2. vmstat - Report virtual memory statistics
3. w - Show who is logged on and what they are doing.
4. uptime - Tell how long the system has been running.
5. ps - display information about the current processes.
6. free - Display amount of free and used memory in the system
7. iostat - Report CPU statistics and input/output statistics for devices and partitions.
8. sar - Collect, report, or save system activity information.
9. mpstat - Report multiprocessors related statistics.
10. pmap - report memory map of a process
11. netstat - Print network connections, routing tables, interface statistics, masquerade connections, and multicast memberships
12. ss - Dump socket statistics
13. iptraf - Interactive IP traffic Monitor
14. tcpdump - Detailed Network Traffic Analysis
15. strace - Trace system calls and signals
16. /Proc file system - Process information pseudo-filesystem.
17. Nagios - Application, Services, Servers And Network Monitoring with Email and SMS alert
18. Cacti - Web-based Monitoring Tool
19. KDE System Guard - Real-time Systems Reporting and Graphing
20. Gnome System Monitor - Real-time Systems Reporting and Graphing
21. nmap - network exploration tool and security auditing.
22. lsof - list open files
23. ntop - web based network traffic probe that shows the network usage, similar to what the popular top Unix command does.
24. Conky - light-weight system monitor for X, that displays any information (uname, uptime, CPU usage, mem usage, disk usage, "top" like process stats, and network monitoring,) on your desktop.
25. GKrellM - Single process stack of system monitors that can used to monitor CPUs, Process, Disk, Network Interface, Memory, File system, Temperature, fan, voltage sensor monitors, and many other things.
26. vnstat - a console-based network traffic monitor. It keeps a log of hourly, daily and monthly network traffic for the selected interface(s).
27. htop - htop is an enhanced version of top, the interactive process viewer, which can display the list of processes in a tree form.
28. mtr - a network diagnostic tool that combines the functionality of the traceroute and ping programs.
29. users - print the user names of users currently logged in to the current host
