In Processes, we mentioned that it was possible to monitor processes; that is what we will cover next. To understand the operations we are going to perform here, it is helpful to know a bit more about them.
As with files, all processes that run on a GNU/Linux system are organized in the form of a tree, and each process has a number (its PID, Process ID), together with the number of its parent process (PPID, Parent Process ID).
This means that there is a process at the top of the tree structure, the equivalent of the root for filesystems: init (see Reference manual), which is always numbered 1. The next section will explain two commands, ps and pstree, which allow you to obtain information on a running process.
Every process in Unix can react to signals sent to it. There exist 31 different signals. For each of these signals, the process can define its own behavior, except for two signals: signal number 9 (KILL), and signal number 19 (STOP).
Signal 9 kills a process irrevocably, without giving it the time to terminate properly. This is the signal you send to a process which is stuck or exhibits other problems. A full list of signals is available using the command kill -l.