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A process is a running instance of a command. For example, there may be one vi command on the system. But if vi is currently being run by 15 different users, that command is represented by 15 different running processes.

A process is identified on the system by what is referred to as a process ID (PID). That PID is unique for the current system. In other words, no other process can use that number as its process ID while that first process is still running. However, after a process has ended, another process can reuse that number.

Along with a process ID number, other attributes are associated with a process. Each process, when it is run, is associated with a particular user account and group account. That account information helps determine what system resources the process can access. For example, a process run as the root user has much more access to system files and resources than a process running as a regular user.

The ability to manage processes on your system is critical for a Linux system administrator. Sometimes, runaway processes may be killing your system's performance. Finding and dealing with processes, based on attributes such as memory and CPU usage, are covered in this chapter.