Friday, November 9, 2012


So, first things first: Using Crontab

- To see a listing of the current user's cronjobs, issue the following command:
root@anneke:~# crontab -l

This will produce, as standard output, something that resembles the following:
*/20 * * * * /home/squire/bin/rebuild-dns-zones
*/40 * * * * /home/squire/bin/delete-session-files >/dev/null 2>&1
*/10 * * * * rm /srv/*

In this example, cron:
Runs the rebuild-dns-zones script every twenty minutes.
Runs the delete-session-files script every forty minutes, and sends all output, including standard error, to /dev/null.
Deletes all files in the /srv/ directory every ten minutes.

To edit the current user's crontab file, issue the following command:
root@anneke:~# crontab -e

This will open a text editor and allow you to edit the crontab.

Basic cron Use

Entries in the crontab file come in a specific format. Each job is described on one and only one line. Each line begins with a specification of the interval, and ends with a command to be run at that interval.

cronjobs are executed with the default system shell, as if run from the command line prefixed with the following command:
/bin/sh -c

You can run any kind of script, command, or executable with cron.

Specifying Dates For cron

The syntax of crontab entries may be a bit confusing if you are new to cron. Each cron line begins with five asterisks:

* * * * *
These represent the interval of repetition with which tasks are processed. In order, the asterisks represent:

Day of month
Day of week

Minutes are specified as a number from 0 to 59. Hours are specified as numbers from 0 to 23. Days of the month are specified as numbers from 1 to 31. Months are specified as numbers from 1 to 12. Days of the week are specified as numbers from 0 to 7, with Sunday represented as either/both 0 and 7.

Special cron Operators

cron also provides a number of operators that allow you to specify more complex repetition intervals. They are:

The "/" operator "steps through" or "skips" a specified units. Therefore "*/3" in the hour field, will run the specified job, at 12:00 am, 3:00am, 6:00am, 9:00am, 12:00pm, 3:00pm, 6:00pm, and 9:00pm. A "*/3" in the "day of month" field, runs the given task on the 3rd, 6th, 9th, 12th, 15th, 18th, 21st, and 29th of every month.
The "," operator allows you to specify a list of times for repetition. Comma separated lists of times must not contain a space.
The "-" operator specifies a range of values. "2-4" in the month field will run a task in Feburary, March, and April. "1-5" in the day of week field will run a task every weekday.
Fields in crontab entries are separated by spaces. If you are using special cron operators, be particularly careful to avoid unintentional spaces in your command.

Special cron Syntax

There are also a number of special cron schedule shortcuts that you can use to specify common intervals to cron. These are specified on the crontab entry in place of the conventional five column date specification. These special interval statements are:

@yearly and @annually both run the specified task every year at 12:00am on the 1st of January. This equivalent to specifying "0 0 1 1 *" on the crontab line.
@daily and @midnight both run the cronjob every day at 12:00am. This is equivalent to the following cron syntax: "0 0 * * *".
@monthly runs the job once a month, on the 1st, at 12:00am. In standard cron syntax this is equivalent to: "0 0 1 * *".
@weekly runs the job once a week at 12:00am on Sunday. This is the same as specifying "0 0 * * 0" on the crontab line.
@hourly runs the job at the top of every hour. In standard cron syntax this is equivalent to: "0 * * * *".
The @reboot statement runs the specified command once, at start up. Generally boot-time tasks are managed by scripts in the /etc/inittab.d files, but @reboot cronjobs may be useful for users who don't have access to edit the init scripts.

Examples of crontab entries

File excerpt:crontab
45 16 1,15 * * /opt/bin/payroll-bi-monthly
45 4 * * 5 /opt/bin/payroll-weekly

In the first example, the /opt/bin/payroll-bi-monthly application is run at 4:45pm (45 16), on the 1st and 15th of every month (1,15). In the second example the /opt/bin/payroll-weekly is run at 4:45am (45 4) every Friday (4).

File excerpt:crontab
1 0 * * * /opt/bin/cal-update-daily
1 0 */2 * * /opt/bin/cal-update

These cronjobs will both run at 12:01am (1 0). The cal-update-daily job will run every day. The cal-update job will run will run every other day.

File excerpt:crontab
*/20 * * * * /home/squire/bin/rebuild-dns-zones
30 */2 * * * /opt/bin/backup-static-files
0 * * * * /opt/bin/compress-static-files
@hourly /opt/bin/compress-static-files

In the first example, the rebuild-dns-zones script runs every twenty minutes. In the second example, the backup-static-files program runs at 30 past the hour, (i.e. the "bottom of the hour") every other hour. In the final two examples, the compress-static-files script runs at the beginning of every hour.

Advanced cron Use

As cron is simply a tool for scheduling jobs, it can be used in a number of different applications and situations to accomplish a wide variety of tasks. Consider the following possibilities:

Running Jobs as Other Users

You can use cron to regularly run tasks as another user on the system. With root access, issue the following command:
root@anneke:~# crontab -u www-data -e

This will allow you to edit the crontab for the www-data user. You can run cronjobs as the root user, or as any user on the system. This is useful if you want to restrict the ability of a script to write to certain files. While the ability to run jobs as system users is extremely powerful, it can sometimes be confusing to manage a large number of crontab files dispersed among a number of system users. Also, carefully consider the security implications of running a cronjob with more privileges than is required.

Redirecting Job Output

By default, cron will send email to the executing user's email box with any output or errors that would normally be sent to the standard output or standard error. If you don't care about the standard output, you can redirect this to /dev/null. Append the following to the end of the line in your crontab file:

This will only redirect output that is sent to "standard out," (e.g. stdout). If your script generates an error, cron will still send the error to your email. If you want to ignore all output, even error messages, append the following to the end of the line in your crontab file:

>/dev/null 2>&1
While this can clean up your email box of unwanted email, redirecting all output to /dev/null can cause you to miss important errors if something goes wrong and a cronjob begins to generate errors.

More Information:

Thanks again to Library Linode for so much provided knowledge  :)