Cron

Timed execution of commands. The cron daemon is checking the crontab every minute to see if anything needs to be done

Format of crontab

minute	hour	day-of-month	month	day-of-week	command
1-59	1-23	1-31	1-12	0-7	doit

In scheduling fields (all except the command) following applies:

• * means each tick is matched.
• */5 means each 5th tick is matched
• A range is defined by a dash. E.g. 2-5 in minute -> execute command on minute 2,3,4 and 5
• A list is defined by comma's E.g. 2,4,6 in minute -> execute command on minute 2, 4 and 6
• For month and day-of-week also names can be used by giving the first 3 characters (case insensitive)
• For cron the week starts on Sunday so 0 is Sunday, 7 is Saturday

Example: Every Sunday at noon execute my script:

00 12 * * sun /usr/local/bin/myscript.sh
00 12 * * 0 /usr/local/bin/myscript2.sh

Who can user cron can be limited in /etc/cron.[allow|deny]

• if cron.allow exists - only users listed into it can use crontab
• If cron.allow does not exist - all users except the users listed into cron.deny can use crontab
• If neither of the file exists - only the root can use crontab
• If a user is listed in both cron.allow and cron.deny - that user can use crontab.

crontab -l

List the current crontab

crontab <crontabfile>

Load the file into the crontab, all existing content will be replaced. In the end a user crontab is stored in /var/spool/cron/crontabs/<username>. Don't edit it there, the cron daemon will only read it if told so by the crontab command.

crontab -e

Edit the current crontab directly in the default editor. Change the default editor by setting the EDITOR environment variable.