CRON(8) BSD System Manager's Manual CRON(8)
NAME
cron -- daemon to execute scheduled commands (Vixie Cron)
SYNOPSIS
cron [-s] [-o] [-x debugflag[,...]]
DESCRIPTION
The cron utility is launched by launchd(8) when it sees the existence of /etc/crontab or files in
/usr/lib/cron/tabs. There should be no need to start it manually. See
/System/Library/LaunchDaemons/com.vix.cron.plist for details.
The cron utility searches /usr/lib/cron/tabs for crontab files which are named after accounts in
/etc/passwd; crontabs found are loaded into memory. The cron utility also searches for /etc/crontab
which is in a different format (see crontab(5)). The cron utility then wakes up every minute, examin-ing examining
ing 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 modification time (or the modifi-cation modification
cation time on /etc/crontab) has changed, and if it has, cron will then examine the modification time
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 modification time of the spool
directory whenever it changes a crontab.
Available 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. If both options -o and -s are specified, the
option specified last wins.
-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 is handled in
one of two ways:
The first case is for the jobs that run at every 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(5) changed after that) or
would run after the change at the next hour. They work as always, skipping the skipped time or
running in the added time, as usual.
The second case is for the jobs that run less frequently. These are executed exactly once;
they are neither skipped, nor executed twice (unless cron is restarted or the user's crontab(5)
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(5).
-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
SEE ALSO
crontab(1), launchctl(1), crontab(5), launchd.plist(5), launchd(8)
AUTHORS
Paul Vixie <paul@vix.com>
BSD December 20, 1993 BSD
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