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:

• Electronic mail. This is not just the mail or SCO Shell email program used to read and send mail (the mail delivery agents or MDAs), but the ``back end'': the programs that spool outgoing mail, work out which host to send it to, dial up the host and transmit the mail to the host's own mail server, then route incoming mail to the correct user's mailbox. (A spooler is a set of service programs that control a queue of files, adding files to it at one end and removing them at the other end for despatch to the destination.) The back end is known as the mail transport agent (or MTA) and is one of the main UNIX subsystems.

• Security and auditing. The first UNIX systems were built on academic research computers; security was not a major concern at first. Later, as the system became a standard for open systems and commercial use, steps were taken to provide tools that allow systems to be secured against accidental damage or deliberate misuse. These subsystems are actually a component of the System Administration tools described below.

• Print services. On a multiuser system, which may be connected to several printers directly or over a network, it is necessary to maintain a queue of print jobs (documents waiting to be printed), which are assigned to printers as they become available. This service is controlled by a print spooler, a set of service programs that maintain the print queue and send files from it to the selected printers.
  

• Terminal services. The UNIX system can be used with almost any serial terminal, and a wide range of other types of terminal. However, before you can use a text editor like vi on a terminal, the UNIX system needs to take control over various aspects of the terminal; cursor positioning, highlighting, and the input and output of characters must all be handled directly. Two services are used by the UNIX system to communicate with character terminals (the ``termcap'' and ``terminfo'' databases), and one service (the X protocol) is used to communicate with bitmapped (graphics) terminals.
  

• Timed execution services. A number of facilities exist that allow the UNIX system to carry out tasks automatically, at a given time. The cron daemon keeps a list of jobs to be started at various times and dispatches them when they are due to run. These jobs include not only user programs (like long print jobs) but system tasks (such as flushing the buffer cache, calling another computer to exchange mail files, and backing up filesystems across a network).