Every process has a unique identifier called a
Process Identifier (PID). Also, every process has a Parent Process Identifier (PPID). There is an exception, init
(or systemd
). The init
process starts all other processes and has a PID of 1. This process is special because it cannot be killed (and any attempt to do so is often fatal).
Similar to files, a process also has access permissions. These are referred to as the Real user and Group IDs. This provides a level of protection by not allowing unprivileged users to access critical operating system tasks. Memory, open files, and other resources are owned by the process and are kept separate from other processes (in most cases).
From the user’s perspective, a process is typically started from the command line or desktop. Take editing a file with vi
, for example. The user starts the session and works interactively with the editor. He can save the file or edit another one. As long as the session is active, there is a PID...