Contents

Linux Crontab Configuration

Linux cron, makes you run commands/scripts on schedule..

fields

(* * * * *)
First Field: minute
Second Field: hour
Third Field: day of the month
Forth Field: month of the year
Fifth Field: day of the week

list

List crontab entries for current user

$ crontab -l

List crontab entries for any user as root

root# crontab -u sysadm -l

edit

Edit crontab entries for current user

$ crontab -e

Edit crontab entries for any user as root

root# crontab -u sysadm -e

See which users have crontab entries as root

root# ls -al /var/spool/cron/

example 1

Run every Sunday at 3am

0 3 * * 0

example 2

Run every 15 mins on Mondays

*/15 * * * 1

example 3

Run at 3am first day of every month

0 3 1 * *

example 4

Run every hour at 00

0 * * * *

example 5

Run every minute

* * * * *
Note

If /etc/cron.allow file exists and contains users, /etc/cron.deny existance dies not matter.
Users in /etc/cron.allow file will be permitted.

If /etc/at.allow file exists and contains users, /etc/at.deny existance dies not matter.
Users in /etc/at.allow file will be permitted.

if /etc/cron.allow file file exists but empty, no users will be permitted.
if /etc/at.allow file file exists but empty, no users will be permitted.

if /etc/cron.allow file does not exits and if /etc/cron.deny file contains users, those users will be denied.
if /etc/at.allow file does not exits and if /etc/at.deny file contains users, those users will be denied.

if /etc/cron.allow file does not exits and /etc/cron.deny file empty, all users will be permitted.
if /etc/at.allow file does not exits and /etc/at.deny file empty, all users will be permitted.

example 6

Run script.sh script two hours later

$ at -f script.sh now+2 hours

example 7

Run script.sh script at 5/15/18 3am

$ at -f script.sh 3:00am 5/15/18

atq

Query at jobs

$ atq -l

atrm

Remote at job number 5

$ atrm 5