| CRON(8) | MidnightBSD System Manager's Manual | CRON(8) |
cron — daemon to
execute scheduled commands (Vixie Cron)
cron |
[-j jitter]
[-J rootjitter]
[-m mailto]
[-n] [-s]
[-o] [-x
debugflag[,...]] |
The cron utility should be started from
/etc/rc or /etc/rc.local. It
will return immediately, so you do not need to start it with '&'.
The cron utility searches
/var/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 and files in
/etc/cron.d and
/usr/local/etc/cron.d which are in a different
format (see
crontab(5)).
The cron utility then wakes up every
minute, examining all stored crontabs, checking each command to see if it
should be run in the current minute. Before running a command from a
per-account crontab file, cron checks the status of
the account with pam(3) and
skips the command if the account is unavailable, e.g., locked out or
expired. Commands from /etc/crontab bypass this
check. 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 from address of
this mail may be set with the MAILFROM environment
variable.
Additionally, cron checks each minute to
see if its spool directory's modification time (or the modification 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:
-j
jittercron will sleep a random number of seconds in the
range from 0 to jitter. This will not affect
superuser jobs (see -J). A value for
jitter must be between 0 and 60 inclusive. Default
is 0, which effectively disables time jitter.
This option can help to smooth down system load spikes during moments when a lot of jobs are likely to start at once, e.g., at the beginning of the first minute of each hour.
-J
rootjitter-j except that it will affect jobs run by the
superuser only.-m
mailtocron mail.
Each crontab(5)
without MAILTO explicitly set will send mail to
the mailto mailbox. Sending mail will be disabled by
default if mailto set to a null string, usually
specified in a shell as '' or
"".-n-sThe 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 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(5) 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(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).
-o-o and -s
are specified, the option specified last wins.-x
debugflag[,...]bitextloadmiscparsprocschtestcronPaul Vixie <paul@vix.com>
| April 15, 2019 | midnightbsd-3.1 |