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)
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CRON(8) System Manager's Manual CRON(8) NAME
cron - daemon to execute scheduled commands (Vixie Cron) SYNOPSIS
cron [-f] [-l] [-L loglevel] DESCRIPTION
cron is started automatically from /etc/init.d on entering multi-user runlevels. OPTIONS
-f Stay in foreground mode, don't daemonize. -l Enable LSB compliant names for /etc/cron.d files -L loglevel Sets the loglevel for cron. The standard logging level (1) will log the start of all the cron jobs. A higher loglevel (2) will cause cron to log also the end of all cronjobs, which can be useful to audit the behaviour of tasks run by cron. Logging will be disabled if the loglevel is set to zero (0). NOTES
cron searches its spool area (/var/spool/cron/crontabs) for crontab files (which are named after accounts in /etc/passwd); crontabs found are loaded into memory. Note that crontabs in this directory should not be accessed directly - the crontab command should be used to access and update them. cron also reads /etc/crontab, which is in a slightly different format (see crontab(5)). Additionally, cron reads the files in /etc/cron.d: it treats the files in /etc/cron.d as in the same way as the /etc/crontab file (they follow the special format of that file, i.e. they include the user field). However, they are independent of /etc/crontab: they do not, for example, inherit environment variable settings from it. The intended purpose of this feature is to allow packages that require finer control of their scheduling than the /etc/cron.{daily,weekly,monthly} directories to add a crontab file to /etc/cron.d. Such files should be named after the package that sup- plies them. Files must conform to the same naming convention as used by run-parts(8): they must consist solely of upper- and lower-case letters, digits, underscores, and hyphens. If the -l option is specified, then they must conform to the LSB namespace specification, exactly as in the --lsbsysinit option in run-parts. Like /etc/crontab, the files in the /etc/cron.d directory are monitored for changes. In general, the admin should not use /etc/cron.d/, but use the standard system crontab /etc/crontab. In contrast to the spool area, files in /etc/cron.d may be symlinks, provided that both the symlink and the file it points to are owned by root. 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 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 children copies of cron running these processes have their name coerced to uppercase, as will be seen in the syslog and ps output. 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. Special considerations exist when the clock is changed by less than 3 hours, for example at the beginning and end of daylight savings time. If the time has moved forwards, those jobs which would have run in the time that was skipped will be run soon after the change. Con- versely, if the time has moved backwards by less than 3 hours, those jobs that fall into the repeated time will not be re-run. Only jobs that run at a particular time (not specified as @hourly, nor with '*' in the hour or minute specifier) are affected. Jobs which are specified with wildcards are run based on the new time immediately. Clock changes of more than 3 hours are considered to be corrections to the clock, and the new time is used immediately. cron logs its action to the syslog facility 'cron', and logging may be controlled using the standard syslogd(8) facility. ENVIRONMENT
If configured in /etc/default/cron in Debian systems, the cron daemon localisation settings environment can be managed through the use of /etc/environment or through the use of /etc/default/locale with values from the latter overriding values from the former. These files are read and they will be used to setup the LANG, LC_ALL, and LC_CTYPE environment variables. These variables are then used to set the charset of mails, which defaults to 'C'. This does NOT affect the environment of tasks running under cron. For more information on how to modify the environment of tasks, consult crontab(5). The daemon will use, if present, the definition from /etc/timezone for the timezone. The environment can be redefined in user's crontab definitions but cron will only handle tasks in a single timezone. SEE ALSO
crontab(1), crontab(5) AUTHOR
Paul Vixie <paul@vix.com> 4th Berkeley Distribution 19 April 2010 CRON(8)