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An overview of the system

System services

User level programs (applications and tools) make use of a repertoire of facilities: the system services. These are sets of programs that provide a service essential to the UNIX system, but which are not intended to interact directly with users. For example, the mmdf mail system is a system service; it comprises a set of programs designed to deliver e-mail to users automatically. When you use the SCO Shell e-mail application to create a mail message and send it, mmdf takes over and determines where the mail is going, then places it in the appropriate spool directory.

Most services are mediated by a daemon. A daemon is a process that runs without human intervention in response to some event. For example, cron responds to the system clock and runs processes that are scheduled for a given time.

System services are not part of the kernel, and can be removed or replaced, though doing so may damage or alter the usability of the system. The common UNIX system services include:


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