# Print current directory
pwd

# List files and directories
ls -la

# Change directory
cd /var/www

# Go to home directory
cd ~

# Go up one level
cd ..

# Create a directory
mkdir /var/www/mysite
mkdir -p /var/www/mysite/public/assets   # create nested directories

# Create an empty file
touch filename.txt

# Copy files
cp source.txt destination.txt
cp -r /source/directory /destination/   # copy directory recursively

# Move or rename files
mv oldname.txt newname.txt
mv file.txt /new/location/

# Delete files
rm filename.txt
rm -r directory/        # delete directory and contents
rm -rf directory/       # force delete (use with caution)

# View permissions
ls -la

# Change ownership
chown user:group filename
chown -R www-data:www-data /var/www/   # recursive ownership

# Change permissions
chmod 755 directory/
chmod 644 file.txt
chmod +x script.sh      # make executable

# Start a service
sudo systemctl start nginx

# Stop a service
sudo systemctl stop nginx

# Restart a service
sudo systemctl restart nginx

# Reload configuration without restart
sudo systemctl reload nginx

# Check service status
sudo systemctl status nginx

# Enable service to start on boot
sudo systemctl enable nginx

# Disable auto-start
sudo systemctl disable nginx

# List all running services
systemctl list-units --type=service --state=running

# Update package lists
sudo apt update

# Upgrade installed packages
sudo apt upgrade -y

# Install a package
sudo apt install nginx -y

# Remove a package
sudo apt remove nginx

# Remove package and configuration
sudo apt purge nginx

# Search for packages
apt search keyword

# Clean up unused packages
sudo apt autoremove

# Update packages
sudo yum update -y

# Install a package
sudo yum install nginx -y

# Remove a package
sudo yum remove nginx

# Search for packages
yum search keyword

# Add a new user
sudo adduser username

# Delete a user
sudo deluser username
sudo deluser --remove-home username   # also remove home directory

# Add user to a group
sudo usermod -aG groupname username

# Grant sudo privileges
sudo usermod -aG sudo username

# Switch to another user
su - username

# List all users
cat /etc/passwd

# Change password
passwd            # current user
sudo passwd username   # another user

# Check disk space usage
df -h

# Check directory size
du -sh /var/www
du -sh /*           # size of each top-level directory

# Find large files (over 100MB)
find / -type f -size +100M -exec ls -lh {} ;

# Check inode usage
df -i

# Check IP address
ip addr show

# Test connectivity
ping google.com -c 4

# Check open ports
ss -tulnp

# Check active connections
ss -tan

# DNS lookup
dig example.com
nslookup example.com

# Download a file
wget https://example.com/file.tar.gz
curl -O https://example.com/file.tar.gz

# Check which process uses a port
sudo lsof -i :80

# List running processes
ps aux

# Find a specific process
ps aux | grep nginx

# Interactive process viewer
top
htop    # more user-friendly (install: sudo apt install htop)

# Kill a process by PID
kill PID
kill -9 PID    # force kill

# Kill by name
killall processname
pkill processname

# View file contents
cat filename.txt

# View with pagination
less filename.txt

# View first/last lines
head -n 20 filename.txt
tail -n 20 filename.txt

# Follow a log file in real time
tail -f /var/log/syslog

# Search within files
grep "search term" filename.txt
grep -r "search term" /directory/   # recursive search

# Edit files
nano filename.txt    # beginner-friendly
vim filename.txt     # powerful but steeper learning curve

# OS information
lsb_release -a
cat /etc/os-release

# Kernel version
uname -r

# System uptime
uptime

# Memory usage
free -h

# CPU info
lscpu
nproc     # number of CPUs

# System hostname
hostname
hostnamectl

# Edit crontab
crontab -e

# List cron jobs
crontab -l

# Cron format: minute hour day month weekday command
# Run daily at 2:30 AM
30 2 * * * /path/to/script.sh

# Run every 5 minutes
*/5 * * * * /path/to/script.sh

# Run every Monday at 9 AM
0 9 * * 1 /path/to/script.sh