We’ve been showing people how to automate various tasks in Linux for quite a while now. Today, we would like to briefly show people how to automate tasks in Linux using Cron jobs.

You can execute crontab if your name appears in the file /usr/lib/cron/cron.allow. If that file does not exist, you can use crontab if your name does not appear in the file /usr/lib/cron/cron.deny. If only cron.deny exists and is empty, all users can use crontab. If neither file exists, only the root user can use crontab. The allow/deny files consist of one user name per line.

export EDITOR=vi ;to specify a editor to open crontab file. (personally, I prefer “nano”)

crontab -e Edit your crontab file, or create one if it doesn’t already exist.

crontab -l Display your crontab file.

crontab -r Remove your crontab file.

crontab -v Display the last time you edited your crontab file. (This option is only available on a few systems.)

Crontab is pretty straight forward… Minute, Hour, Day of the Week, Day of the Month, Month are all options that you can set…


* * * * * command to be executed
- - - - -
| | | | |
| | | | +----- day of week (0 - 6)
| | | +------- month (1 - 12)
| | +--------- day of month (1 - 31)
| +----------- hour (0 - 23)
+------------- min (0 - 59)


0 3 * * * /usr/local/sbin/dumpdatabases

This is my scheduled database backup. This task is scheduled to run every day at 3AM. After the jump is a breakdown of everything.

Minute	Hour	DOM	Month	DOW	Full File Location
0	3	*	*	*	/usr/local/sbin/dumpdatabases

Minute: 0-59 are valid
Hour: 24 hour clock (army time)
Day Of Month: 1-31 are valid
Month: 1-12 are valid (1 is January, 2 is February)
Day Of Week: 0-6 are valid (0 is Sunday, 1 is Monday)
Full File Location: Make sure you put the FULL path to the script you’re automating.

What is the * for? This means to run it on every option for that field. In this example it is run every day of every week of every month at 3:00AM.

To collect the cron execution execution log in a file:
30 18 * * * rm /home/joe/tmp/* > /home/joe/cronlogs/clean_tmp_dir.log

Filed under: howto, linux