1. top – Display Linux Tasks
This is a very used tool by any Linux administrator because it provides real-time information about the running tasks, system and about the tasks which are managed by the kernel.
More examples about how to use the “top” utility can be found here: http://www.cyberciti.biz/faq/tag/top-command/
2. vmstat – Report Virtual Memory Statistics
The command “vmstat” provides very useful statistics that relates to: CPU activity, virtual memory, kernel threads, IO blocks, kernel threads, processes, etc. For more examples about how to use vmstat, you can read: http://www.cyberciti.biz/faq/tag/vmstat-command/
3. lsof – List Open Files
The full name of this command is self-explanatory. Basically, the “lsof” command helps a sysadmin to find the open files and the processes that keeps them opened. You can read more about it here: http://www.catonmat.net/blog/unix-utilities-lsof/
4. tcpdump – Dump Traffic On a Network
This tool is a network packet analyzer and a sniffer that can be used to capture the packets transferred through a specific network interface. A detailed tutorial can be found here: http://www.danielmiessler.com/study/tcpdump/
5. netstat – Network Statistics
When you want to use “netstat” it means that you’re looking for information about the inbound/outbound in terms of interface statistics. Many of us already know it from Windows. More useful practical examples using netstat can be found here: http://www.cyberciti.biz/tips/tag/netstat
6. iostat – IO Statistics
The name of this tool is also self-explanatory. It is designed to provide statistics about the input/output of different storage devices and it also provide info about the CPU status. Some useful examples on how to use this tool can be found here: http://linux.101hacks.com/unix/iostat/
In order to make this tool available, you have to install the sysstat package from the repository.