NAME
cron - daemon to execute scheduled commands (Vixie Cron)
SYNOPSIS
cron
[-f]
[-l]
[-L
R loglevel ]
DESCRIPTION
cron
is started automatically from /etc/init.d on entering multi-user
runlevels.
OPTIONS
-f
Stay in foreground mode, don't daemonize.
-l
Enable LSB compliant names for /etc/cron.d files
-L loglevel
Sets the loglevel for cron. The standard logging level (1) will log
the start of all the cron jobs. A higher loglevel (2) will cause
cron to log also the end of all cronjobs, which can be useful to
audit the behaviour of tasks run by cron. Logging will be disabled
if the loglevel is set to zero (0).
NOTES
cron
searches its spool area (/var/spool/cron/crontabs) for crontab
files (which are named after accounts in
/etc/passwd); crontabs found are loaded into memory. Note that
crontabs in this directory should not be accessed directly -
the
crontab
command should be used to access and update them.
cron
also reads /etc/crontab, which is in a slightly different format (see
R crontab(5) ).
Additionally,
cron
reads the files in /etc/cron.d: it
treats the files in /etc/cron.d as in the same way as the /etc/crontab file (they
follow the special format of that file, i.e. they include the
user
field). However, they are independent of /etc/crontab: they do not, for
example, inherit environment variable settings from it. The intended purpose
of this feature is to allow packages that require
finer control of their scheduling than the /etc/cron.{daily,weekly,monthly}
directories to add a crontab file to /etc/cron.d. Such files
should be named after the package that supplies them. Files must
conform to the same naming convention as used by
R run-parts(8) :
they
must consist solely of upper- and lower-case letters, digits, underscores,
and hyphens. If the
-l
option is specified, then they must conform to the LSB namespace specification,
exactly as in the
--lsbsysinit
option in
R run-parts .
Like /etc/crontab, the files in the /etc/cron.d directory are
monitored for changes. In general, the admin should not use /etc/cron.d/,
but use the standard system crontab /etc/crontab.
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). The children copies of cron running these
processes have their name coerced to uppercase, as will be seen in the
syslog and ps output.
Additionally,
cron
checks each minute to see if its spool directory's modtime (or the modtime
on
R /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
R crontab (1)
command updates the modtime of the spool directory whenever it changes a
crontab.
Special considerations exist when the clock is changed by less than 3
hours, for example at the beginning and end of daylight savings
time. If the time has moved forwards, those jobs which would have
run in the time that was skipped will be run soon after the change.
Conversely, if the time has moved backwards by less than 3 hours,
those jobs that fall into the repeated time will not be re-run.
Only jobs that run at a particular time (not specified as
@hourly, nor with '*' in the hour or minute specifier) are
affected. Jobs which are specified with wildcards are run based on the
new time immediately.
Clock changes of more than 3 hours are considered to be corrections to
the clock, and the new time is used immediately.
cron
logs its action to the syslog facility 'cron', and logging may be
controlled using the standard syslogd(8) facility.
SEE ALSO
crontab(1), crontab(5)
AUTHOR
Paul Vixie <paul@vix.com>