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Info Node: (libc.info)Process Persona

(libc.info)Process Persona


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The Persona of a Process
========================

   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 also
important.

   Both the real and effective user ID can be changed during the
lifetime of a process.  Note: Why Change Persona.

   For details on how a process's effective user ID and group IDs affect
its permission to access files, see Note: Access Permission.

   The effective user ID of a process also controls permissions for
sending signals using the `kill' function.  Note: 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 ID.


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