NAME
cron
—
daemon to execute scheduled commands
(ISC Cron V4.1)
SYNOPSIS
cron |
[-n ] [-x
debugflags] |
DESCRIPTION
cron
is normally started during system
boot by rc.d(8) framework, if cron is switched on in
rc.conf(5).
It will return immediately so you don't have to start it with ‘&’.
cron
searches
/var/cron/tabs for crontab files which are named
after accounts in /etc/passwd. Crontabs found are
loaded into memory. cron
also searches for
/etc/crontab which is in a different format (see
crontab(5)). Finally cron
looks for crontabs
in /etc/cron.d if it exists, and executes each file
as a crontab.
When cron
looks in a directory for
crontabs (either in /var/cron/tabs or
/etc/cron.d) it will not process files that:
- Start with a ‘.’ or a ‘#’.
- End with a ‘~’ or with “.rpmsave”, “.rpmorig”, or “.rpmnew”.
- Are of zero length.
- Their length is greater than
MAXNAMLEN
.
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).
Events such as START
and
FINISH
are recorded in the
/var/log/cron log file with date and time details.
This information is useful for a number of reasons, such as determining the
amount of time required to run a particular job. By default, root has an
hourly job that rotates these log files with compression to preserve disk
space.
Additionally, cron
checks each minute to
see if its spool directory's modtime (or the modtime on
/etc/crontab or /etc/cron.d)
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.
The following options are available:
-x
- This flag turns on some debugging flags. debugflags
is comma-separated list of debugging flags to turn on. If a flag is turned
on,
cron
writes some additional debugging information to system log during its work. Available debugging flags are:- sch
- scheduling
- proc
- process control
- pars
- parsing
- load
- database loading
- misc
- miscellaneous
- test
- test mode - do not actually execute any commands
- bit
- show how various bits are set (long)
- ext
- print extended debugging information
-n
- Stay in the foreground and don't daemonize
cron
.
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).
FILES
- /var/cron/tabs
cron
spool directory- /etc/crontab
- system crontab file
- /etc/cron.d/
- system crontab directory
- /var/log/cron
- log file for cron events
SEE ALSO
AUTHORS
Paul Vixie ⟨vixie@isc.org⟩