MAN 8 cron

NAME

cron - daemon to execute scheduled commands (ISC Cron V4.1)

SYNOPSIS

 cron [-l load_avg] [-n]

DESCRIPTION

Cron should be started from /etc/rc or /etc/rc.local. It will return immediately, so you don't need to start it with '&'.

The -n option changes this default behavior causing it to run in the foreground. This can be useful when starting it out of init.

Cron searches /var/spool/cron/crontabs for crontab files which are named after accounts in /etc/passwd; crontabs found are loaded into memory. Cron also searches for /etc/crontab and the files in the /etc/cron.d/ directory, which are in a different format (see crontab(5)). Cron then wakes up every minute, examining all stored crontabs, checking each command to see if it should be run in the current minute. When executing commands, any output is mailed to the owner of the crontab (or to the user named in the MAILTO environment variable in the crontab, if such exists).

Additionally, cron checks each minute to see if its spool directory's modtime (or the modtime on /etc/crontab) has changed, and if it has, cron will then examine the modtime on all crontabs and reload those which have changed. Thus cron need not be restarted whenever a crontab file is modified. Note that the Crontab(1) command updates the modtime of the spool directory whenever it changes a crontab.

Daylight Saving Time and other time changes

Local time changes of less than three hours, such as those caused by the start or end of Daylight Saving Time, are handled specially. This only applies to jobs that run at a specific time and jobs that are run with a granularity greater than one hour. Jobs that run more frequently are scheduled normally.

If time has moved forward, those jobs that would have run in the interval that has been skipped will be run immediately.

Conversely, if time has moved backward, care is taken to avoid running jobs twice.

Time changes of more than 3 hours are considered to be corrections to the clock or timezone, and the new time is used immediately.

SIGNALS

On receipt of a SIGHUP, the cron daemon will close and reopen its log file. This is useful in scripts which rotate and age log files. Naturally this is not relevant if cron was built to use syslog(3).

CAVEATS

In this version of cron, /etc/crontab must not be readable or writable by any user other than root. In other words, it should be mode 0600.

SEE ALSO

crontab(1), crontab(5)

AUTHOR

Paul Vixie <vixie@isc.org>