# List directory contents (detailed, including hidden files)
ls -la
 
# Change directory
cd /path/to/directory
cd ..  # Move up one directory
cd ~   # Move to home directory
 
# Create a new directory
mkdir new_folder
mkdir -p path/to/nested/folder  # Create parent directories if needed
 
# Remove files and directories
rm filename.txt
rm -r foldername      # Remove directory recursively
rm -rf foldername     # Force remove recursively (use with caution!)
 
# Copy and move
cp source.txt dest.txt
cp -r source_folder dest_folder
mv oldname.txt newname.txt
 
# Read files
cat filename.txt      # Print entire file
less filename.txt     # Scroll through file
tail -n 50 file.log   # Read last 50 lines
 
# Search for text within files
grep "search term" filename.txt
grep -r "search term" /path/to/search  # Recursive search

# Update package lists and upgrade existing packages
sudo apt update && sudo apt upgrade -y
 
# Install a new package
sudo apt install nginx
 
# Add a new user
sudo adduser newusername
 
# Grant user sudo privileges
sudo usermod -aG sudo newusername
 
# Check firewall status (UFW)
sudo ufw status
 
# Allow SSH through the firewall and enable it
sudo ufw allow OpenSSH
sudo ufw enable

# Check permissions
ls -l
 
# Change permissions (chmod)
chmod 755 script.sh   # Owner: rwx, Group: r-x, Others: r-x
chmod 644 file.txt    # Owner: rw-, Group: r--, Others: r--
chmod +x script.sh    # Make a file executable for everyone
 
# Change ownership (chown)
sudo chown user:group filename.txt
sudo chown -R user:group directory/  # Change recursively

# Connect to a remote server
ssh user@hostname_or_ip
 
# Connect using a specific port
ssh -p 2222 user@hostname_or_ip
 
# Connect using a specific private key
ssh -i ~/.ssh/custom_key.pem user@hostname_or_ip
 
# Generate a new SSH key pair
ssh-keygen -t ed25519 -C "your_email@example.com"
 
# Copy your public key to a remote server for passwordless login
ssh-copy-id user@hostname_or_ip
 
# Securely copy files from local to remote (SCP)
scp local_file.txt user@hostname_or_ip:/path/to/remote/

# View running processes (interactive)
top
htop  # (Requires installation, much better UI than top)
 
# List running processes
ps aux
 
# Find the Process ID (PID) of a specific program
pgrep nginx
 
# Kill a process by its PID
kill 1234
 
# Force kill a stubborn process
kill -9 1234
 
# Kill all processes with a specific name
killall nginx

# Test connectivity to a host
ping google.com
 
# Show network interfaces and IP addresses
ip a
ifconfig  # (Older command, still widely used)
 
# Check active listening ports
netstat -tulpn
ss -tulpn
 
# Make an HTTP request / Download a file
curl -I https://example.com   # Fetch headers only
curl -O https://example.com/file.zip  # Download file
 
# Query DNS records
dig example.com
nslookup example.com