# Display and manage the top processes
top

# Interactive process viewer (top alternative)
htop

# Display processor related statistics
mpstat 1

# Display virtual memory statistics
vmstat 1

# Display I/O statistics
iostat 1

# Display the last 100 syslog messages  (Use /var/log/syslog for Debian based systems.)
tail 100 /var/log/messages

# Capture and display all packets on interface eth0
tcpdump -i eth0

# Monitor all traffic on port 80 ( HTTP )
tcpdump -i eth0 'port 80'

# List all open files on the system
lsof

# List files opened by user
lsof -u user

# Display free and used memory ( -h for human readable, -m for MB, -g for GB.)
free -h

# Execute "df -h", showing periodic updates
watch df -h