free              # Display memory usage
free -h           # Human-readable memory usage
ps                # List current processes
ps aux            # Detailed process list
top               # Interactive process viewer
htop              # Enhanced process viewer

uptime            # System uptime and load
w                 # Who is logged in
uname -a          # System information
hostname          # Display hostname
lshw              # Hardware configuration
history           # Command history
tty               # Terminal device info

man [command]     # Manual pages
info [command]    # Software documentation
help [command]    # Built-in help
whatis [command]  # Brief description

pwd               # Print working directory
cd [dir]          # Change directory
cd ..             # Go to parent directory
cd ~              # Go to home directory
cd -              # Go to previous directory

ls                # List files
ls -l             # Long format
ls -a             # Show hidden files
ls -la            # Long format with hidden
ls -R             # Recursive listing
ls -h             # Human-readable sizes

touch [file]      # Create/update file
cp [src] [dest]   # Copy file
cp -r [src] [dest] # Copy directory
mv [src] [dest]   # Move/rename
rm [file]         # Remove file
rm -r [dir]       # Remove directory
rm -f [file]      # Force remove
rm -rf [dir]      # Force remove directory

mkdir [dir]       # Create directory
mkdir -p [path]   # Create nested directories
rmdir [dir]       # Remove empty directory

chmod 755 [file]  # Change permissions
chmod +x [file]   # Make executable
chown user [file] # Change owner
chgrp group [file] # Change group

file [file]       # Determine file type
stat [file]       # File statistics
lsof              # List open files

cat [file]        # Display entire file
head [file]       # Display first 10 lines
head -n 20 [file] # Display first 20 lines
tail [file]       # Display last 10 lines
tail -f [file]    # Follow file updates
less [file]       # Page through file
more [file]       # Page through file

grep [pattern] [file]     # Search for pattern
grep -i [pattern] [file]  # Case-insensitive search
grep -r [pattern] [dir]   # Recursive search
grep -n [pattern] [file]  # Show line numbers
egrep [pattern] [file]    # Extended regex

sort [file]       # Sort lines
sort -n [file]    # Numerical sort
uniq [file]       # Remove duplicates
cut -d: -f1 [file] # Extract fields
wc [file]         # Word/line count
wc -l [file]      # Line count only
sed 's/old/new/g' [file] # Find and replace
tr 'a-z' 'A-Z'   # Translate characters

nano [file]       # Simple editor
vi [file]         # Vi editor
vim [file]        # Vi improved

ps                # List processes
ps aux            # Detailed process list
top               # Interactive process monitor
htop              # Enhanced process monitor
kill [PID]        # Terminate process
kill -9 [PID]     # Force kill process
killall [name]    # Kill by process name

command &         # Run in background
jobs              # List background jobs
fg %1             # Bring job to foreground
bg %1             # Send job to background
nohup command &   # Run immune to hangups

sudo command      # Run as superuser
sudo -s           # Root shell
su [user]         # Switch user
sudo -u [user] command # Run as specific user
passwd            # Change password
whoami            # Current username
who               # Logged in users
id                # User and group info

useradd [user]    # Add user
adduser [user]    # Interactive user creation
usermod -aG [group] [user] # Add user to group
userdel [user]    # Delete user
groupadd [group]  # Create group

sudo apt update   # Update package list
sudo apt upgrade  # Upgrade packages
sudo apt install [pkg] # Install package
sudo apt remove [pkg]  # Remove package
sudo apt purge [pkg]   # Remove with config
sudo apt autoremove    # Remove unused
apt search [term] # Search packages
dpkg -l           # List installed packages

systemctl status [service]  # Service status
systemctl start [service]   # Start service
systemctl stop [service]    # Stop service
systemctl restart [service] # Restart service
systemctl enable [service]  # Enable at boot
systemctl disable [service] # Disable at boot

shutdown -h now   # Shutdown immediately
shutdown -r now   # Restart immediately
reboot            # Restart system
halt              # Halt system
sync              # Write buffers to disk

ip addr           # Show IP addresses
ip link           # Show network interfaces
ifconfig          # Interface configuration
ping [host]       # Test connectivity
ping -c 4 [host]  # Ping 4 times

ssh user@host     # SSH connection
scp [file] user@host:[path] # Secure copy
wget [url]        # Download file
curl [url]        # Transfer data
netstat -tulpn    # Network connections
ss -tulpn         # Socket statistics

iwconfig          # Wireless configuration
iwlist scan       # Scan for networks

sudo ufw enable   # Enable firewall
sudo ufw disable  # Disable firewall
sudo ufw status   # Firewall status
sudo ufw allow [port] # Allow port
sudo ufw deny [port]  # Deny port
sudo ufw allow from [ip] # Allow IP

tar -cvf archive.tar [files]  # Create tar
tar -czvf archive.tar.gz [files] # Create tar.gz
tar -cjvf archive.tar.bz2 [files] # Create tar.bz2
zip archive.zip [files]       # Create zip

tar -xvf archive.tar          # Extract tar
tar -xzvf archive.tar.gz      # Extract tar.gz
tar -xjvf archive.tar.bz2     # Extract tar.bz2
unzip archive.zip             # Extract zip

gzip [file]       # Compress with gzip
gunzip [file.gz]  # Decompress gzip
bzip2 [file]      # Compress with bzip2
bunzip2 [file.bz2] # Decompress bzip2

df -h             # Disk space usage
du -h             # Directory space usage
du -sh [dir]      # Total directory size
du -h --max-depth=1 # Size of subdirectories

mount             # Show mounted filesystems
mount [device] [dir] # Mount device
umount [dir]      # Unmount
fdisk -l          # List disk partitions
lsblk             # List block devices

echo $VAR         # Display variable
export VAR=value  # Set environment variable
env               # Show all variables
unset VAR         # Remove variable
alias ll='ls -la' # Create alias
unalias ll        # Remove alias

bash              # Start new bash shell
exit              # Exit shell
clear             # Clear screen
source [file]     # Execute script in current shell
. [file]          # Same as source

vmstat            # Virtual memory stats
iostat            # I/O statistics
dmesg             # Kernel messages
journalctl        # System logs

find [dir] -name [pattern]    # Find by name
find [dir] -type f -size +1M  # Find large files
locate [file]     # Find in database
updatedb          # Update locate database
which [command]   # Command location
whereis [command] # Binary, source, manual

awk '{print $1}' [file]       # Print first column
paste [file1] [file2]         # Merge files
split -l 100 [file]           # Split by lines
diff [file1] [file2]          # Compare files
comm [file1] [file2]          # Compare sorted files

crontab -e        # Edit cron jobs
crontab -l        # List cron jobs
at 10:00          # Schedule one-time job
atq               # List scheduled jobs

date              # Current date/time
cal               # Calendar
bc                # Calculator
time [command]    # Time command execution
watch [command]   # Run command repeatedly
tree              # Directory tree
neofetch          # System info with ASCII art

# Find and delete files older than 30 days
find /path -type f -mtime +30 -delete

# Monitor log file in real-time
tail -f /var/log/syslog | grep ERROR

# Find largest files
du -ah / 2>/dev/null | sort -rh | head -20

# Backup directory
tar -czvf backup-$(date +%Y%m%d).tar.gz /path/to/dir

# Search and replace in multiple files
find . -type f -name "*.txt" -exec sed -i 's/old/new/g' {} \;

# System resource usage
ps aux --sort=-%cpu | head -10  # Top CPU users
ps aux --sort=-%mem | head -10  # Top memory users

graph LR
  A[Basic Commands] --> B[File System]
  B --> C[Text Processing]
  C --> D[Shell Scripting]
  D --> E[System Admin]
  E --> F[Advanced Topics]