4.1.3. Process attributes
A process has a series of characteristics:
The process ID or PID: a unique identification number used to refer to the process.
The parent process ID or PPID: the number of the process (PID) that started this process.
Nice number: the degree of friendliness of this process toward other processes (not to be confused with process priority, which is calculated based on this nice number and recent CPU usage of the process).
Terminal or TTY: terminal to which the process is connected.
User name of the real and effective user (RUID and EUID): the owner of the process. The real owner is the user issuing the command, the effective user is the one determining access to system resources. RUID and EUID are usually the same, and the process has the same access rights the issuing user would have. An example to clarify this: the browser mozilla in /usr/bin is owned by user root :
theo:~> ls -l /usr/bin/mozilla -rwxr-xr-x 1 root root 4996 Nov 20 18:28 /usr/bin/mozilla* theo:~> mozilla &  26595 theo:~> ps -af UID PID PPID C STIME TTY TIME CMD theo 26601 26599 0 15:04 pts/5 00:00:00 /usr/lib/mozilla/mozilla-bin theo 26613 26569 0 15:04 pts/5 00:00:00 ps -af
When user theo starts this program, the process itself and all processes started by the initial process, will be owned by user theo and not by the system administrator. When mozilla needs access to certain files, that access will be determined by theo 's permissions and not by root 's.
Real and effective group owner (RGID and EGID): The real group owner of a process is the primary group of the user who started the process. The effective group owner is usually the same, except when SGID access mode has been applied to a file.