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Clock server (UNIX)
cron [-d crondir] [-v] &
- -d crondir
- Specifies that cron use the named directory instead of /var/spool/cron.
- Turn on verbose mode. Log and diagnostic messages are written to standard error as cron operates.
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:
- Each cron command assumes it has exclusive use of this directory.
- If present, this file lists the only users authorized to have their crontab run. By default, all users are authorized. The cron.deny list (below) overrides the setting of the cron.allow list.
- If present, this file lists users who aren't authorized to have their crontab run. This list overrides the list of users authorized (the cron.allow file).
- The periodic commands to be run are read out of files found in this directory.
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.
- cron was successfully and cleanly terminated by a SIGTERM or SIGPWR signal.
- An error occurred. A diagnostic message will have been written to standard error.