====== Automating tasks with cron ======

**One of the main tasks of the systems admnistrator is to automate tasks that need to be accomplished on a regular basis.**

===== cron =====

cron is a scheduling daemon for UNIX and Linux systems. It searches for files in the directories /etc/cron.hourly, /etc/cron.daily, /etc/cron.weekly, /etc/cron.monthly, as well as user files in /var/spool/cron. cron wakes every minute, examining the files and executing the appropriate commands. It also examines the files /etc/cron.allow and /etc/cron.deny, if they exist.

|**Directory and file locations used for cron**||
|/etc/crontab 	        |contains the time to run various cron directories|
|/etc/cron.hourly 	|contains files to be run hourly|
|/etc/cron.daily 	|contains files to be run daily|
|/etc/cron.weekly 	|contains files to be run weekly|
|/etc/cron.monthly 	|contains files to be run monthly|
|/etc/cron.allow 	|lists users allowed to use crontab|
|/etc/cron.deny 	|lists users denied to use crontab|

cron.allow and cron.deny may not be present all on systems.

===== Editing a crontab file =====

crontab files are edited using the crontab command. You must be root to view or edit another users crontab file.

**crontab -l** lists the contents of the current user's crontab file. \\ 
**crontab -l user** lists the contents of the crontab file of user.

**crontab -r** removes the current user's crontab file. \\ 
**crontab -r user** removes the crontab file for user.

**crontab -e** opens the current user's crontab file in vi for editing. Be sure to look up [[:tutorials:using:working_with_plain_text_files#vi_and_vim|some basic moves in vi]], if you have not used it before.\\ 
**crontab -e user** opens the crontab file of user in vi for editing.

A typical line in a crontab file will resemble the following:

  30 2 * * * find / -name core -exec rm {} \;

This will find and delete all core files each night at 2:30.

The fields in a crontab entry (Hint-copy and paste all but the last line of this into your crontab for a handy reference when you edit):

<code>
# minute (0-59),
# |      hour (0-23),
# |      |       day of the month (1-31),
# |      |       |       month of the year (1-12 or Jan-Dec),
# |      |       |       |       day of the week (0-6, 0=Sunday, or Sun-Sat).
# |      |       |       |       |       commands

  30     2       *       *       *       find / -name core -exec rm {} \;
</code>

There are several ways to define values:

|, 	|multiple values|
|- 	|a range of values|
|* 	|all possible values|

If the first field contains **0,15,30,45** this would indicate it would run every 15 minutes. To run Monday through Wednesday, the fifth column should contain 1-3. And if the fifth column is a * it will run every day.
Examples

Run the command ''df -h'' and send the output to the file ///root/dfusage// every Sunday Night at 2:30 am.

  30 2 * * 0 df -h > /root/dfusage