Section 8: cron
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CRON(8) CRON(8)
NAME
cron - daemon to execute scheduled commands (Vixie Cron)
SYNOPSIS
cron
DESCRIPTION
cron is started automatically from /etc/init.d on entering
multi-user runlevels.
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 dif-
ferent format (see crontab(5)). Additionally, cron reads
the files in /etc/cron.d; see the DEBIAN SPECIFIC section
below for more details.
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 out-
put 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 mod-
ified. Note that the crontab(1) command updates the mod-
time 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 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 immedi-
ately.
20 December 1993 1
CRON(8) CRON(8)
DEBIAN SPECIFIC
cron treats the files in /etc/cron.d as extensions to the
/etc/crontab file (they follow the special format of that
file, i.e. they include the user field). The intended pur-
pose of this feature is to allow packages that require
finer control of their scheduling than the
/etc/cron.{daily,weekly,monthly} directories allow 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 run-parts(8):
they must consist solely of upper- and lower-case letters,
digits, underscores, and hyphens. Like /etc/crontab, the
files in the /etc/cron.d directory are monitored for
changes.
SEE ALSO
crontab(1), crontab(5)
AUTHOR
Paul Vixie
20 December 1993 2