cron - daemon to execute scheduled commands (Vixie Cron)
cron
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 different 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 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 has its 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 /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.
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 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 purpose 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.
crontab(1), crontab(5)
Paul Vixie <[email protected]>
4th Berkeley Distribution 20 December 1993 CRON(8)
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