The cron and at services enable sysadmins to schedule tasks to run at a specific time in the future. The at service specifies a one-time task that runs at a certain time. The cron service can schedule tasks on a repetitive basis, such as daily, weekly, or monthly.
The crond daemon is the background service that enables cron functionality.
The cron service checks for files in the /var/spool/cron and /etc/cron.d directories and the /etc/anacrontab file. The contents of these files define cron jobs that are to be run at various intervals. The individual user cron files are located in /var/spool/cron, and system services and applications generally add cron job files in the /etc/cron.d directory.
Crontab The cron utility runs based on commands specified in a cron table (crontab). Each user, including root, can have a cron file. These files don't exist by default, but can be created in the /var/spool/cron directory using the crontab -e command that's also used to edit a cron file. Using the crontab command not only allows you to edit the command, it also restarts the crond daemon when you save and exit the editor. The crontab command uses Vi as its underlying editor, because Vi is always present
New cron files are empty, so commands must be added from scratch, see the example file below:
# crontab -e
SHELL=/bin/bash
MAILTO=root@example.com
PATH=/bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/usr/local/sbin
# For details see man 4 crontabs
# Example of job definition:
# .---------------- minute (0 - 59)
# | .------------- hour (0 - 23)
# | | .---------- day of month (1 - 31)
# | | | .------- month (1 - 12) OR jan,feb,mar,apr ...
# | | | | .---- day of week (0 - 6) (Sunday=0 or 7) OR sun,mon,tue,wed,thu,fri,sat
# | | | | |
# * * * * * user-name command to be executed
# backup using the rsbu program to the internal 4TB HDD and then 4TB external
01 01 * * * /usr/local/bin/rsbu -vbd1 ; /usr/local/bin/rsbu -vbd2
# Set the hardware clock to keep it in sync with the more accurate system clock
03 05 * * * /sbin/hwclock --systohc
# Perform monthly updates on the first of the month
# 25 04 1 * * /usr/bin/dnf -y updateThe directory /etc/cron.d is where some applications, such as SpamAssassin and sysstat, install cron files.
The anacron program performs the same function as crond, but it adds the ability to run jobs that were skipped, such as if the computer was off or otherwise unable to run the job for one or more cycles.As soon as the computer is turned on and booted, anacron checks to see whether configured jobs missed their last scheduled run. If they have, those jobs run immediately, but only once.
The anacron program provides some easy options for running regularly scheduled tasks. Just install your scripts in the /etc/cron.[hourly|daily|weekly|monthly] directories, depending how frequently they need to be run.
These single-word time shortcuts can be used to replace the five fields usually used to specify times. The @ character is used to identify shortcuts to cron. The list below, taken from the crontab(5) man page, shows the shortcuts with their equivalent meanings.
- @reboot : Run once after reboot.
- @yearly : Run once a year, ie. 0 0 1 1 *
- @annually : Run once a year, ie. 0 0 1 1 *
- @monthly : Run once a month, ie. 0 0 1 * *
- @weekly : Run once a week, ie. 0 0 * * 0
- @daily : Run once a day, ie. 0 0 * * *
- @hourly : Run once an hour, ie. 0 * * * *
These shortcuts can be used in any of the crontab files, such as those in /etc/cron.d.
For more information, the man pages for cron, crontab, anacron, anacrontab, and run-parts all have excellent information and descriptions of how the cron system works.
