System users are those used to run services such as DNS, mail, web servers, and so forth. The reason for this is security; if all services ran as the superuser, they could act without restriction.
Examples of system users are daemon,
operator, bind (for
the Domain Name Service), news, and
www.
nobody is the generic unprivileged
system user. However, it is important to keep in mind that the
more services that use nobody, the more
files and processes that user will become associated with, and
hence the more privileged that user becomes.
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