Viewing the cron table

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

crontab -l

00 01 * * * bash /etc/cron.daily/Example.sh

If there are no crontab entries, the /var/spool/cron directory will be empty, or the /var/spool/cron/username file will be empty, and be 0 bytes.

ls -l /var/spool/cron

-rw-------. 1 root root    0 Aug  7 21:20 root

. On the other hand, if there are crontab entries, the /var/spool/cron/username file can be viewed to view each users cron table.

cat /var/spool/cron/root

00 01 * * * bash /etc/cron.daily/Example.sh

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.

[john.doe@server1 ~]# 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:

00 01 * * * bash /home/JohnDoe/scripts/Example.sh

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

00 01 * * * /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:

00 01 * * 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:

00 01 * * 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

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

Refer to Linux Files - cron log.