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CRON(8) FreeBSD System Manager's Manual CRON(8)


[править] NAME

    cron -- daemon to execute scheduled commands (Vixie Cron)

[править] SYNOPSIS

    cron [-j jitter] [-J rootjitter] [-m mailto] [-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 mem-
    ory.  The cron utility also searches for /etc/crontab which is in a dif-
    ferent 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).
    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 mail-
            box.  Sending mail will be disabled by default if mailto set to a
            null string, usually specified in a shell as  or "".
    -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 inter-
            val.  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

[править] FILES

    /etc/crontab     System crontab file
    /etc/pam.d/cron  pam.conf(5) configuration file for cron
    /var/cron/tabs   Directory for personal crontab files

[править] SEE ALSO

crontab(1) • pam(3) • crontab(5) • pam.conf(5)

[править] AUTHORS

    Paul Vixie <>

FreeBSD 9.0 June 29, 2008 FreeBSD 9.0

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