At any time, each process has an effective user ID, a effective
group ID, and a set of supplementary group IDs. These IDs
determine the privileges of the process. They are collectively
called the persona of the process, because they determine “who it
is” for purposes of access control.
Your login shell starts out with a persona which consists of your user
ID, your default group ID, and your supplementary group IDs (if you are
in more than one group). In normal circumstances, all your other processes
inherit these values.
A process also has a real user ID which identifies the user who
created the process, and a real group ID which identifies that
user's default group. These values do not play a role in access
control, so we do not consider them part of the persona. But they are
Both the real and effective user ID can be changed during the lifetime
of a process. See Why Change Persona.
For details on how a process's effective user ID and group IDs affect
its permission to access files, see Access Permission.
The effective user ID of a process also controls permissions for sending
signals using the kill function. See Signaling Another Process.
Finally, there are many operations which can only be performed by a
process whose effective user ID is zero. A process with this user ID is
a privileged process. Commonly the user name root is
associated with user ID 0, but there may be other user names with this
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