Cron Jobs Link to heading
The Cron daemon is a built-in Linux utility that reads the crontab (cron table) file and executes commands and scripts at predetermined times and intervals
crontab -e
Documentation Link to heading
Uses:
- updating software
- creating backups
- clearing caches
- running arbitrary tasks at specific times
Syntax format:
MIN HOUR DOM MON DOW CMD
Execute a script on January 1st at 9 AM, regardless of which day of the week falls on January 1st.
0 9 1 1 \* /path/to/your_script.sh
For example, the following command tells Cron to execute the backup.sh script, located in the root directory, every day at midnight:
0 0 * * * /root/backup.sh
| Field | Possible Values | Syntax | Description |
|---|---|---|---|
| Minute Within the Hour (MIN) | 0 – 59 | 7 * * * * | The cron job is initiated every time the system clock shows 7 in the minute’s position. |
| Hour of the Day (HOUR) | 0 – 23 | 0 7 * * * | The cron job runs any time the system clock shows 7 AM (7 PM would be coded as 19). |
| Day of the Month (DOM) | 1 – 31 | 0 0 7 * * | The day of the month is 7, which means that the job runs every 7th day of the month. |
| Month of the Year (MON) | 1 - 12 or JAN - DEC | 0 0 0 7 * | The numerical month is 7, which determines that the job runs only in July. The Month field value can be a number or the month abbreviation. |
| Day of the Week (DOW) | 0 - 7 or SUN - SAT | 0 0 * * 7 | 7 in the current position means that the job would only run on Sundays. The Day of the Week field can be a number or the day abbreviation. |
Operators Link to heading
An asterisk (_)substitutes all possible values for a time field. For example, when used in the day field, it indicates that the task should be executed every day.A comma (,)is used to separate individual values within a field. For example, 20,40 in the minute field runs the task at 20 and 40 minutes past the hour.A dash (-)defines a range of values in a single field. Entering 15-18 in the minute field instructs Cron to run a task every minute between and including the 15th and 18th minute.A forward slash (/)divides a field value into increments. For example, _/30 in the minute field means that the task is run every 30 minutes.