Clock server (UNIX)
cron [-d crondir] [-v] &
The cron server schedules commands to be run at specified times, without user intervention. This server supports user-specific cron entries, and runs continuously. The server must be run in the background.
The cron server assumes it has sole use of the /var/spool/cron directory. Therefore, you can run only one cron server per filesystem containing that directory. You typically run the cron server on the network server.
Commands are specified by instructions found in crontab files, which are accessed via the crontab utility.
To minimize overhead, cron examines the contents of the files in /var/spool/cron/crontabs when it first comes up, and then reexamines only those that have been changed via the crontab utility.
Errors cause diagnostic messages to be written to standard error. If -v is specified, log messages are written to the standard error.
The cron utility uses data read from the following:
The cron utility normally runs indefinitely. However, it terminates early if errors are encountered in startup, errors are encountered in reading the crontabs files, or if it's terminated by a signal.