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7.5.2. Monitoring Activity in Real Time
top is an interactive tool that displays a list of currently running processes. The default sorting is based on the current amount of processor use and can be obtained with the P key. Other sort orders include a sort by occupied memory (M key), by total processor time (T key), and by process identifier (N key). The k key kills a process by entering its process identifier. The r key changes the priority of a process.
When the system seems to be overloaded, top is a great tool to see which processes are compet- ing for processor time or consuming too much memory. In particular, it is often interesting to check if the processes consuming resources match the real services that the machine is known to host. An unknown process running as the ”www-data” user should really stand out and be inves- tigated since it’s probably an instance of software installed and executed on the system through a vulnerability in a web application.
top is a very flexible tool and its manual page gives details on how to customize its display and adapt it to your personal needs and habits.
The gnome-system-monitor graphical tool is similar to top and it provides roughly the same fea- tures.