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 users crontab file. crontab -l user lists the contents of the crontab file of user.

crontab -r removes the current users crontab file. crontab -r user removes the crontab file for user.

crontab -e opens the current users crontab file in vi for editing. 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):

# 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 {} \;

There are several ways to define 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