
    local date=$(date +%Y%m%d)

    if [ ! -d "$source" ]; then
        echo "Error: Source directory not found"
        return 1
    fi

    tar -czf "$destination/backup-$date.tar.gz" "$source"
    return 0
}

if backup_directory "/home/user/docs" "/backup"; then
    echo "Backup successful"
else
    echo "Backup failed"
fi

#!/bin/bash

# Basic argument usage
echo "Script name: $0"
echo "First argument: $1"
echo "Second argument: $2"
echo "Number of arguments: $#"
echo "All arguments: $@"

# Check argument count
if [ $# -lt 2 ]; then
    echo "Usage: $0 <source> <destination>"
    exit 1
fi

SOURCE=$1
DEST=$2
echo "Copying from $SOURCE to $DEST"

# Default values for optional arguments
NAME=${1:-"Guest"}  # Use $1 or default to "Guest"
echo "Hello, $NAME"

# Reading user input
read -p "Enter your name: " USERNAME
echo "Hello, $USERNAME"

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

# Input with timeout
if read -t 5 -p "Enter choice (5 sec timeout): " CHOICE; then
    echo "You chose: $CHOICE"
else
    echo "Timeout! Using default."
fi

# Yes/No confirmation
read -p "Continue? (y/n): " ANSWER
if [[ "$ANSWER" =~ ^[Yy]$ ]]; then
    echo "Continuing..."
else
    echo "Aborted."
    exit 1
fi

# Processing all arguments
echo "Processing files:"
for file in "$@"; do
    if [ -f "$file" ]; then
        echo "  Found: $file"
    else
        echo "  Not found: $file"
    fi
done

# Using shift to process arguments
while [ $# -gt 0 ]; do
    echo "Processing: $1"
    shift  # Move to next argument
done

#!/bin/bash

# Exit on error, undefined variables
set -e
set -u
set -o pipefail

# Function to handle errors
error_exit() {
    echo "Error: $1" >&2
    exit 1
}

# Check if file exists
FILE="data.txt"
[ -f "$FILE" ] || error_exit "File $FILE not found"

# Test command success
if ping -c 1 google.com &> /dev/null; then
    echo "Internet available"
else
    echo "No internet connection" >&2
    exit 1
fi

# Using $? to check exit code
grep "pattern" file.txt
if [ $? -eq 0 ]; then
    echo "Pattern found"
else
    echo "Pattern not found"
fi

# Short-circuit operators
mkdir /tmp/mydir || error_exit "Failed to create directory"
cd /tmp/mydir || error_exit "Failed to change directory"

# Command succeeds, do next action
rm temp.txt && echo "File removed successfully"

# Cleanup function with trap
cleanup() {
    echo "Cleaning up..."
    rm -f /tmp/temp_*
}
trap cleanup EXIT

# Error handler
error_handler() {
    local line=$1
    echo "Error on line $line" >&2
    cleanup
    exit 1
}
trap 'error_handler $LINENO' ERR

# Critical section
critical_operation() {
    # Temporarily disable exit-on-error
    set +e
    risky_command
    local status=$?
    set -e

    if [ $status -ne 0 ]; then
        echo "Warning: risky_command failed"
        # Continue anyway
    fi
}

# Exit with appropriate code
if [ "$SUCCESS" = true ]; then
    exit 0
else
    exit 1
fi