NAME
cron - daemon to execute scheduled commands (Vixie Cron)
SYNOPSIS
cron [-f] [-l] [-L 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 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 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 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.
In contrast to the spool area, files in /etc/cron.d may be symlinks,
provided that both the symlink and the file it points to are owned by
root.
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 /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 logs its action to the syslog facility 'cron', and logging may be
controlled using the standard syslogd(8) facility.
ENVIRONMENT
If configured in /etc/default/cron in Debian systems, the cron daemon
localisation settings environment can be managed through the use of
/etc/environment or through the use of /etc/default/locale with values
from the latter overriding values from the former. These files are read
and they will be used to setup the LANG, LC_ALL, and LC_CTYPE
environment variables. These variables are then used to set the charset
of mails, which defaults to 'C'.
This does NOT affect the environment of tasks running under cron. For
more information on how to modify the environment of tasks, consult
crontab(5).
The daemon will use, if present, the definition from /etc/timezone for
the timezone.
The environment can be redefined in user's crontab definitions but cron
will only handle tasks in a single timezone.
SEE ALSO
crontab(1), crontab(5)
AUTHOR
Paul Vixie <paul@vix.com>