Ensure the cron daemon in running
Before using cron, ensure the cron daemon is enabled and running. The ps command can be used to determine if your system is using init or systemd. If PID 1 is init, then you will use the service command. If PID 1 is systemd, then you will use the systemctl command.
If your system is using systemd, use the systemctl command to start and enable crond.
systemctl start crond systemctl enable crond
If your system is using init, use the service command to start and enable crond.
service crond start service crond enable
Viewing the cron table
The crontab -l (list) command can be used to view the entries in crontab. If the -u (user) option is not included, the current users cron table will be displayed.
Or the -u option can be used to display a certain users cron table.
crontab -l -u john.doe
Something like this could be returned.
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
Edit cron table
The crontab -e (edit) command can be used to edit the current users cron table, or the -u option can be included to specify a specifc user. In this example, John Doe's cron table will be edited.
[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
Crontab can be used to run bash shell scripts, or commands. For example, here is how you would schedule the reboot command to occur daily at 1:00 am.
00 01 * * * /sbin/reboot
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
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, on the hour and then 15 minutes, then 30 minutes, then 45 minutes after the hour.
*/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
Variables can be defined in the cron table. In this example, the FOO variable contains a value of bar. In this example, bar would be appended to out.txt once every minute.
FOO="bar" * * * * * echo $FOO >> out.txt
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:
Refer to Linux Files - cron log.