Performance monitoring

top - Interactive process monitoring tool

# top

lsof - List open files and sockets/pipes as well

# lsof

htop - Advanced interactive process monitoring tool

# htop

iotop - Display real time Disk I/O and process

# iotop

tcpdump - Network packet capture (Sniffer)

# tcpdump -n -i eth0 net 10.1.1.0/24 and host 10.2.2.2 and port 80
# tcpdump -n -i eth0 'host 10.1.1.1 and  (port 80 or 443)'
# tcpdump -w - | tee capture.pcap | tcpdump -r -   (write to file and standard output the same time)

iftop - Real time network bandwidth monitoring tool

# iftop

Also check nload, trafshow, iptraf, pftop

vmstat - Display statistics of virtual memory, kernel, disks, process, I/O blocks and CPU activity

# vmstat
# vmstat -s  (other visualization)
# vmstat -S m (show MB instead of KB)

iostat - Display statistics of CPU, disk I/O and NFS

# iostat -m -n -N -x 1

Additional tools: iperf for traffic generation, and apache mod_status

And Dstat / Collectl - Powerful and feature rich command line utilities that monitors CPU, memory, disk and network