cron(8) [suse man page]
CRON(8) System Manager's Manual CRON(8) NAME
cron - daemon to execute scheduled commands (ISC Cron V4.1) SYNOPSIS
cron [-l load_avg] [-n] DESCRIPTION
Cron should be started from /etc/rc or /etc/rc.local. It will return immediately, so you don't need to start it with '&'. The -n option changes this default behavior causing it to run in the foreground. This can be useful when starting it out of init. Cron searches /var/spool/cron for crontab files which are named after accounts in /etc/passwd; crontabs found are loaded into memory. Cron also searches for /etc/crontab and the files in the /etc/cron.d directory, which are in a different format (see crontab(5)). 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 execut- ing 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 modified. Note that the Crontab(1) command updates the modtime of the spool directory whenever it changes a crontab. Daylight Saving Time and other time changes Local time changes of less than three hours, such as those caused by the start or end of Daylight Saving Time, are handled specially. This only applies to jobs that run at a specific time and jobs that are run with a granularity greater than one hour. Jobs that run more fre- quently are scheduled normally. If time has moved forward, those jobs that would have run in the interval that has been skipped will be run immediately. Conversely, if time has moved backward, care is taken to avoid running jobs twice. Time changes of more than 3 hours are considered to be corrections to the clock or timezone, and the new time is used immediately. PAM Access Control On SUSE LINUX systems, crond now supports access control with PAM - see pam(8). A PAM configuration file for crond is installed in /etc/pam.d/crond . crond loads the PAM environment from the pam_env module, but these can be overriden by settings in the crontab file. SIGNALS
On receipt of a SIGHUP, the cron daemon will close and reopen its log file. This is useful in scripts which rotate and age log files. Naturally this is not relevant if cron was built to use syslog(3). CAVEATS
In this version of cron, /etc/crontab must not be writable by any user other than root. No crontab files may be links, or linked to by any other file. No crontab files may be executable, or be writable by any user other than their owner. SEE ALSO
crontab(1), crontab(5), pam(8) AUTHOR
Paul Vixie <vixie@isc.org> 4th Berkeley Distribution 10 January 1996" CRON(8)
Check Out this Related Man Page
CRON(8) BSD System Manager's Manual CRON(8) NAME
cron -- daemon to execute scheduled commands (Vixie Cron) SYNOPSIS
cron [-j jitter] [-J rootjitter] [-m mailto] [-s] [-o] [-x debugflag[,...]] DESCRIPTION
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 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). 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 "". -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 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 SEE ALSO
crontab(1), pam(3), crontab(5), pam.conf(5) AUTHORS
Paul Vixie <paul@vix.com> BSD
June 29, 2008 BSD