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Describe detour used in Unix user ids and process ids.

User IDs (UIDs):

  • In Unix, each user is given a unique number called a User ID (UID).
  • This UID is crucial because it determines who can access which files.

Types of UIDs in Processes:

  • Real UID: Identifies the owner of the process, defining file access.
  • Effective UID: Usually the same as Real UID but can change, allowing non-privileged users special access.
  • Saved UID: Used when a powerful process needs to temporarily do less important tasks.

Subjects and Principals:

  • A program running on behalf of someone is a “subject.”
  • The one on whose behalf the subject runs is a “principal.”
  • Principals can have one or more subjects working for them.


  • Anything a subject interacts with is an “object.”
  • Examples are files, directories, or memory segments.

User Accounts and UID 0:

  • Each user has a UID, and 0 is for the super user (System admin).
  • Users can belong to various groups, and processes are linked to UID/GID pairs.

Mapping Users to Principals:

  • Users map to principals in a one-to-many way.
  • While users may have multiple principals, each principal connects uniquely to a user, ensuring responsibility for their actions.

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