
Since Linux is a true multi-user system, the principles of both users and groups are important to the internal operation of the system, and also to the day-to-day use of the system. Linux handling of users and groups is the reason it is one of the most secure operating systems.
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Physical Users are people who use the system physically. They have user IDs which are usually unique, as well as group IDs.
Logical Users are not tied to a person, but instead are tied to a system function that are used by applications to perform specific tasks. Daemons, mail, news are examples of logical users.