Bash Scripting Basics: A Beginner's Guide

#!/bin/bash

echo "Hello, World!"

chmod +x hello.sh
./hello.sh

#!/bin/bash

# Assigning variables (no spaces around =)
name="Alice"
age=30

# Using variables (use quotes to handle spaces)
echo "$name is $age years old"

# Command substitution
files=$(ls)
echo "Files in current directory: $files"

# User input
read -p "Enter your name: " username
echo "Hello, $username"

#!/bin/bash

# If-else
if [ "$1" = "hello" ]; then
    echo "Hello to you too!"
elif [ "$1" = "bye" ]; then
    echo "Goodbye!"
else
    echo "I don't understand"
fi

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

if [ -d "/home/user" ]; then
    echo "Directory exists"
fi

# Numeric comparisons
count=5
if [ "$count" -gt 3 ]; then
    echo "Count is greater than 3"
fi

#!/bin/bash

# For loop over a list
for name in Alice Bob Charlie; do
    echo "Hello, $name"
done

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

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

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

#!/bin/bash

# Define a function
greet() {
    local name="$1"  # $1 is the first argument
    echo "Hello, $name!"
}

# Function with return value
add() {
    local sum=$(( $1 + $2 ))
    echo "$sum"  # Use echo to "return" a value
}

# Call functions
greet "Alice"
result=$(add 5 3)
echo "5 + 3 = $result"

#!/bin/bash

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

#!/bin/bash

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

# Read CSV
while IFS=',' read -r name age city; do
    echo "$name is $age years old and lives in $city"
done < "data.csv"

#!/bin/bash

# Exit on error
set -e

# Exit on undefined variable
set -u

# Print commands before executing (debug mode)
set -x

# Check exit status
if ! mkdir "/backup"; then
    echo "Error: Could not create backup directory"
    exit 1
fi

#!/bin/bash

BACKUP_DIR="/backup/$(date +%Y-%m-%d)"
mkdir -p "$BACKUP_DIR"

tar -czf "$BACKUP_DIR/home.tar.gz" /home/user
tar -czf "$BACKUP_DIR/etc.tar.gz" /etc

echo "Backup completed: $BACKUP_DIR"

#!/bin/bash

LOG_FILE="/var/log/app.log"
MAX_SIZE=10485760  # 10MB in bytes

file_size=$(stat -f%z "$LOG_FILE" 2>/dev/null || stat --printf="%s" "$LOG_FILE" 2>/dev/null)

if [ "$file_size" -gt "$MAX_SIZE" ]; then
    mv "$LOG_FILE" "$LOG_FILE.$(date +%Y%m%d-%H%M%S)"
    touch "$LOG_FILE"
    echo "Log rotated"
fi