Linux cron is a very useful tool for running scheduled background jobs at a specific time and/or day on an ongoing basis.
This tutorial will explain how to run a cronjob and explain how it works. By defining items in the cron table, called crontab, you can schedule (or run) any script or program to run on almost any sort of schedule.
The basics:
Every user has their own cron table (crontab), those scheduled scripts or programs run as that user, so take in account with regards to that users permissions.
You may edit a users crontab by entering the following command in SSH:
crontab -e
You can list what your current crontab is by using the following command:
crontab -l
Root can view other users crontab by using this command:
crontab -u <username>
Crontab formatting:
Formatting the correct entries in crontab are:
* * * * * command to be executed- - - - -¦ ¦ ¦ ¦ ¦¦ ¦ ¦ ¦ ¦¦ ¦ ¦ ¦ +----- day of week (0 - 6) (0 is Sunday, or use names)¦ ¦ ¦ +---------- month (1 - 12)¦ ¦ +--------------- day of month (1 - 31)¦ +-------------------- hour (0 - 23)+------------------------- min (0 - 59)
As an example, if we want to run a script every day at 4:30AM we would use:
30 4 * * * /path/to/your/script
You may suppress email messages after the cron has run by entering > /dev/null 2>&1 at the end of the entry, like so:
30 4 * * * /path/to/your/script > /dev/null 2>&1
Here are some other examples of different times for your cronjob:
#Run command at 7:00am each weekday [mon-fri]00 07 * * 1-5 (/path/to/your/script);
#Run command on 1st of each month, at 5:30pm30 17 1 * * (/path/to/your/script);
#Run command at 8:00am,10:00am and 2:00pm every day with email suppression00 8,10,14 * * * (/path/to/your/script > /dev/null 2>&1);
#Run command every 5 minutes during market hours*/5 6-13 * * mon-fri (/path/to/your/script);
Some other things to think about, to use with crontab.
Special characters:
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 forward slash (/) to specify a repeating range.
For example: */5 for every five minutes, hours, days
You can use dashes (-) to specify ranges.
For example: mon-fri, or 9-17
Special entries:
There are several special entries that you may also use in crontab, some of 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 server reboots. 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.
Example's of some special entries:
@reboot munin start@reboot exim restart
Here is the complete list of special entries you may use:
@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 * * * *
Script output:
If there is any output from your script or command it will be sent to that user's e-mail account, on that server. 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:
MAILTO="admin@yourserver.tld"
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
This concludes the crontab tutorial.
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