Cron is a daemon that executes scheduled commands. Cron is started automatically from /etc/init.d on entering multi-user runlevels. 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.
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. Conversely, 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 wild cards 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.
In Debian and Redhat cron treats the files in /etc/cron.d as extensions to the /etc/crontab file (they follow the special format of that file, i.e. they include the user field). 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 allow to add a crontab file to /etc/cron.d. Such files should be named after the package that supplies 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. Like /etc/crontab, the files in the /etc/cron.d directory are monitored for changes.
You can only use Bourne shell commands can be used in crontab entries. This means you can't use any extentions recognized by bash and other modern shells. You should use absolute path names for commands like /bin/ls. This is to insure you call the correct command.
Crontab is the program used to install, deinstall or list the tables used to drive the cron(8) daemon in Vixie Cron. Each user can have their own crontab, and though these are files in /var/spool/cron/crontabs, they are not intended to be edited directly.
If the /etc/cron.allow file exists, then you must be listed therein in order to be allowed to use this command. If the /etc/cron.allow file does not exist but the /etc/cron.deny file does exist, then you must not be listed in the /etc/cron.deny file in order to use this command. If neither of these files exists, then depending on site-dependent configuration parameters, only the super user will be allowed to use this command, or all users will be able to use this command. For standard Debian systems, all users may use this command.
If the -u option is given, it specifies the name of the user whose crontab is to be tweaked. If this option is not given, crontab examines "your" crontab, i.e., the crontab of the person executing the command. Note that su(8) can confuse crontab and that if you are running inside of su(8) you should always use the -u option for safety's sake.
The first form of this command is used to install a new crontab from some named file or standard input if the pseudo-filename ``-'' is given.
The -l option causes the current crontab to be displayed on standard output.
The -r option causes the current crontab to be removed.
The -e option is used to edit the current crontab using the editor specified by the VISUAL or EDITOR environment variables. The specified editor must edit the file in place; any editor that unlinks the file and recreates it cannot be used. After you exit from the editor, the modified crontab will be installed automatically.
The format of the crontab file is as follows:
#Minute (0-59) Hour (0-23) Day of Month (1-31) Month (1-12 or Jan-Dec) Day of Week (0-6 or Sun-Sat) Command
0 2 12 * 0,6 /usr/bin/find
This line executes the "find" command at 2AM on the 12th of every month that a Sunday or Saturday falls on.
Here are some more examples of crontab lines. Use the command "crontab -e" to edit your crontab file.
This line executes the "ping" command every minute of every hour of every day of every month. The standard output is redirected to dev null so we will get no e-mail but will allow the standard error to be sent as a e-mail. If you want no e-mail ever change the command line to "/sbin/ping -c 1 192.168.0.1 > /dev/null 2>&1".
* * * * * /sbin/ping -c 1 192.168.0.1 > /dev/null
This line executes the "ping" and the "ls" command every 12am and 12pm on the 1st day of every 2nd month. It also puts the output of the commands into the log file /var/log/cronrun.
0 0,12 1 */2 * /sbin/ping -c 192.168.0.1; ls -la >>/var/log/cronrun
This line executes the disk usage command to get the directory sizes every 2am on the 1st thru the 10th of each month. E-mail is sent to the user executing the jobs e-mail address. Remember if your executing this command as root to go into your /etc/aliases file and put your e-mail in there. You could also put a .forward file in root's home dir to forward to another e-mail address.
0 2 1-10 * * du -h --max-depth=1 /
This line is and example of running a cron job every month, on Mondays whose dates are between 15-21. This means the third Monday only of the month at 4 a.m.
0 4 15-21 * 1 /command
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