Crontab
Overview
Cron is driven by a crontab (cron table) file, a configuration file that specifies shell commands to run periodically on a given schedule. The crontab files are stored where the lists of jobs and other instructions to the cron daemon are kept. Users can have their own individual crontab files and often there is a system-wide crontab file (usually in /etc or a subdirectory of /etc) that only system administrators can edit.
Each line of a crontab file represents a job, and looks like this:
# ┌───────────── minute (0 - 59) # │ ┌───────────── hour (0 - 23) # │ │ ┌───────────── day of the month (1 - 31) # │ │ │ ┌───────────── month (1 - 12) # │ │ │ │ ┌───────────── day of the week (0 - 6) (Sunday to Saturday; # │ │ │ │ │ 7 is also Sunday on some systems) # │ │ │ │ │ # │ │ │ │ │ # * * * * * command to execute
The syntax of each line expects a cron expression made of five fields, followed by a shell command to execute.
Logging
Cron execution logs can be saved to specific file using rsyslog service.
- In Ubuntu uncomment in
/etc/rsyslog.d/50-default.confthe line:
#cron.* /var/log/cron.log- and restart rsyslog service:
sudo systemctl restart rsyslog
- In Debian uncomment in
/etc/rsyslog.confthe line:
#cron.* /var/log/cron.log
Configuration
systemctl edit cron.servicesystemctl status cron
/etc/cron.deny (echo ALL >>/etc/cron.deny) /etc/cron.allow
/var/spool/cron/crontabs/
Examples
Execute every minute:
* * * * * /scripts/script.sh 45 18 * * * /scripts/script.sh @reboot /scripts/script.sh
Related terms
See also
cron,crontab,systemctl list-timers, anacron, Kubernetes scheduler, Systemd services,CronJobsStack,cron.schedule,crontab -l,0x004b,at, /etc/cron.deny, /etc/cron.allow
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Source: wikiversity
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