The cron utility runs continuously in the background, taking care of scheduled system tasks and user requests at the appropriate time.
You might not know it,
but your Mac does quite a bit on its own behind your back — or
under your fingertips, I should say. Your system regularly purges
itself of outdated, space-hogging log files, updates system databases
so utilities like
locate on the command line
for details) can work effectively, and performs several other
maintenance tasks that keep your system running lean and mean.
It does so by means of a task-scheduling utility called
cron (as in chronological). The
cron command launches automatically at system
startup and runs continuously in the background. It keeps a list of
what needs to happen when and consults this list each and every
minute of each and every day, at least while your machine is awake.
When it notices it’s time to perform some duty, it
does so quietly in the background.
The lists are
crontab files associating a particular action
with a timetable. Each user account can have its own
crontab file. The system itself has a special
crontab, found in the
directory; it belongs to the
superuser, or root, account and takes care of actions requiring the
kind of system access allowed only to root [Hack #50].
The format of a
file might appear rather esoteric at first, but it’s
really rather simple. For example, Figure 5-20 ...