Linux

pwd               # Print working directory
ls -la            # List all files with details
ls -lh            # Human-readable sizes
cd /path/to/dir   # Change directory
cd ~              # Go home
cd -              # Go back to previous dir
tree              # Visual directory tree

touch file.txt            # Create empty file
mkdir -p a/b/c            # Create nested dirs
cp file.txt backup.txt    # Copy file
cp -r dir/ backup/        # Copy directory
mv old.txt new.txt        # Move/rename
rm file.txt               # Delete file
rm -rf dir/               # Delete directory (careful!)
ln -s /path/to/file link  # Symbolic link

find / -name '*.log'          # Find by name
find . -type f -mtime -7      # Files modified in last 7 days
find . -size +100M            # Files larger than 100MB
locate filename               # Fast search using index
which python3                 # Find command location
whereis bash                  # Find binary, source, man page

# chmod: change mode
chmod 755 script.sh      # rwxr-xr-x
chmod +x script.sh       # Add execute
chmod -w file.txt        # Remove write
chmod -R 644 dir/        # Recursive

# chown: change owner
chown user:group file
chown -R www-data /var/www

# Permission bits: 4=read, 2=write, 1=execute
# 7=rwx  6=rw-  5=r-x  4=r--

ps aux                    # All running processes
top                       # Live process monitor
htop                      # Interactive monitor
kill PID                  # Send SIGTERM
kill -9 PID               # Force kill (SIGKILL)
pkill nginx               # Kill by name
jobs                      # List background jobs
bg %1                     # Resume job in background
fg %1                     # Bring job to foreground
nohup cmd &               # Run immune to hangup

# grep: search
grep 'error' app.log
grep -r 'TODO' ./src          # Recursive
grep -i 'warning' file        # Case-insensitive
grep -n 'pattern' file        # Show line numbers

# sed: stream editor
sed 's/old/new/g' file.txt    # Replace all
sed -i 's/foo/bar/g' file     # In-place edit

# awk: field processing
awk '{print $1, $3}' file     # Print columns 1 and 3
awk -F: '{print $1}' /etc/passwd  # Custom delimiter

cat file.txt              # Print entire file
less file.txt             # Paginate (q to quit)
head -20 file.txt         # First 20 lines
tail -20 file.txt         # Last 20 lines
tail -f app.log           # Follow log in real-time
wc -l file.txt            # Count lines
sort file.txt             # Sort lines
uniq -c sorted.txt        # Count unique lines

ip addr                   # Show IP addresses
ifconfig                  # Legacy: network interfaces
ping google.com           # Test connectivity
curl -I https://site.com  # HTTP headers
wget https://file.com/f   # Download file
ss -tulnp                 # Show open ports
netstat -tulnp            # Legacy: open ports
ssh user@host             # Connect via SSH
scp file user@host:/path  # Copy file over SSH
rsync -av src/ user@h:/d  # Sync directories

df -h                     # Disk space by filesystem
du -sh /var/log           # Size of a directory
du -sh * | sort -rh       # Sort dirs by size
free -h                   # Memory usage
uname -a                  # Kernel & system info
uptime                    # System uptime & load
lscpu                     # CPU info
lsblk                     # Block devices