FreeKB - Schedule reoccurring jobs using CRONTAB in Linux

Creating a crontab job

The crontab command with the -e (edit) option can be used to edit the cron table. Each user has their own cron table. Editing the cron table as root will edit root's cron table.

[root@server1 ~]# crontab -e

Editing the cron table as John.Doe will edit John's cron table.

[JohnDoe@server1 ~]# crontab -e

If a non-root user has permission to use sudo, using sudo will run the script as root.

[JohnDoe@server1 ~]# sudo crontab -e

Hourly, Daily, Weekly, Monthly, Reboot

The @ symbol can be used to run a command or script once every hour, once every day, once every week, once every month, or when the server is restarted.

@hourly  bash /home/JohnDoe/scripts/Example.sh
@daily   bash /home/JohnDoe/scripts/Example.sh
@weekly  bash /home/JohnDoe/scripts/Example.sh
@monthly bash /home/JohnDoe/scripts/Example.sh
@reboot  bash /home/JohnDoe/scripts/Example.sh

The sleep option can be used to wait a while before running a script after reboot. In this example, Example.sh is run 1 minute after reboot.

@reboot sleep 60 && bash /home/JohnDoe/scripts/Example.sh

Specific date time

You can specify an exact date and time to run a command or script. The syntax of the lines in the cron table is: <minute> <hour> <day of the month> <month> <day of the week> <command or script to execute>. For example, to run Example.sh every day at 1:00 am:

0 1 * * * bash /home/JohnDoe/scripts/Example.sh

Or, to remove every file in the /tmp directory every day at 1:00 am.

0 1 * * * /bin/rm /tmp/*

The first 5 fields of crontab are:

minute (0-59)

hour (0-23)

day of the month (1-31)

month (1-12)

day of the week (0-7)

0 = Sunday

1 = Monday

2 = Tuesday

3 = Wednesday

4 = Thursday

5 = Friday

6 = Saturday

7 = Sunday

Ranges

Each field can accept a range. For example, to run Example.sh at 1:00 am, Monday through Friday:

0 1 * * 1-5 bash /etc/cron.daily/Example.sh

Comma separated lists

Fields can also accept a comma separated list. For example, to run Example.sh at 1:00 am on Saturday and Sunday:

0 1 * * 6,7 bash /etc/cron.daily/Example.sh

Once every x minutes

All astericks can be used to run a cron job once every minute.

* * * * * bash /etc/cron.daily/Example.sh

Or, in this example, Example.sh will be run once every 15 minutes.

*/15 * * * * bash /etc/cron.daily/Example.sh

Or, in this example, Example.sh will be run once every 15 minutes as long as minute is 2-59. In other words, Example.sh will not run at 00 or 01 minutes.

2-59/15 * * * * bash /etc/cron.daily/Example.sh

Viewing the cron table

The crontab -l (list) command can be used to view the entries in crontab.

~]# crontab -l
0 1 * * * bash /etc/cron.daily/Example.sh

Likewise, the /var/spool/cron/crontab/username file can be viewed to view each users cron table.

~]# cat /var/spool/cron/crontab/root
0 1 * * * bash /etc/cron.daily/Example.sh

Removing cron tables

The crontab -r (remove) command can be used to remove a users entire cron table. For example, to remove user1 cron table:

~]# crontab -r

Cron log

The cron log is typically located at /var/log/cron.

~]# tail /var/log/cron
mm dd hh:mm:ss hostname CROND[pid]: (user) CMD (/etc/cron.daily/Example.sh)

The syslog or rsyslog daemon control how events are logged to /var/log/cron. If you want to change logging, such as disabling logging, you'll edit the syslog or rsyslog configuration file and then restart the syslog or rsyslog daemon.

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