Every user as well as administrator of the linux system very often needs to execute some programs on regular basis.
For example, an administrator may need to monitor a disk usage of a system. In this case a cron scheduler is very handy tool to achieve this. Let's say that root needs to execute /usr/local/sbin/backup.sh script every Sunday at 2:36AM he would edit his crontab file as shown on the figure below:
# crontab -e
The format is simple, 6 fields separated with spaces or tabs. The rest of the line is the command, and it's parameters to be executed. The sixth field - user name (in blue) is used only in the system wide cron scheduler.
Crontab Example 1: This crontab example runs updatedb command 35 minutes past every hour.
35 * * * * updatedb
Crontab Example 2: This crontab example runs /usr/local/bin/diskusage.sh every 5 minutes (e.g. 0, 5, 10, 15, ...).
*/5 * * * * /usr/local/bin/diskusage.sh
Crontab Example 3: This crontab example runs /usr/local/bin/diskusage.sh at 1:25 AM, 1:50 AM every Tuesday and on 15th of every month.
25,50 1 15 * 2 /usr/local/bin/diskusage.sh
Crontab Example 4: This crontab example runs /usr/local/bin/diskusage.sh at 2:00 PM on 10th of March, June, September and December.
00 14 10 3,6,9,12 * /usr/local/bin/diskusage.sh
Crontab Example 6: This crontab example runs /usr/local/bin/diskusage.sh every 5 minutes during the 5 working days (Monday - Friday), every week and month.
*/5 * * * 1-5 /usr/local/bin/diskusage.sh
Crontab Example 7: This crontab example runs /usr/local/bin/diskusage.sh every minute during the 4-th hour only in Sunday, every week and month. This is every minute from 0:00 till 0:59, then from 4:00 till 4:59, etc.
* */4 * * sun /usr/local/bin/diskusage.sh
As a Linux administrator you can also use predefined cron directories:
/etc/cron.d /etc/cron.daily /etc/cron.hourly /etc/cron.monthly /etc/cron.weekly
If root wishes to run backup.sh script once a week he will place backup.sh script into /etc/cron.weekly directory.
Every user can edit, view or remove his own crontab file. If the root user needs to change someone else's crontab file he must add '-u' option to specify the user name. To edit crontab file for user foobar we can use command:
# crontab -u foobar -e
Remove foobar's crontab file:
# crontab -u foobar -r
To view foobar's crontab content:
# crontab -u foobar -l