A cron is a time-based scheduler which runs a command or script on your server automatically. Cron jobs are used for scheduling jobs (commands or scripts) to run periodically at fixed times, dates, or intervals.
It is basically a task scheduler daemon (or process) that runs under a Linux / Unix-like OS. It wakes up every minute and checks planned tasks in CRON TABLE aka crontab. Crontab is a configuration file containing shell commands to run periodically on a given schedule.
Before you start, lets have a look at a few examples of cron job time scheduling and the allowed cron job operators:
┌───────────── min (0 – 59)
│ ┌────────────── hour (0 – 23)
│ │ ┌─────────────── day of month (1 – 31)
│ │ │ ┌──────────────── month (1 – 12)
│ │ │ │ ┌─ day of week (0 – 6) (Sunday=0 or 7) OR sun,mon,tue,wed,thu,fri,sat)
│ │ │ │ │
│ │ │ │ │
* * * * * /path/to/your/script.sh
* * * * * => Run every minute
0 * * * * => Run every Hour
0 0 * * * => Run every midnight
0 0 0 * * => Run every Month
0 0 0 0 * => Run every Weekdays
You can also use one of these special strings in place of the time/date fields.
| Entry | Description | Equivalent to | Example |
|---|---|---|---|
@yearly (or @annually) |
Run once a year at midnight on January 1 | 0 0 1 1 * |
@yearly php /home/example_username/mail.php |
@monthly |
Run once a month at midnight on the first day of the month | 0 0 1 * * |
@monthly php /home/example_username/mail.php |
@weekly |
Run once a week at midnight on Sunday morning | 0 0 * * 0 |
@weekly php /home/example_username/mail.php |
@daily (or @midnight) |
Run once a day at midnight | 0 0 * * * |
@daily php /home/example_username/mail.php |
@hourly |
Run once an hour at the beginning of the hour | 0 * * * * |
@hourly php /home/example_username/mail.php |
@reboot |
Run at startup (of the cron daemon) | @reboot |
@reboot php /home/example_username/mail.php |
