Thread: crontab explained
11-04-2006, 05:08 AM #1Disabled
- Join Date
- Oct 2006
cron is a utility that you can use to schedule and automate tasks. By defining items in the cron table, called crontab, you can schedule any script or program to run on almost any sort of schedule
Each user has their own crontab, the scheduled scripts run as that user take this in account with regards to permissions. To edit the crontab use the following command:
$ crontab -e
$ crontab -l
#crontable -u linuxweb
The following is the format entries in a crontab must be.
MIN HOUR MDAY MON DOW COMMAND 5 * * * *
MIN = Minute 0-60 HOUR = Hour [24-hour clock] 0-23 MDAY = Day of Month 1-31 MON = Month 1-12 OR jan,feb,mar,apr ... DOW = Day of Week 0-6 OR sun,mon,tue,wed,thu,fri,sat COMMAND = Command to be run Any valid command-line
Here are a few examples, for crontab entry
#Run command at 7:00am each weekday [mon-fri] 00 07 * * 1-5 (/sbin/diskusages.sh);Code:
#Run command on 1st of each month, at 5:30pm 30 17 1 * * (/home/backup.sh);Code:
#Run command at 8:00am,10:00am and 2:00pm every day 00 8,10,14 * * * (/sbin/sendstatus.sh);Code:
#Run command every 5 minutes during market hours */5 6-13 * * mon-fri get_stock_quote.script
Special Characters in Crontab
You can use an asterisk in any category to mean for every item, such as every day or every month.
You can use commas in any category to specify multiple values. For example: mon,wed,fri
You can use dashes to specify ranges. For example: mon-fri, or 9-17
You can use forward slash to specify a repeating range. For example: */5 for every five minutes, hours, days
There are several special entries, some which are just shortcuts, that you can use instead of specifying the full cron entry.
The most useful of these is probably @reboot which allows you to run a command each time the computer gets reboot. This could be useful if you want to start up a server or daemon under a particular user, or if you do not have access to the rc.d/init.d files.
# restart freevo servers @reboot freevo webserver start @reboot freevo recordserver start
Entry Description Equivalent To
@reboot Run once, at startup. None @yearly Run once a year 0 0 1 1 * @annually (same as @yearly) 0 0 1 1 * @monthly Run once a month 0 0 1 * * @weekly Run once a week 0 0 * * 0 @daily Run once a day 0 0 * * * @midnight (same as @daily) 0 0 * * * @hourly Run once an hour 0 * * * *
If there is any output from your script or command it will be sent to that user's e-mail account, on that box. Using the default mailer which must be setup properly.
You can set the variable MAILTO in the crontab to specify a separate e-mail address to use. For example:
11-19-2006, 03:06 PM #2Aspiring Evangelist
- Join Date
- Mar 2006
If you dont wish to recieve any output, even though the script you're running returns some (for example a script runing every 5 mins and returning some output would quickly spam your email account), you can add this to the end of the script command: > /dev/null 2>&1
#Run command every 5 minutes during market hours */5 6-13 * * mon-fri get_stock_quote.script > /dev/null 2>&1