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CRON(8) |
FreeBSD 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
jitter
- Enable time jitter. Prior to executing commands,
cron 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
- Enable time jitter for superuser jobs. The same as
-j except that it will affect jobs run by the
superuser only.
-m
mailto
- Overrides the default recipient for
cron 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
- Do not daemonize; run in foreground instead.
-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 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
- 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.
-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
- /etc/crontab
- System crontab file
- /etc/cron.d
- Directory for optional/modularized system crontab files.
- /etc/pam.d/cron
- pam.conf(5)
configuration file for
cron
- /usr/local/etc/cron.d
- Directory for third-party package provided crontab files.
- /var/cron/tabs
- Directory for personal crontab files
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