Daemon to execute scheduled commands (Vixie Cron).
Syntax
cron [-s] [-o] [-x debugflag[,...]]
Options
-o Disable the special handling of situations when the GMT offset of
the local timezone changes, to be compatible with the old (default) behavior.
-s Enable special handling of situations when the GMT offset of the
local timezone changes, such as the switches between the standard
time and daylight saving time.
The jobs run during the GMT offset changes time as intuitively
expected. If a job falls into a time interval that disappears
(for example, during the switch from standard time) to daylight
saving time or is duplicated (for example, during the reverse
switch), then it’s handled in one of two ways:
The first case is for the jobs that run every at hour of a time
interval overlapping with the disappearing or duplicated interval.
In other words, if the job had run within one hour before the GMT
offset change (and cron was not restarted nor the crontab changed
after that) or would run after the change at the next hour.
They work as always, skip the skipped time or run in the added time
as usual.
The second case is for the jobs that run less frequently. They are
executed exactly once, they are not skipped nor executed twice
(unless cron is restarted or the user’s crontab is changed during
such a time interval). If an interval disappears due to the GMT offset
change, such jobs are executed at the same absolute point of time
as they would be in the old time zone.
For example, if exactly one hour disappears, this point would be
during the next hour at the first minute that is specified for
them in crontab.
-x debugflag[,...]
Enable writing of debugging information to standard output.
One or more of the following comma-separated debugflag identifiers
must be specified:
bit currently not used
ext make the other debug flags more verbose
load be verbose when loading crontab files
misc be verbose about miscellaneous one-off events
pars be verbose about parsing individual crontab lines
proc be verbose about the state of the process, including all of
its offspring
sch be verbose when iterating through the scheduling algorithms
test trace through the execution, but do not perform any actions
If both options -o and -s are specified, the option specified last wins.
Although cron(8) and crontab(5) are officially supported under Darwin, their functionality has been absorbed into launchd(8), which provides a more flexible way of automatically executing commands. See launchctl for more information.
Cron should be started from /etc/rc or /etc/rc.local. It will return immediately, so you don’t need 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). 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).
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 command updates the modtime of the spool directory whenever it changes a crontab.
Directory for personal crontab files:
/usr/lib/cron/tabs
”Time is what prevents everything from happening at once” ~ John Archibald Wheeler
Local man page: cron - Command line help page on your local machine.
crontab - Schedule a command to run at a later time.
Lingon GUI